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D. degree in Physics I l’) 1,”me {kl ‘8 'TSCL V\Q/ @ajor Professor’s SiQtature September 21, 2006 Date MSU is an Affirmative Action/Equal Opportunity institution LIBRARY Michit "ate UfllVCiotty A- .<-.--.-o-0—0-.-.-o-c—n-o—o-o—o-c—o-u—-o-.-.-.-a-c-.—o—-—--—n—.--o-o-s-o-o-o-u-.—-—--—o-u—--o-o—.-—-—-—-—o--o--o-— PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 2/05 p:/CIRCIDateDue.indd-p.1 RARE ISOTOPE PRODUCTION By Michal Mocko A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Physics and Astronomy 2006 ABSTRACT RARE ISOTOPE PRODUCTION By Michal Mocko Projectile fragmentation is one of the prominent methods of rare isotope beam production. A series of projectile fragmentation experiments have been performed us— ing 40Ca, 48Ca, 58Ni, and 64Ni primary beams at 140 MeV/ u and 86Kr at 64 MeV/u. Two targets: 9Be and 181Ta were used in the present experiments in order to inves- tigate the target and projectile dependence of the fragmentation production cross sections. The study resulted in 1740 measured cross sections created in reactions of 10 systems. The fitted parameters of the momentum distributions are compared to the sys- tematics and parameterizations derived from the high energy projectile or target frag- mentation reactions. Extracted width parameter corresponding to the “pure” frag— mentation component of the momentum distribution is in good agreement with the limiting fragmentation models, and consistent with the relativistic energy projectile fragmentation experiments. The fragmentation cross sections are reproduced well by the empirical parameterization EPAX, while discrepancies in tails of the isotopic dis- tributions and for elements close to the neutron-rich projectiles were observed. Fragmentation cross section target dependence is more complex than the trend suggested by the limiting fragmentation framework. Nevertheless the rather small enhancement of the fragmentation cross section when using 181Ta target does not translate to the production yields because of relatively small atom density in tanta- lum versus beryllium material. The projectile dependence of the fragmentation cross section resembles isoscaling for light fragments but is more complicated for heavier fragments. Calculations by three different theoretical models are presented. A macroscopic geometrical Abrasion-Ablation model results in excellent reproduction of the cross section distribution, with only two fitted parameters for each reaction system. A macroscopic-microscopic Heavy Ion Phase Space Exploration (HIPSE) model yields very good description of the fragment velocity distributions for all reaction systems involving 9Be target, but fails in the case of 181Ta target showing the limits of its ap— plicability. A more SOphisticated microscopic Antisymmetrized Molecular Dynamics (AMD) calculation can only be applied to reaction systems containing 9Be target. The AMD does not reproduce the fragment velocity distributions. However, its de- scription of the fragmentation cross sections is similar in quality as the HIPSE model comparison. to my wife Veronika iv ACKNOWLEDGMENTS I would like to start by thanking my Ph.D. supervisor Professor M. Betty Tsang for providing the opportunity to do my dissertation research at the NSCL. I was fortunate enough to join a new project on projectile fragmentation that she had started only a short time before I came to MSU. I really appreciate her determination and persistence in whatever she does: starting with the experiments and ending with the data analysis. Her ability to get things “organized” and “done” is admirable. I have learned a lot about doing data analysis by working with her. I would like to thank her for the continuing support and guidance during my stay at MSU. Being a part of the HiRA group headed by Professor W. G. Lynch really broadened my experience, by giving me the ability to help to work on development of a new detector system like the High Resolution Array. Even though I worked on a different project than the rest of the HiRA, they never let me feel isolated. It has been my privilege to work with a group of people like them. Over the years the group had its share of postdocs coming through, and all of them are people I enjoyed working and interacting with. Any question or problem related to my data analysis or issues with PAW were always answered by Giuseppe Verde. I enjoyed having Marc-Jan van Goethem as a friend and colleague, and being able to discuss anything from the most compelling physics questions to chatting over a cup of good coffee. Michael Famiano is the most hard working person I have met in my life. Despite his very busy schedule he got me started with ob ject-oriented data analysis using the ROOT framework, and he always found time to address the endless problems I experienced in the beginning. All of this probably would not be possible without my very good friend and “coffee buddy” Mark Wallace. He has been greatly helpful since my joining of the HiRA group. He always found time to help or explain things I could not grasp. I really enjoyed having him at RIKEN for one of our projectile fragmentation experiments. Life would have been much tougher without our regular 10 o’clock coffee. I am thankful to Ingo Wiedenhover for allowing me to work with him and intro- ducing me to the A1900 fragment separator during the busy commissioning period. I am grateful for the Opportunity to work with Andreas Stolz. He helped me to un- derstand the A1900 and has always been there when I needed help, whether it be the analysis and understanding the data acquisition or any kind of physics question. He even squeezed the proofreading of this manuscript to his already very busy schedule! I would also like to thank other members of the A1900 group at the NSCL: Thomas Baumann, Thomas Ginter, Mauricio Portillo and Mathias Steiner. I am indebted to Prof. Hiroyoshi Sakurai and Nori Aoi for their invaluable help and providing very hospitable environment for me and my colleagues during our frag- mentation experiment at RIPS fragment separator at RIKEN. I want to acknowledge all postdocs, graduate students and staff scientists working at Radioactive Isotope Physics Laboratory at RIKEN for their help and support during my stay in Wako— shi. I am grateful to Mikhail and Lyudmila Andronenko and Hui Hua for their help during the data analysis. I really appreciate the help of Sergei Lukyanov, who carefully checked every single momentum distribution (out of 1740) presented in this disserta- tion. Oleg Tarasov’s help was indispensable, especially during the first fragmentation experiments and I greatly acknowledge his help during my studies. I would like to acknowledge also other members of the HiRA group like Andrew Rogers, Franck Delaunay, Jenny Lee, Wanapeng Tan for their support and help dur- ing my graduate studies. I acknowledge the help of fellow graduate students Jeremy Armstrong and Ivan Brida with reading the manuscript of this dissertation. The dissertation would not be complete without the extensive calculations I have done in order to understand the experimental data. I enjoyed interacting and working with many theorists, and this experience definitely deepened and broadened my un- derstanding of the physics of nuclear reactions. I hereby want to express my thanks and gratitude to Akira Ono for his help with the AMD code, Denis Lacroix for in- troducing me to his HIPSE model, Bob Charity for many useful discusions and help using GEMINI, and Pawel Danielewicz for his support and fruitful discussions. Last, but not least I want to thank my parents Miroslav and Anna Mocko for always being there for me and for their continuous support and help. I am grateful to my wife Veronika for her endless love, support and belief in me. vii Contents 1 Introduction 1 1.1 Rare isotope beam production ...................... 6 1.2 Projectile fragmentation reactions .................... 8 1.3 Organization of dissertation ....................... 11 2 Experimental setup 12 2.1 Method of measurement ......................... 12 2.2 Particle identification ........................... 15 2.3 Experimental setup for the N SCL experiments ............. 16 2.3.1 Primary beam and reaction targets ............... 19 2.3.2 A1900 Fragment separator .................... 20 2.3.3 Detectors at the A1900 fragment separator ........... 23 2.3.4 Magnetic rigidity settings .................... 30 2.3.5 Data acquisition system ..................... 31 2.4 Experimental setup for RIKEN experiment ............... 34 2.4.1 Primary beam and reaction targets ............... 34 2.4.2 RIPS fragment separator ..................... 35 2.4.3 Detectors at the RIPS fragment separator ........... 36 2.4.4 Magnetic rigidity settings .................... 39 2.4.5 Data acquisition system ..................... 4O 3 Data analysis 42 3.1 Primary beam charge state distributions ................ 42 3.2 Calibration of beam intensity monitors ................. 44 3.2.1 NSCL experiments ........................ 44 3.2.2 RIKEN experiment ........................ 48 3.3 Particle identification ........................... 49 3.3.1 NSCL experiments ........................ 49 3.3.2 RIKEN experiment ........................ 51 3.4 Cross section analysis ........................... 53 3.4.1 Differential cross sections ..................... 53 3.4.2 Momentum distribution fitting procedure ............ 54 3.4.3 Evaluation of fragmentation production cross section ..... 55 3.4.4 Transmission correction evaluation ............... 59 3.5 Error analysis ............................... 69 viii 4 Experimental results 4.1 Momentum distributions ......................... 4.2 4.3 4.1.1 4.1.2 Widths of the momentum distributions ............. Centroids of the momentum distributions ............ Fragmentation production cross sections ................ Cross section comparisons ........................ 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 EPAX parameterization ..................... Comparison to EPAX ...................... Comparison to other data .................... Target dependence ........................ Projectile dependence ...................... 5 Comparison to models 5.1 Reaction models .............................. 5.2 5.3 5.4 5.5 5.6 5.1.1 5.1.2 5.1.3 Abrasion Ablation model ..................... Heavy Ion Phase Space Exploration ............... Antisymmetrized Molecular Dynamics ............. Primary fragment distributions ..................... Excitation energy ............................. Evaporation codes ............................ 5.4.1 5.4.2 LisFus evaporation code ..................... Statistical evaporation code GEMINI .............. Cross section distributions ........................ Velocity distributions ........................... 6 Summary and conclusions A Fitting results Results for 40Ca projectile ........................ Results for 48Ca projectile ........................ Results for 58Ni projectile ........................ Results for 64Ni projectile ........................ Results for 86Kr projectile ........................ A.1 A2 A3 A4 A5 Bibliography ix 70 7O 72 77 84 86 86 91 92 110 114 119 120 120 123 130 137 139 144 144 147 150 164 171 178 178 185 195 205 217 224 List of Figures 1.1 Nuclear landscape. ............................ 2 1.2 Schematic representation of an ISOL (left) and an in-flight Projectile Fragmentation (right) technique to produce rare isotope beams. . . . 7 1.3 Projectile fragmentation reaction in a two step Abrasion-Ablation model. 10 2.1 Layout of the experimental areas at the NSCL. ............ 17 2.2 A schematic of the Coupled Cyclotron Facility at the N SCL. ..... 17 2.3 Calculated yields of different isotope species for the Coupled Cyclotron Facility. .................................. 18 2.4 Detailed view of the A1900 fragment separator. ............ 22 2.5 Simulated fragment distributions. .................... 22 2.6 Simulation of N = Z setting for 58Ni+9Be reaction using LISE++. . . 26 2.7 Principle of operation of an avalanche counter ............. 27 2.8 Parallel Plate Avalanche Counter ..................... 27 2.9 Detector setups in the A1900 used for our fragmentation experiments. 28 2.10 Photographs of the two beam intensity monitoring devices. ...... 29 2.11 Magnetic rigidity settings for Ca primary beams. ........... 31 2.12 Magnetic rigidity settings for Ni primary beams ............. 32 2.13 The electronic diagram used for the A1900 fragmentation experiments. 33 2.14 Layout of RIKEN Accelerator Research Facility. ............ 35 2.15 RIPS setup used for the fragmentation experiment. .......... 37 2.16 Beam intensity monitor installed at the target position ......... 38 2.17 Magnetic rigidity coverage for the 86Kr runs .............. 39 2.18 Block schematic of the electronic circuit used for the 86Kr fragmenta- tion experiment at RIKEN. ....................... 41 3.1 Primary beam charge state distributions for 58"“‘Ni and 86Kr primary beams plus 9Be and 181Ta targets ..................... 43 3.2 Faraday cups Z001 and 2014 in the extraction beam line and the target box, shown relative to the K1200 cyclotron and the A1900 fragment separator. ................................. 45 3.3 NaI(Tl) beam monitor calibration for all primary beams. ....... 46 3.4 Ban beam monitor calibration for primary beams mentioned in the text. .................................... 47 3.5 Uncalibrated and calibrated particle identification spectrum. ..... 50 3.6 Uncalibrated particle identification spectrum of fragments created in reaction 86Kr+181Ta ........................... 51 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 Particle identification spectrum for a 2.07 Tm magnetic rigidity setting using the 86Kr primary beam. ...................... Momentum distributions of 50V and 50Mn created in fragmentation of 64Ni on 9Be target. ............................ Systematics of the centroids and variances of the momentum distribu- tions for the reaction 64Ni+9Be. ..................... Coordinate system used in ion optical calculations of transmission cor- rection .................................... Angular acceptance of the A1900 and RIPS fragment separators as calculated by M OCADI in (,6 versus 9 plane. .............. Comparison of angular transmission calculated using LISE++ and MOCADI .................................. Calculations of the primary beam emittance ellipse for all beams de- livered by the K1200 cyclotron in stand alone mode during the 19903. Angular transmission correction as a function of fragment mass number for the A1900 and RIPS fragment separators. ............. Angular distributions in the target plane for 44Ca and 59Co fragments from the 64Ni+QBe reaction. ....................... Final transmission correction, 5, for 40"‘8Ca primary beams ....... Final transmission correction, 5, for 58"54Ni and 86Kr primary beams. . Examples of the momentum distributions. ............... Width of the right side of the momentum distribution 03 for the 40"‘E‘Ca and 58"“‘Ni primary beams and the two reaction targets Be and Ta. Width of the left side of the momentum distribution 0L for the 40"‘8Ca and 58"“‘Ni primary beams and the two reaction targets Be and Ta. Width of the left 0L and right on sides of the momentum distribution for the 86Kr primary beam on 9Be and 181Ta reaction targets. Relative deviations from the projectile velocity (vp/vp — 1) for frag- ments with complete momentum distributions identified in the frag- mentation of 40"“‘Ca isotopes on 9Be and 181Ta targets. ........ Relative deviations from the projectile velocity (Up/12p — 1) for all frag- ments with complete momentum distributions identified in the frag- mentation of 58NM isotopes on 9Be and 181Ta targets. ........ Relative deviations from the projectile velocity (vp/vp — 1) for all frag- ments with complete momentum distributions identified in the frag- mentation of 86Kr primary beam on 9Be and 181Ta targets. ...... Nuclides identified in the fragmentation of 40Ca projectile on two targets. Nuclides identified in the fragmentation of 480a projectile on two targets. Nuclides identified in the fragmentation of 58Ni projectile on two targets. Nuclides identified in the fragmentation of 64Ni projectile on two targets. Nuclides identified in the fragmentation of 86Kr projectile on two targets. Isotopic distributions for elements 5 S Z S 20 produced in the 40Ca+9Be reactions at 140 MeV/ u. ......................... reactions at 140 MeV/ u. ......................... xi 53 56 57 59 61 62 63 64 66 67 68 71 76 78 79 81 82 83 95 96 97 98 99 100 Isotopic distributions for elements 5 3 Z S 20 produced in the 4°Ca+181Ta 101 4.15 Isotopic distributions for elements 5 S Z S 20 produced in the 48Ca+9Be reactions at 140 MeV/u. ......................... 102 4.16 Isotopic distributions for elements 5 S Z S 20 produced in the 48Ca+181Ta reactions at 140 MeV/ u. ......................... 4.17 Isotopic distributions for elements 5 S Z S 28 produced in the 58Ni+9Be reactions at 140 MeV/u. ......................... 103 104 4.18 Isotopic distributions for elements 5 S Z S 28 produced in the 58Ni+181Ta reactions at 140 MeV/u. ......................... 4.19 Isotopic distributions for elements 5 S Z S 28 produced in the 64Ni+9Be reactions at 140 MeV/ u. ......................... 105 106 4.20 Isotopic distributions for elements 5 S Z S 28 produced in the 64Ni+181Ta reactions at 140 MeV/ u. ......................... 107 4.21 Isotopic distributions for elements 25 S Z S 36 produced in the 86Kr+9Be reactions at 64 MeV/ u. .................... 108 4.22 Isotopic distributions for elements 25 S Z S 36 produced in the 86Kr+181Ta reactions at 64 MeV/ u. ................... 109 4.23 Target ratios of the fragmentation cross sections, aTa(A, Z) /aBe(A, Z), - of fragments 8 S Z S 18 for two projectiles 40Ca and 48Ca. ...... 112 4.24 Target ratios of the fragmentation cross sections 0T8(A, Z) /aBe(A, Z) of fragments 10 S Z S 26 for two projectiles 58Ni and 64N i. ...... 112 4.25 Target ratios of the fragmentation cross sections 0T3(A, Z) /aBe(A, Z) of fragments 25 S Z S 36 for 86Kr projectile ............... 113 4.26 Ratios of cross sections 043(N, Z) /040(N, Z) of fragments created in 4"3Ca and 40Ca reactions on 9Be (top panel) and 181Ta (bottom panel) targets .................................... 116 4.27 Ratios of cross sections 064(N, Z) /0‘58(N, Z) of fragments created in 64Ni and 58Ni reactions on 9Be (top panel) and 181Ta (bottom panel) targets .................................... 117 5.1 N ucleus-nucleus potential VATAP as a function of the relative distance. 125 5.2 A schematic representation of the quantum branching for multichannel reactions [115]. .............................. 133 5.3 Examples of the time evolution of the 86Kr+9Be collisions at 64 MeV/u.136 5.4 Primary fragment isotopic distributions for 40"‘8Ca+9Be reaction system. 138 5.5 Primary fragment isotopic distributions for 58"54Ni+9Be reaction system. 139 5.6 Excitation energy per nucleon, E*/A, as a function of the mass num- ber of the primary fragments in different models for reaction systems involving 181Ta. .............................. 141 5.7 Excitation energy per nucleon, E*/A, as a function of the mass num- ber of the primary fragments in different models for the 48Ca+181Ta reaction system ............................... 142 5.8 Excitation energy per nucleon, E*, as a function of the mass number of the primary fragment different models for 86Kr beam and 9Be and 181Ta targets. ............................... 143 5.9 Two Abrasion-Ablation calculations compared for 40"‘E‘Ca projectiles with 9Be target ............................... 146 xii 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17 5.18 5.19 5.20 5.21 5.22 5.23 5.24 5.25 Influence of the level density parameterization on the final isotope dis- tributions in 40Ca+9Be reaction ...................... Influence of the level density parameterization on the final isotope dis- tributions in 48Ca+9Be reaction ...................... Fragmentation cross sections of 40Ca+9Be compared to theoretical mod- els. ..................................... Fragmentation cross sections of 48Ca+9Be compared to theoretical mod— els. ..................................... Fragmentation cross sections of 58Ni+QBe compared to theoretical mod- els. ..................................... Fragmentation cross sections of 64 N i+9Be compared to theoretical mod- els. ..................................... Fragmentation cross sections of 86Kr+9Be compared to theoretical mod- els. ..................................... Fragmentation cross section isotopic distributions of 40Ca+181Ta reac- tion are compared to theoretical models. ................ Fragmentation cross sections of 48Ca+181Ta compared to theoretical models. .................................. Fragmentation cross sections of 58Ni+181Ta compared to theoretical models. .................................. Fragmentation cross sections of 64Ni+181Ta compared to theoretical models. .................................. Fragmentation cross sections of 86Kr+181Ta compared to theoretical models. .................................. Deviation from the projectile velocity (Up/Up — 1) in percent for frag- ments in the 40"“‘Ca and 58MN i projectile and 9Be target collisions. Deviation from the projectile velocity (’UF/‘Up — 1) in percent for frag- ments in the 48Ca projectile on 181Ta target collisions .......... Deviation from the projectile velocity (vp/vp — 1) in percent for frag- ments in the 86Kr projectile and 9Be, 181Ta target collisions. ..... Results of the HIPSE calculations in terms of the excitation energy per nucleon, E*/A, (left) and the fragment velocity (right) for the 483Ca+181Ta reaction system ........................ IMAGES IN THIS DISSERTATION ARE PRESENTED IN COLOR xiii 148 149 153 154 155 156 157 158 159 160 161 162 165 166 166 168 List of Tables 2.1 List of primary beams used in our experiments. ............ 19 2.2 List of targets used in the series of experiments at the NSCL. . . . . 21 2.3 Fundamental parameters of the A1900 fragment separator [55]. . . . . 21 2.4 The magnetic rigidity, Bp, values of N = Z settings, used for particle ID calibration ................................ 24 2.5 Primary beam energy measured and target thickness measurement in the 86Kr experiment. ........................... 35 2.6 Fundamental parameters of the RIPS fragment separator [35]. . . . . 36 3.1 Values of 00 and 0'0 parameters in Equation (3.5) used in the angular transmission correction calculations for different primary beam target combinations. ............................... 69 4.1 Goldhaber reduced width parameters for all reaction systems. . . . . 74 4.2 Number of fragments and pick-up (including exchange) cross sections measured for all reaction systems ..................... 85 4.3 Best fit values of the isoscaling parameters for all investigated systems. 118 5.1 Best fit values for the K and S parameters of the excitation energy in the Abrasion-Ablation model. ...................... 122 5.2 Values of HIPSE adjustable parameters as determined by Lacroix [111]. 129 A.1 Fitting results for the reaction system 40Ca+9Be. ........... 178 A2 Fitting results for the reaction system 40Ca+181Ta. .......... 181 A3 Fitting results for the reaction system 48Ca+9Be. ........... 185 A4 Fitting results for the reaction system 48Ca+181Ta. .......... 190 A5 Fitting results for the reaction system 58Ni+9Be ............. 195 A6 Fitting results for the reaction system 58Ni+181Ta ............ 199 A.7 Fitting results for the reaction system 64Ni+9Be ............. 205 A8 Fitting results for the reaction system 64Ni+181Ta ............ 210 A9 Fitting results for the reaction system 86Kr+9Be. ........... 217 A.10 Fitting results for the reaction system 86Kr+181Ta ............ 221 xiv Chapter 1 Introduction The nucleus is the core of the atom. Containing more than 99.9% of the atom’s mass, the nucleus defines its center and influences the chemical properties of the atom through its electrical charge. A nucleus is composed of Z protons and N neutrons which determine its mass number A = N + Z. Since the protons and neutrons have similar mass and behave similarly in the realm of the strong interaction they are called nucleons. They themselves have a sub-structure, quarks and gluons, with the gluons holding the quarks together in a tightly bound entity [1—3]. This results in the attractive force between nucleons as the residue of the quark and gluon interactions [4]. Nuclei come in a large variety of combinations of protons and neutrons. However, due to fundamental laws of nature, which are still being investigated, not all com- binations are possible. Figure 1.1 illustrates the landscape of those nuclei that we presently think might exist in a map spanned by the number of protons on one axis and the number of neutrons on the other. This landscape shows several thousands of nuclei that are expected to be bound by the strong force. Of these, slightly fewer than 300 isotopes make up the assortment of the 82 stable elements (marked as black squares in Figure 1.1) that exist in nature. When displayed in the representation of a nuclear landscape, these stable isotopes lie along a slightly curved line, called the line of stability (or the valley of stability) [6]. There are thousands of unstable nuclides Nuclear Landscape ‘ 7 less than 300 stable ] proton number Z terra incognita known nuclei A V 2 8 neutron number N Figure 1.1: Nuclear landscape —— nuclei shown in proton versus neutron number rep- resentation [5]. which are bound against the release of protons and neutrons, but they are subject to a [7] and/or 6 [8] decay. Some of the unstable nuclei have very long half lives and can be found on Earth. Others are man-made, while thousands more have not been discovered. The yellow region in Figure 1.1 indicates short lived nuclei that have been produced in laboratories. By adding either protons or neutrons one moves away from the valley of stability, finally reaching what is referred to as the drip lines [9] where the nuclear binding ends because the forces between the neutrons and protons are no longer strong enough to hold these particles together. Many thousands of radioactive nuclei with very small or very large N / Z ratios are yet to be explored. In the nuclear landscape they form the term incognita indicated in green [10]. The proton drip line has been reached experimentally for all odd Z nuclei up to Z = 91 [11], because the long range Coulomb repulsion among the protons prohibits formation of extremely proton-rich nuclides. In contrast, the neutron drip line is considerably further away from the valley of stability [11], hence harder to approach. Except for the lightest nuclei where it has been reached experimentally, the neutron drip line is estimated on the basis of nuclear models, therefore it is very uncertain due to the extrapolations involved. The red lines in Figure 1.1 show the “magic numbers” known around the valley of stability [12]. However, since the structure of nuclei is expected to change significantly as the drip lines are approached, we do not know how nuclear shell structure evolves at the extreme nuclear asymmetries. For decades, nuclear physics experiments have been performed with reactions be- tween beams and targets of stable nuclei found in nature. Basic properties of the atomic nuclei have been discovered and many fundamental models developed (e. g., Weizsacker mass formula [13], shell model [6], etc.). In order to understand nuclear matter, we need to explore and study not only the properties of stable nuclei, but also those close to the limits of nuclear existence. These investigations of unstable nuclei pose a great experimental challenge. Such studies at the extremes will provide important insights into the structure of nuclei, the dynamics of nuclear reactions, the underlying symmetries of nature, and the nucleus as a fundamental many-body system governed by the strong interaction. With recent developments in heavy-ion accelerators and rare isotope beam production many new surprising phenomena have been observed in unstable nuclei (e. g., neutron halo [14], neutron and proton skins of nuclei far from stability [15,16], large deformations of neutron rich isotopes [17], etc.). Further study of the unexplored regions of the nuclear landscape will provide answers to many important questions outlined in the “Scientific opportunities with Fast Fragmentation Beams from RIA” [18]: o What is the origin of the elements in the cosmos? 0 Where are the limits of nuclear existence? 0 What are the properties of nuclei with extreme N /Z ratios? 0 What are the properties of bulk neutron-rich matter under extreme conditions of temperature and density? 0 How must existing theoretical models be changed to describe the properties of rare isotopes? o How are the prOperties of rare isotopes related to the basic nucleon-nucleon interaction? Projectile fragmentation reactions serve as an important tool for producing the rare isotope beams used in a wide variety of experiments looking for answers to the above mentioned questions. The presented dissertation explores the fragmentation reaction mechanisms, to better understand and optimize the production of rare iso- topes. It allows experimenters to better estimate and optimize the yields of the rare ion beams available for their studies. Understanding fragmentation reactions is not only important for the fundamental science experiments investigating new phenomena with exotic isotope species, but is also needed in many applications. Three concrete applications of fragmentation reac- tions will be mentioned here briefly: space radiation, heavy-ion therapy and nuclear waste transmutation. Space radiation consists of many components, from which the high energy and high-charge ions constitute about 1%. High energy highly charged ions possess signif- icantly higher ionizing power this means they have a greater potential for radiation- induced damage. Since nuclear fragmentation is an important reaction mechanism for these relativistic heavy ions, better understanding and modeling of these processes is very important [19]. Realistic simulations of the radiation damage to equipment and humans are essential for the success of any space missions that last an extensive period of time. Realizing that there is a lack of experimental data, NASA joined efforts with the Brookhaven National Laboratory in building the NASA Space Radiation Labora- tory (NSRL) in Brookhaven, New York [20]. The new facility will study the reactions of heavy ions in various biological materials (tissues, cells or DNA in solution). It also evaluates industrial samples for their suitability in spacecraft shielding. Heavy-ion therapy is a new emerging technique to treat tumors resistant to other conventional treatment methods. Close to the end of a heavy-ion track, there is a well localized energy deposition by the ion. This maximum in the stopping power is called the Bragg peak. When positioned properly, the peak concentrates the dam- age to the tumor tissue and minimizes the radiobiological effects to the surrounding tissue [21]. Fragmentation of the heavy-ion beams used to treat tumors may produce lighter isotopic species that may penetrate deeper into the healthy tissue. The en- ergy deposited by the beam and the produced ions causes a severe damage to both helices of the DNA of a cell nucleus, also referred to as “double strand breaks” [22]. Therefore, energy and mass of the treatment means used are constrained by the num- ber of the fragment species created in the treatment tissue. Better understanding of the fragmentation reaction mechanism improves calculations of the radiation dam- age inflicted by the fragments of the treatment heavy-ion beam. A pilot program of treatment using relativistic ions of 12C was started in 1997 at GSI in Darmstadt, Germany [23]. Based on the results of this project, a new cancer therapy center [24] is under construction in Heidelberg, Germany. Nuclear waste is predominantly comprised of used fuel discharged from oper- ating reactors. Almost all issues related to risks arising from long-term disposal of spent nuclear fuel are attributable to about 1% of its content [25]. This small fraction consists of transuranic elements and long-lived isotopes of iodine and technetium. 'I‘ransmutation [26] of these isotopes by a proton beam can decrease the toxic nature of the spent fuel below that of natural uranium ore, reducing the challenges associ- ated with long-term storage. In order to Optimize the nuclear waste transmutation it is very important to study and understand the spallation (target fragmentation) reaction mechanism. Realizing the difficulties connected with long-term storage of the nuclear waste materials, large collaborations such as the Advanced Fuel Cycle Initiative at Los Alamos National Laboratory [27] have been formed to identify the relevant cross sections and thereby address the scientific questions concerning the nuclear waste transmutation as a solution of the nuclear waste problem. These three examples of nuclear fragmentation applications are by no means ex- haustive; they only illustrate the broad impact of the fragmentation reaction research in human society. This dissertation, however, focuses mainly on the study of cross sections and the reaction mechanism of projectile fragmentation reactions in basic science. 1.1 Rare isotope beam production The two most prominent techniques to produce rare isotope beams are Isotope Separa- tion On-Line (ISOL) and in-flight Projectile Fragmentation (PF) [28]. The underlying principle of the production of the exotic nuclei is to transform the stable nuclei into unstable species using nuclear reactions. In an ISOL facility the radioactive nuclei are produced essentially at rest in a thick target by bombardment with particles from a driver accelerator. After ionization and mass separation, the nuclei of rare isotopes are accelerated in a post-accelerator (see the left panel of Figure 1.2). A wide range of primary beams including thermal neutrons [29], high energy protons [30], intermediate energy heavy ions [31] can be used as projectiles. For the thick target construction high Z materials are used such as tantalum or uranium. Similarly a range of different types of post-accelerators are being used including cyclotrons [32], linacs [33], and tandem accelerators [34]. Post-accelerators — optimized for high quality beams — provide easy energy variability, high energy precision and small emittances. These characteristics are re- Isotope Separation On-Line Projectile Fragmentation _ isobar/isotope .. . . IL... p se ration Pa fragment ] If .- ~, separator production _7 e tar et “ driver thiCk. hot target 9094' tn 9 acce'eratOF a°°° 9'3 0 heavy ion radioactive beam d' ct' b accelerator ra Ioa we earn Experiment Experiment Figure 1.2: Schematic representation of an ISOL (left) and an in-flight Projectile Fragmentation (right) technique to produce rare isotope beams. quired by many experiments in nuclear structure and nuclear astrophysics. Another advantage of ISOL facilities is the high luminosity that can be achieved through the combination of a thick production target and a very intense primary beam. The time delay resulting from the stopping, ionization and extraction of the radioactive fragments from the production target and forming a secondary ion beam limits the production of particularly short-lived rare isotope beams. The extraction of the rare isotope species from the target material is done using chemical methods which make the whole process charge, Z, dependent. Modern day facilities that use this production technique include: the REX-ISOLDE facility [33] at CERN in Geneva, Switzerland and the ISAC facility [30] at TRIUMF in Vancouver, Canada. In an in—flight PF facility, as illustrated in the right panel of Figure 1.2, an en- ergetic heavy-ion beam is fragmented when it interacts with a (production) target. The desired reaction products are subsequently transported to an experiment after mass, charge and momentum selection in a fragment separator. In—flight fission of very heavy beams, and also charge exchange and transfer reactions, have been used as an alternative to projectile fragmentation. The high energy that the fragments au- tomatically carry from the primary beam in this production method, eliminates the need for post-acceleration. The in-flight production also means that the experiments with radioactive fragments as a secondary beam can be done promptly. Because they are only delayed by the flight time through the separator and beam line system, typi- cally < 1 us, production of very short-lived exotic nuclides is possible. Contemporary facilities using the PF method of rare isotope beam production include: Riken Accele- rator Research Facility (RARF) [35] at RIKEN in Wako—shi, Japan and the Coupled Cyclotron Facility (CCF) [36] at NSCL in East Lansing, MI. These two techniques of rare isotope beam production are, in many respects, complementary. Different classes of experiments impose very different requirements on rare isotope beams in terms of velocity, momentum and angular spread and pu- rity, allowing experimenters to choose the most suitable secondary beam production method possible. 1.2 Projectile fragmentation reactions Nuclear reaction [37] is the single most important technique to probe the fundamen- tal properties of nuclear matter. Nuclear collisions are usually classified according to the impact parameter and the incident energy of the projectile. We associate periph- eral collisions with large impact parameters and central collisions with small impact parameters. In terms of the kinetic energy the collisions are usually categorized as low (< 20 MeV/u), intermediate (20—200 MeV/u) and high energy (>200 MeV/u). Projectile fragmentation reactions are peripheral collisions of heavy ions at interme- diate to high energies, when the projectile breaks up — fragments into one or more heavy ions — on impact with the target nucleus. More central collisions of heavy ions in intermediate to high energy regime, also referred to as multifragmentation reactions [38], occur when the interaction region of the reaction system shatters into many smaller pieces (i. e. clusters). From the experimental viewpoint, two classes of fragmentation reactions are recog- nized: target fragmentation, more commonly referred to as spallation, and projectile fragmentation, as described above. These two reaction categories are identical in the center of mass frame of reference, allowing one to use the same theoretical treatment and model description. However, the experimental techniques used in their study and the historical evolution were quite different. Target fragmentation reactions have been investigated for more than four decades and large number of data sets are available. Many of the fragmentation models, pa- rameterizations and systematics are based on the target spallation experimental data. Projectile fragmentation reactions were experimentally inaccessible until the 19703 when high-energy heavy-ion beams became available [39,40]. The first pioneering pro- jectile fragmentation experiments at relativistic energies with 40Ar and 48Ca beams were carried out at Bevalac at Lawrence Berkeley Laboratory (LBL) [41,42]. These experiments showed the potential of this method to produce rare isotope beams, and many fragment separators have been constructed (e. g., A1200 (Analysis 1200) at MSU [43], F RS (FRagment Separator) at GSI [44], RIPS (RIken Projectile fragment Separator) at RIKEN [35]) utilizing this technique. The first attempts to describe the projectile fragmentation reactions were done at LBL. In 1973 Bowman, Swiatecki and Tsang introduced the Abrasion-Ablation (AA) reaction model [45]. Which forms the basis of our understanding of the fragmentation reactions. This simple model approximates the reaction in two stages; Figure 1.3 illustrates the two very distinct processes. The nuclei are treated as perfect spheres colliding on classical parallel trajectories separated by the impact parameter, b. Any alteration from the straight path is neglected in the model because it assumes that the projectile is moving with relativistic velocity. At the end of the abrasion stage the overlap (participant) region of two spherical nuclei is removed. In its simplest form, no treatment of the participant region is necessary to explain the data using the AA model. The abrasion stage ends with a deformed and highly excited prefragment which decays in the second ablation stage by evaporating light clusters, nucleons and gamma radiation. The second step of the AA model is much slower (z 10‘16-10“18 8, depending on the excitation energy) as compared to the abrasion step (a: 10‘23 s). Since its introduction in 1973, many different versions of the AA model have been developed. Many of these AA models differ in the second ablation stage. There is one common feature of all the AA models: the inability to predict the excitation energy of the prefragment. Most of the models rely on various parameterizations of the excitation energy and in many cases it is taken as a free parameter. The details of the AA model used in this dissertation are explained in Section 5.1.1. projectile prefragment e O * e E . v 0 t O 0 target Abrasion Ablation (deexcitation) Figure 1.3: Projectile fragmentation reaction in a two step Abrasion-Ablation model. While there are many puzzling aspects to the fragmentation phenomenon, it does display some simplifying characteristics at high incident energies. For exam- ple, many experimental observables in peripheral collisions at high incident energies ( >200 MeV/u), such as charge or multiplicity distributions, vary little with incident energy and target mass. This “limiting fragmentation” behavior forms the basis for the EPAX parameterization used to calculate the secondary beam rates in many ra- dioactive beam facilities and even the rates predicted for the next generation rare isotope facility [46]. This parameterization assumes that the isotopic distributions and their dependence on the isospin of the projectile and target are consistent with limiting fragmentation. However, EPAX derives its results from a careful empirical fit to a limited data set of production cross sections measured under a wide variety of experimental conditions. As the parameterization is not based upon a specific theory for projectile fragmentation, EPAX is better at interpolating between measured data points taken under similar conditions than at predicting the production of an isotope 10 further away from the valley of stability. The original EPAX parameterization [47] included only the target spallation data; it was revisited in 2000 [48] and improved by including, the contemporary projectile fragmentation data available. The threshold energy for the limiting fragmentation is not uniquely defined and fragmentation data in the intermediate energy range are rather scarce. Very little data exist to examine the detailed dependence of isotopic distributions on target and beam in the intermediate energy regime. As Siimmerer et al. [48] pointed out: “It would be interesting, however, to compare EPAX also to cross sections obtained at lower incident energies once high-quality data become available”. The present study in this dissertation provides high-quality and comprehensive projectile fragmentation data at intermediate energy available from the Coupled Cyclotron Facility at the National Superconducting Cyclotron Laboratory at Michigan State University. 1.3 Organization of dissertation The dissertation is organized in the following way. Chapter 2 introduces the exper- imental method used for the fragmentation measurements. The description of the fragment separators, the A1900 at the NSCL and RIPS at RIKEN, along with ex- perimental details of the measurements are also included. Chapter 3 describes the principal steps taken in the analysis of the experimental data, emphasis is given to the discussion of the fragmentation production cross section extraction. In Chapter 4 the experimental data are presented, starting with the discussion of the momentum distributions and ending with the cross sections of fragments and nucleon pick-up reactions measured in our experiments. Three different theoretical models of frag- mentation reactions are introduced in Chapter 5, along with the comparisons to the experimental data. Conclusions of the dissertation are summarized in Chapter 6. The measured experimental cross sections of fragments and nucleon pick-up products are listed in the Appendix. 11 Chapter 2 Experimental setup The present experiments were carried out at two laboratories -— NSCL and RIKEN. A series of four fragmentation experiments with 40"‘8Ca and 58MM projectiles at 140 MeV/ u using the A1900 fragment separator were carried out at the NSCL. The frag- mentation experiment using the 86Kr primary beam at 64 MeV/ u was done at RIKEN using the RIPS fragment separator. In this chapter a description of the method of measurement is followed by introduction of the particle identification techniques com- mon to all experiments. Since the details of the experimental setup differ, the frag— ment separators and the detector setups for the NSCL and RIKEN measurements are discussed separately. 2.1 Method of measurement One of our goals is to perform comprehensive measurements of fragmentation produc- tion cross sections produced in the reactions of 5 different projectiles in intermediate energy regime. The emitted fragments were collected and identified with a fragment separator. In order to obtain the final production cross sections, one must understand how to correctly reconstruct the fragment distributions in angular and momentum coordinate spaces. The angular distributions are not directly measured in our exper- 12 iments and are taken as an efficiency correction from simulations and are referred to as the transmission correction in Chapter 3. The momentum distributions of frag- ments are measured in the magnetic field of the fragment separator. Because of the limited momentum acceptance of current fragment separators (z :t(2—3)% around the central momentum) the momentum distributions must be measured by multiple magnetic settings of the fragment separator. In doing so, one can use two methods of obtaining the momentum distributions running the fragment separator with its full momentum acceptance or running with a narrow momentum acceptance. The full momentum acceptance of the fragment separator is generally used in pro- duction mode to maximize the production yield of the rare isotope beams. We chose the narrow momentum acceptance mode for our experiments since the main objec- tives of our experiments were to measure comprehensive sets of fragmentation cross sections minimizing the associated uncertainties. The following issues have been con- sidered in choosing the narrow momentum acceptance mode: primary beam charge states, transmission characteristics, particle identification, number of magnetic set- ting, optimum use of the beam intensity and the electronic dead time. The energetic heavy ions can pick-up, exchange or loose electrons when they tra- verse a foil of a given material composition. These complex interactions depend on the heavy-ion energy and charge of both projectile and target. The heavy ions then emerge from the foil in different charge states —— differing by number of electrons they possess. Fragment production rate is approximately 103—104 lower than the primary beam intensity or its charge states. If the full momentum acceptance of the fragment separator is used, regions of the magnetic rigidity that accepts the charge states must be blocked Off. This large difference precludes the measurements of fragments and the primary beam (or its charge states) in one magnetic rigidity, Bp, setting. If narrow momentum acceptance is used we can map the Bp space between the charge states of the primary beam resulting in a better Optimization of the beam intensity and a reduction of the data acquisition dead time. 13 In order to measure the absolute fragmentation cross sections, we need to under- stand the transmission characteristics of the fragment separator. These characteristics express the efficiency of our measurement. With the full momentum acceptance we are also filling the spatial acceptance of the fragment separator. Optical abberations start to play more important role for particles traveling farther away from the optical axis Of the separator. On the other hand if we restrict ourselves to a narrow momen- tum acceptance we are limited to particles traveling very close to the optical axis of the separator and therefore minimizing the effects of the Optical abberations. The experimental measurement of the transmission is very difficult because it requires ac- curately knowing the cross sections. The transmission is determined by an ion-optical calculation. Transmission for particles within the narrow momentum acceptance of the fragment separator can be simulated more accurately. This consideration favors narrow momentum acceptance mode. With wide momentum acceptance, particle identification requires a measurement of the magnetic rigidity at the dispersive image plane, to correct for the different particle trajectories. In narrow momentum acceptance mode no additional position sensitive detectors are required at the dispersive image plane, because the particles are bound to very similar trajectories and the momentum is determined by the magnetic rigidity setting of the fragment separator. This consideration again favors narrow momentum acceptance mode. One disadvantage of narrow momentum acceptance is that it requires more mag— netic settings of the fragment separator compared to the wide momentum acceptance to cover the same range in Bp. But with narrow momentum acceptance we gain other advantages: measurement between the charge states of the primary beam, easier par- ticle identification, and better utilization of the beam time (data acquisition dead time characteristics). 14 2.2 Particle identification Particle identification was obtained by using the Bp—ToF -AE-TK E method [44] on an event-by-event basis. The magnetic rigidity, Bp, was given by the magnetic setting of the fragment separator. Fragment time of flight, ToF, energy loss , AE, and total kinetic energy, TK E , were measured by charged-particle detectors. The determination of these four quantities Bp, ToF, AE, TK E defines unambiguously the momentum, p, mass and charge numbers A and Z along with the charge state, Q, for every ion. The magnetic rigidity, Bp, of a charged particle, can be expressed as a function of the A/Q ratio: Bp = flvuc x (2-1) a, where c is the speed of light in vacuum, B = v/c is the velocity of the charged particle, 7 = 1/ W is the relativistic factor and u is the atomic mass unit. The time of flight, T 0F , of a charged particle through a distance, L, is given by its velocity, 6, as follows L ToF = 53' (2.2) The Bethe-Bloch formula [49] connects the differential energy losses, —dE/d:r, of a charged particle, with nuclear charge, Z, and velocity, 6, in a material with the mean ionization potential, I: dE 47re4n 2 2mec2 62 2 'E’W'Z'lln( I 'l—wl‘fil’ (2‘3) where e is elementary charge, and n is the electron density of the material. Hence, the energy loss of a charged particle in a thin detector can be expressed as: AE = K1Z2 x [1 — $111 (IX—2%)] , (2.4) where K1 and K2 are parameters containing all medium specific and universal con- 15 stants. The total kinetic energy, TK E , of a charged particle is given by its mass number, A, and the relativistic factor 7: TKE = (’y — 1) X Auc2. (2.5) This gives one uniquely the mass, A, provided that one knows the velocity, ,6. The A/Q ratio for a given charged particle with the magnetic rigidity, Bp, is obtained by expressing the 6 from Equation (2.2) and inserting it into Equation (2.1). The energy loss, AE, in a thin layer of matter, is proportional to the square of nuclear charge (Equation (2.4)), providing the resolution of nuclear charge, Z. Finally, the relation between the total kinetic energy, TK E, and particle mass, A, (Equation (2.5)) is utilized to calculate the charge state, Q, using the A / Q ratio from Equation (2.1). The general Bp—ToF -AE—TK E identification technique can be reduced to the Bp—ToF-AE method, when Q = Z for all charged particles of interest (i. e. all ions of interest are fully stripped of electrons). In this special case the determination of the total kinetic energy, TK E, is not necessary, because the measurement of the magnetic rigidity, Bp, yields the A/Z ratio (Equation (2.1)). The nuclear charge, Z, is calculated from Equation (2.4) based on the energy loss AE, while the particle velocity, 6, is determined from the time of flight, ToF, in Equation (2.2). 2.3 Experimental setup for the NSCL experiments In 1999—2001 the National Superconducting Cyclotron Laboratory(NSCL) underwent a major upgrade [36]. The two superconducting cyclotrons (K500 and K1200) had previously been used to accelerate heavy-ion beams individually. In the new Coupled Cyclotron Facility (CCF) [36, 51,52], the two cyclotrons are coupled so the facility 16 ECR K500 N2 N3 N4 N15 N6 SRF Clean room Cryogenic plant Figure 2.1: Layout Of the experimental areas at the NSCL. The Coupled Cyclotron Fa- cility consisting of the K500 and K1200 cyclotrons and the A1900 fragment separator is shown with respect to the experimental vaults N2—N6 and Sl—S3. Figure 2.2: A schematic of the Coupled Cyclotron Facility at the NSCL [50]. RT-ECR and SC-ECR are Room Temperature and Superconducting ECR ion sources. The K500 and K1200 cyclotrons are connected by the coupling line. The A1900 fragment separator is enclosed in the dashed-line area. is able to deliver higher beam intensities at higher energies. Figure 2.1 shows the layout of the experimental building at the NSCL housing the CCF. The CCF, as shown in more detailed view in Figure 2.2, consists of the K500 cyclotron, K1200 cyclotron and the A1900 fragment separator. The main function of the CCF is to produce and deliver rare-isotope beams (RIB) to any of the 8 experimental vaults (labeled 81—83 and N2—N6 in Figure 2.1). The CCF is able to accelerate all stable ion 17 beams, from hydrogen to uranium, prepared by either the Room Temperature [53] or Superconducting [54] Electron Cyclotron Resonance ion sources, also referred to as RT-ECR and SC-ECR in Figure 2.2. To date, 20 primary beams from oxygen to bismuth have been developed. The CCF uses the projectile fragmentation technique to produce secondary beams of exotic isotopes. Since 2001 more than 330 secondary beams have been delivered to various experiments. To produce a rare isotope beams, first a stable beam (such as 400a, 48Ca, etc.) is accelerated in the K500 cyclotron to energies of approximately 10— 20 MeV/u (z 0.2 c) (Figure 2.2). As the beam is injected into the K1200 cyclotron, it passes through a carbon stripper foil increasing its charge state substantially to maximize its energy in the final stage of acceleration. For ions with mass number A S 136 the maximum energy attained in the K1200 is approximately 120—140 MeV/u (z 0.5 c). The accelerated heavy-ion (primary) beam strikes a production target, Yield #/8 I>1OZ l> 10 I> 10 I> .01_5 l> 10 Proton Number I I 01 O Neutron Number Figure 2.3: Calculated yields of different isotope species for the Coupled Cyclotron Facility at the NSCL—MSU [36]. Nuclides are plotted in proton versus neutron number representation. The colors represent the yield in number of particles per second in logarithmic scale. 18 creating various isotopic species lighter (fragmentation) or slightly heavier (nucleon pick-up) than the primary beam nucleus. The A1900 fragment separator works as an “isotopic filter” —— separating and transporting specific rare isotope(s) to different beam lines for experiments [50]. Figure 2.3 displays calculated rates (by EPAX [48]) for different rare isotopes produced by the CCF [36] in the proton versus neutron number (nuclear chart) representation. 2.3.1 Primary beam and reaction targets In the study carried out at the N SCL we used four primary beams: 40"‘8Ca and 58"MN i. Basic characteristics of these beams, like charge state and energy, are listed in Table 2.1. The primary beam intensity was controlled by a series of meshes (attenuators), allowing experimenters to attenuate primary beam intensity from a factor of 1 to 106, with a step size of approximately a factor of 3. Beam spot in the target plane was approximately 2 x 2 mm2 (determined using a CCD camera looking at a viewing plate covered with phosphorous material, scintillating upon interaction with charged particles). Table 2.1: List of all primary beams used in our experiments. Nominal energies and charge states of the primary beams are given at the exit of ECR, K500 and K1200 respectively. Energy in the A1900 is determined by measuring the Bp. Ion ECR Charge state E (MeV/ u) Charge state E (MeV/ u) E (MeV/ u) K500 K500 K1200 Nominal A1900 40Ca8+ 8+ 12.36 19+ 140.00 140.81 “Ca“ 8+ 12.23 19+ 140.00 141.96 58Ni11+ 11+ 12.35 27+ 140.00 140.96 64Ni11+ 11+ 12.35 27+ 140.00 141.23 One of the goals of our experiments was to investigate the target dependence of the fragment yields. In our measurements we used two target materials — Be (beryllium) and Ta (tantalum). Due to its relatively large nuclear number density, beryllium is a commonly used material for production targets at fragmentation facilities. Beryllium 19 forms a mechanically stable solid material from which a sturdy self-supporting target can be easily manufactured, with relatively good heat transfer and radiation hardness. The tantalum target was chosen to investigate the dependence of the fragmentation cross section and the production yield dependence on the nuclear charge and neutron- proton asymmetry of the target material. Table 2.2 lists all beam-target combinations used in our fragmentation experi- ments at the NSCL along with energy of the primary beam before, in the middle, and after the target material. All combinations of two targets, 9Be, and 181Ta and four pri- mary beams, 40' 48Ca, and 58"“‘Ni, were used in our fragmentation experiments carried out at the NSCL. The 9Be target thickness was chosen as a reasonable compromise between maximizing the yield of the Observed fragments and minimizing the effects of the energy loss, angular and energy straggling on the final fragment momentum distributions. In order to ensure these effects were comparable for both targets, the thickness of the 181Ta target was chosen such that the energy losses Of all primary beams were similar to the 9Be target (z 4—9 MeV/ u). The data could then be taken with both targets using the same magnetic settings, thus, minimizing the number of settings required in the experiments. 2.3.2 A1900 Fragment separator Since its commissioning in 2001 the A1900 fragment separator have been used rou- tinely to separate and deliver rare isotOpe beams to experiments at the NSCL. In our experiments the A1900 was used to collect and identify the emitted fragments. Figure 2.2 depicts the A1900 fragment separator as part of the CCF at the NSCL. A more detailed view of the A1900 is shown in Figure 2.4. The A1900 consists of 4 superconducting dipoles D1—D4, with a radius of 3 m and a bending angle of 45°. The A1900 uses 24 quadrupoles grouped in 8 cryostats. The A1900 fragment separator was constructed with a considerably larger (10x) angular acceptance than its predecessor the A1200 device [43]. Angular and mo- 20 Table 2.2: List of targets used in the series of experiments at the NSCL. Measured thickness is determined by measuring energy losses of the primary beam in the target material. Energy of the primary beam before, in the middle and after the target material are also given. Beam Target Measured Energy material thickness before middle after target target target (mg/cm2) (MeV/u) (MeV/u) (MeV/u) Be 103 138.41 135.98 40 C“ Ta 221 ”0'81 137.48 134.11 Be 105 139.93 137.89 48 ca Ta 228 ”1'96 139.12 136.24 Be 104 137.68 134.35 58 - N1 Ta 226 ”0'96 136.35 131.64 . Be 105 138.24 135.20 64 N1 Ta 225 14123 137.08 132.85 Table 2.3: Fundamental parameters of the A1900 fragment separator [55]. [ Parameter [ A1900 ] Max. rigidity (Tm) 6 Solid angle (msr) 8 Momentum acceptance (%) 5 Dispersion (mm/%) 59.5 Resolving power 2915 mentum acceptances determine the overall efficiency for collecting the fragmentation products. While we used the narrow momentum acceptance mode in our measure— ments, the large momentum acceptance is usually utilized in the production mode, when one wants to transmit a large fraction of the desired fragment to an experiment. Fundamental parameters characterizing the A1900 fragment separator are listed in Table 2.3. The principle of operation of the A1900 fragment separator in the production mode is illustrated below. The exotic fragments are produced by the fragmentation of the primary beam in a production target and have the initial beam velocity (left panel of Figure 2.5). The mixture of unreacted primary and secondary ions is bent by the D1 21 Figure 2.4: Detailed View of the A1900 fragment separator. Bending dipoles are labeled D1—D4. The five focal planes are labeled as target, image 1, image 2, image 3 and focal plane. Eight cryostats each housing 3 quadrupoles one is also indicated. .117 50Co pro ons neutrons , i‘ -4 . aneutrons Figure 2.5: Distribution of fragments in the chart of nuclides (proton versus neutron number) at three positions along the axis of the A1900 fragment separator: target, dispersive image, focal plane, respectively from left to right. Simulation is provided for the production of 50Ca in fragmentation of 58Ni on a 9Be target, with momentum acceptance, 1%, target thickness, 300 mg/cmz, and degrader thickness, 200 mg/cm2. Simulation was done using LISE++ [56]. and D2 dipoles to select a single magnetic rigidity, Bp, using a momentum slit (middle panel of Figure 2.5). Isotopic selection is completed by passing the ions through an energy-degrading “wedge”, placed after the momentum slit in the intermediate image plane (image 2 in Figure 2.4). Ions enter the degrader with a single Bp but may have different charges, Q, and atomic mass numbers, A, and exit with different momenta that depend on Q and A. A second dispersive beam line then provides, in most cases, isotopic separation (right panel of Figure 2.5). The nature and the thickness of the production target and the energy degrader, as well as the sizes of momentum apertures, are parameters that are adjusted to control the secondary beam intensity and purity [50]. We used the A1900 fragment separator in stand alone mode with narrow mo- 22 mentum acceptance (0.2% in dp/ p). Particle identification was achieved using the detectors mounted in the focal plane detector box. We tried to minimize all factors that introduce uncertainties to our measurement. There was no degrader material used in the intermediate image plane in order to diminish the transmission uncertain- ties. (The wedge was used only during the measurement of 50Ca produced in 58Ni+9Be reaction, as discussed in Section 4.3.2.) 2.3.3 Detectors at the A1900 fragment separator Particle identification detectors It is a good approximation to assume that all fragments produced in our experiments at 140 MeV/u are predominantly fully stripped of electrons (Q = Z). (From the charge state distribution measurement for the primary beams we expect less than 2% contribution from hydrogen-like charge states for all projectile-target combinations except nickel projectiles on a tantalum target where it is less than 5%. ) This means we can use the simpler particle identification method, Bp—ToF-AE, discussed in Section 2.2. In this case the measurement of three quantities, the magnetic rigidity, Bp, time of flight, ToF, and energy loss, AE, suffice to identify the various species. Since we used the narrow momentum acceptance mode (0.2% in dp/ p), the Bp values are given by the magnetic field settings of the fragment separator. For the time of flight (T 0F in Equation (2.2)) detector we used a plastic scintillator with thickness of 100 mm and an area, 150 x 100 mm”, placed at the focal plane as the last detector. The radio frequency (RF) pulse of the cyclotron is used as a time reference. The plastic scintillator (SCIN) provided an event trigger and the TOF start signal. The time of flight was stopped by the RF pulse. The time structure of the primary beam produced by the CCF spans approximately 2—4 ns due to multi—turn extraction in the K1200 cyclotron. Nevertheless, we are able to resolve the mass for all fragments in our experiments. The length of the time of flight path (the distance 23 from target to the focal plane position along the fragment separator axis) was taken 35.48 m. Table 2.4: The magnetic rigidity, Bp, values of N = Z settings, used for particle ID calibration for every primary beam discussed in the text. Primary beam B p(Tm) 40Ca 3.12 4”Ga 3.20 58Ni 3.60 64Ni 3.30 For energy loss measurement (AE in Equation (2.4)) we used a 500 pm thick silicon PIN (Positive Intrinsic Negative) detector. The active area Of the PIN detector is 50 x 50 mm2. A PID spectrum is obtained by plotting the energy loss, AE, versus the time of flight, TOF. Figure 2.6 displays a simulated PID spectrum for the 58Ni+9Be reaction using LISE++ [56]. The individual fragments are displayed as well separated groups of events forming characteristic bands in horizontal and vertical directions. All nu- clides with a constant neutron excess, N — Z, form vertical bands in Figure 2.6. The slightly tilted horizontal bands correspond to isotopes of individual elements. The leftmost vertical band corresponds to fragments with N = Z, the neighboring band corresponding to N — Z = 1, etc. (The time of flight was simulated such that the more neutron-rich fragments are located to the right of the N — Z = 0 band.) By recognizing these characteristic features of the PID spectrum we can easily identify nuclides with different N — Z. Identification of the element bands is done by locating “the hole”, which corresponds to particle unbound 8Be nucleus, as shown in the mag- nified region of the PID spectrum in Figure 2.6 allows one to identify all fragments. For reference, the isotope of 30P and the band corresponding to isotopes of calcium (Z = 20) are labeled in Figure 2.6. The magnetic rigidity setting for a given reaction allowing the observation of the N — Z = 0 band is also referred to as the “N = Z setting”. The N = Z magnetic rigidity settings of the A1900 fragment separator used 24 in the PID calibrations for all primary beams are listed in Table 2.4. The AE and TOF detectors are calibrated using the fragments from the N = Z setting, to the calculated values of TOF and AE using the LISE++ code [56]. First the AE and T 0F values are calculated using LISE++ for the fragments identified in the N = Z setting. Then the channel values of AE and TOF are extracted from the experimental spectrum and the coefficients of a linear calibration from channels to MeV and ns are Obtained for AE and TOF, respectively. Next, the coefficients, K1 and K2, of Equation (2.4) are fitted to reproduce the nuclear charge numbers of all identified fragments in the N = Z setting. After this calibration the Equation (2.1), (2.2) and (2.4) are used to calculate the momentum, nuclear charge and mass numbers for all fragments in our analysis. The PID calibration was done for each projectile separately. Position sensitive detectors The detection of the position of particles traveling through the fragment separator is important for diagnostic purposes (measurement of the positions and angles ensures that the magnetic rigidity tune corresponds to the ion optical calculation). In the case of wide/ full momentum acceptance, position measurement gives the momentum of fragments. A total of four position sensitive Parallel Plate Avalanche Counters (PPAC) detectors is installed in the intermediate image (2) and the focal plane (2) along the fragment separator axis. The principle of the PPAC detectors is described here. A charged particle ionizes the gas in the active volume of the detector. The electrons freed in the ionization process are multiplied in a cascade (avalanche), by ionizing surrounding gas molecules, in high electric field in the active volume as shown in Figure 2.7. PPACs used in the A1900 fragment separator have segmented cathodes, providing the position sensitivity. Figure 2.8 shows two different (front and side) views of a PPAC detector. Each detector consists of two separate chambers sharing one anode 25 I I I I l I I I I l m W r ,. a; , . _ . g 100— It . 1 . he ‘ O ' > , III * I O . l I § ' b ' I ' . . g * 53 5o— 2 . ‘ ‘— " _ ”3 4. _N-Z—1 . C. _ , at. ‘ 300 O . 9 .. . j ’\ _ -.. . z . ‘1 r? > a: BB; . , ‘ ,/ .. .. % i: . ._ ‘ . ‘ 0 II m: l L I L L A I 1 . g . Q 235 240 245 250 255 x, - TOF (ns) _. I .E _ O " .» e "‘ l I ] ' "’ ' . ~ 2:20 - i- . . 4:. "' O ‘_ *_ III 4. .— 1 "' - 100 —L 6.? I: 1 ', . .. _ _ : - ‘ «up. .. _ _ {:1 I l I 1-]-l ['14 + I r1 I I'I I I I I I‘d 930 240 250 260 270 280 TOF (ns) Figure 2.6: The PID spectrum of a simulated of N = Z setting for 58Ni+9Be reaction at 140 MeV/u using LISE++ [56] for the A1900 fragment separator. Parameters of the simulation: Bp=3.6 Tm, 0.2% momentum acceptance. 26 Ionizing particle \ Anode E :\° ° Gas 3. Cathode Figure 2.7: Principle of Operation of an avalanche counter. The charged particle ionizes the gas in sensitive volume of the detector between the anode and cathode. The electrons freed in this process are multiplied in an avalanche, creating an electric signal in the cathode. foil. Segmented cathodes have evaporated strips connected to the readout electronics. Two different designs of PPAC detectors are used in the A1900, referred to as I2 PPACs and PP PPACs. III-I-I-I-I-I-II-I- :19: x 1;: c.1pz;,gmxw~ 335,12:ij flavzg-g Figure 2. 8: The front (left panel) )and side (right panel) view of a Parallel Plate Avalanche Counter (PPAC). 1 — cathode plane with horizontal strips, 2 ~ common anode plane, 3 — cathode plane with vertical strips A pair of I2 PPAC detectors is installed at the dispersive image of the A1900 on a retractable platform. Each detector has an active area of 400 x 100 mm2 corre— sponding to 160 x 32 strips. Every strip is individually read out using the Front End 27 Figure 2.9: Detector setups in the A1900 used for our fragmentation experiments. Beam intensity monitors (NaI(Tl) and Ban) shown in the target area (TA), momen- tum slit at intermediate image (12) and the particle identification setup (PIN and Scin) at the focal plane (PP). The position sensitive PPACO and PPACl were used to track the fragment trajectories. Electronics (FEE) boards. This arrangement of the read out allows experimenters to use the detectors up to the particle rates of 1 MHz. The ion Optics of the A1900 fragment separator is set up such that the particles with different momentum have different spatial positions in the horizontal plane. Hence, the measurement of position at the intermediate image is equivalent to the measurement of momentum. In our experiments the I2 PPACs were used only for the measurements of momentum of the primary beam and its charge states (see analysis of the charge state distributions in Section 3.1). These detectors were removed from the beam during our fragment cross section measurements, because they were not needed in narrow momentum acceptance mode. Each of the two focal plane PPAC (FP-PPAC) detectors have an active area of 100 x 100 mm2 with resistive read-outs. The strips are interconnected by a resistor chain and read out by two channels on both ends. Hence one PPAC detector provides signals in 5 channels: left, right, up, down and anode. These detectors were used for the tracking all fragments during our fragmentation experiments (Figure 2.9). rIracking of the fragments at two different positions along the fragment separator axis at the FP (before and after the PIN detector) allowed us to interpolate the paths to ensure that all fragments were hitting the active area of the PIN detector (Figure 2.9). 28 I“ I 1.2; '11—? .' in I ' A1900 ‘- triplet J l2. _ . i. in" [‘2' I :1, iv a ,_..._ chamber Figure 2.10: Photographs of the two beam intensity monitoring devices. Left panel: four NaI(Tl) detectors attached to a ladder; Right panel: Ban detector in a lead fortress (used to shield the detector from the background gamma radiation). Primary beam intensity monitors For overall normalization of fragment yields, precise continuous measurement of pri- mary beam intensity is required. In order to calculate absolute fragmentation cross sections we needed to know the absolute primary beam intensity. Intensities of the primary beam ranged from 106 to 1010 particles per second (pps) during our produc— tion runs. The absolute beam intensity is measured by a Faraday Cup (FC) which stops the primary beam, at the CCF. As the direct measurement by a charged—particle detector is not possible, because of the high rates of the primary beam used in our experiments (> 106 pps), we used an indirect method to determine the beam intensity by monitoring the radiation created by beam-target interactions. The flux of light particles and gamma radiation emitted in the interactions Of the primary beam with the production target is proportional to the beam intensity. This feature provides means to monitor the primary beam intensity continuously. Two detector systems of light particles and gamma radiation were used in order to cover the primary beam intensity range used in our experiments (approximately 4 orders of magnitude). The first detector configuration was an array of four N aI(Tl) crystals; the second one was a single Ban crystal coupled to a photomultiplier tube (Figure 2.9). 29 The N aI(T l) array consists of 4 crystals mounted on a ladder which is attached to the outside of the target vacuum chamber (left panel of Figure 2.10) approximately 30 cm from the target at an angle of approximately 45°. The bias voltages and dis— criminator levels were set up to detect the high energy charged particles emitted in the reaction of the primary beam and target. The BaF2 detector was shielded from the surrounding gamma radiation in the target area by a lead fortress. It was placed on the floor behind the target chamber (right panel of Figure 2.10) approximately 130 cm from the target at an angle of approximately 72°. The bias voltage and the discriminator level were set up to detect the gamma radiation created in the beam-target interactions. Calibration of the absolute beam intensity monitor was performed using a Fara- day Cup. The FC was placed in the extraction channel of the K1200 cyclotron (Z001) in the case of 58Ni fragmentation, and the PC was placed in the target box of the A1900 fragment separator for 40"“3Ca and “Ni primary beams. For the NaI(Tl) array the background count due to radiation produced by activated target area has been subtracted. This rate was determined by simply measuring the ambient radiation in the target area with no beam on target. Since the beam intensity monitoring tech- nique relies on nuclear reactions of the projectile and target nuclei, it is necessary to perform the beam intensity monitor calibration for each beam-target combination. For every beam-target combination we performed 3 beam monitor calibrations (at the beginning, middle and end of an experiment). With the three calibration mea- surements we confirmed that changes (e.g., due to activation of the target area, beam spot position) during our experiment were negligible. 2.3.4 Magnetic rigidity settings Measurement of the momentum distributions for a wide range of fragments was carried out by systematically scanning across the magnetic rigidity settings of the A1900 fragment separator. Figure 2.11 and 2.12 show magnetic rigidity, Bp, settings used 30 for all the primary beams with the 9Be and 181Ta targets. The individual points in Figure 2.11 and 2.12 represent the magnetic rigidity settings of the A1900 fragment separator. The horizontal bars display the momentum acceptance for individual Bp settings. For 40Ca we took measurements between 3.2—4.2 Tm in 33 steps and for 48Ca we scanned region between 3.2—5.1 Tm in 50 settings. For 58Ni and “Ni beams we took data between 3.25—4.3 Tm in 26 steps and 3.3—4.5 Tm in 34 settings, respectively. The gaps in the Bp coverage correspond to the primary beam and its charge states rigidity (Figure 2.11 and 2.12). The primary beam intensity was optimized at each magnetic rigidity such that the counting rate of the PIN detector was approximately 700—900 counts per second. The detection efficiency of the PIN detector is close to 100% when the counting rates are below 1 kHZ. 2.3.5 Data acquisition system Figure 2.13 shows a schematic diagram Of the electronic circuit used in the A1900 fragmentation experiments. The trigger was produced by the focal plane scintillator T j T I r I fir I I I I I l I I I ' ' r BeL—uuuuunnuu HHHHHHHHHHHNHHHHHHHHHHHH1 Ta—uuuuuunu HHHHHHHHHHHHHHHHHHHHHHHHH— J L 1 1 .4 I l a l l l 1 L a l 3.2 3.4 3.6 3.8 4 4.2 Bp(Trn) 1'...l.....1.'...'.uerthiurvir..l...... Be—IIIIIIIIIIIIIIIIIIIIIIIIIIIIIII linen-ulnllnnlu III—II Ta—Ielulnll IIIIIIIIIIIIIII IIIIIIIIIIII III-luau II —I .L.;.l.;.l;..llamleggnnl...l...l...lL4 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8 5 Bp(Tm) Figure 2.11: Magnetic rigidity settings for Ca primary beams. Top panel for 40Ca and bottom panel for 48Ca. The error bars in horizontal direction indicate the 0.2% in dp/ p momentum acceptance in the Bp space. 31 Bel-I'D! H H H H H H H H HH H HHH HHH H H H H H —] k P L +4 I l I I 1 . I . 1 . . 1 3.2 3.4 3.6 3.8 4 4.2 39 (Tm) ' I T I I I If I ' lIII'IIfo‘rI Bel—IIIIIHIIIIIHHIIIIHHHHHHHHHHHHH HHHHHHHH-A TaI—IHIHHIIIIIHHHHHIINIIHHNN HHNHHHHHH—I I 4 J 14 L m l I I I l I I I I I I j . I I I l 3.4 3.6 3.8 4 4.2 4.4 . BP (Tm) Figure 2.12: Magnetic rigidity settings for Ni primary beams. Top panel for 58Ni and bottom panel for “Ni. The error bars in horizontal direction indicate the 0.2% in dp/p momentum acceptance in the Bp space. signals (labeled as FP SCI N&S in Figure 2.13). The time of flight was measured by the F P SCI with respect to the Cyclotron Radio Frequency (RF). The focal plane PPAC (Parallel Plate Avalanche Counter) detectors with standard readout provide signals in 5 channels (up, down, left, right and anode signals). All channels from the two PPAC detectors are denoted by using thick line in Figure 2.13. Timing and analog signals were digitized using standard CAMAC (Computer Au- tomated Measurement And Control) Analog to Digital Converters (ADC), Charge to Digital Converters (QDC), and Time to Digital Converters (TDC). The CAMAC bus was interfaced through the Versa Module Eurocard (VME) to CAMAC Branch Driver (CES CBD8210) and controlled by a personal computer (PC) running the standard NSCL data acquisition system NSCLDAQ [57] in the Linux Operating system. The data were collected and distributed by the SpectroDaq server [57]. The NSCLDAQ al- lows users to run multiple simultaneous online analyses using the NSCL SpecTcl [58], plus archive the data on the network filesystem. The final back-up of the experimental data was stored in a 100 GB Digital Linear Tape (DLT). 32 [ CFD L, > Trigger FPSCI —~~—b~—@§E \ QDC N&S »—] Gate Generator ] > Scaler ADC CYC. {E— TAC - RF PIN —[E—i Shaper FP PPACl —[®—J Shaper > ADC PPAC2 Q [— cm NaI —[E—L%aper |—[CFD]— Z ——>Sca1er BaF 41 Shaper [—[ CFD 1r +>Scaler — 5+5 signals — 4 signals Figure 2.13: Schematic diagram of the electronic diagram used for the A1900 frag- mentation experiments. 33 2.4 Experimental setup for RIKEN experiment Projectile fragmentation experiment using 86Kr beam was performed at RIKEN (The Institute of Physical & Chemical Research) Accelerator Research Facility (RARF) in Wako-shi, Japan. The layout of the RARF facility at RIKEN is shown in Figure 2.14. The beam of 86Kr was accelerated in the Ring Cyclotron (K540 in Figure 2.14) and delivered to experimental area D (Figure 2.14). The fragments were analyzed and identified using the RIKEN Projectile-fragment Separator (RIPS). Figure 2.14 shows the RIPS fragment separator located across two adjacent rooms D and E6. More detailed view of the RIPS is shown in Figure 2.15. Primary produc- tion targets were located in the target position of RIPS (Figure 2.15). Projectile-like fragments produced in interactions of the primary beam with the target nuclei were collected in the RIPS fragment separator and transported to doubly achromatic focal plane F2 (Figure 2.15). Detector setups at focal planes F2 and F3 allow us to identify all fragments reaching F2. In order to cover the full momentum space we changed the magnetic settings of the two dipole magnets D1 and D2. 2.4.1 Primary beam and reaction targets Ions of 86Kr30+ were accelerated in the K540 ring cyclotron at RIKEN. The primary beam energy, determined to be 63.77 MeV/ u, was measured by a direct measurement of the magnetic rigidity using the RIPS. The beam intensity, which was adjusted by a beam attenuator at the exit of the Electron Cyclotron Resonance (ECR) ion source varied from 106 to 1011 particles per second (pps), during our experiment. Beam spot size on the target was approximately 1 mm in diameter. For the fragmentation experiments with the 86Kr primary beam, two target mate- rials 9Be and 181Ta were used. Energy of the primary beam before, in the middle, and after the target is listed in Table 2.5, along with the measured thickness of the target materials from energy loss consideration. The target thickness was chosen as a reason- 34 able compromise between minimizing the distortion of the momentum distributions and maximizing the yields Of reaction products. Table 2.5: Primary beam energy measured and target thickness measurement in the 86Kr experiment. Energy before, in the middle and after the target is also listed. Target Thickness measured Energy before middle after (mg/cmz) (MeV/u) (MeV/ u) (MeV/ u) 9Be 96 63.77 57.90 51.60 181Ta 153 63.77 58.02 51.89 2.4.2 RIPS fragment separator The fundamental parameters characterizing the RIPS fragment separator are listed in Table 2.6. Since its commission in the beginning of the 19903, it has been routinely Figure 2.14: Layout of RIKEN Accelerator Research Facility (RARF) [35]. The K540 cyclotron is shown with respect to the experimental areas labeled as E1—E6. The RIPS fragment separator is located across the two adjacent rooms D and E6. 35 used to deliver a variety of radioactive beams for experiments. Configuration of the RIPS fragment separator in the RARF facility is shown in Figure 2.14. It consists of two 45° dipole magnets, twelve quadrupoles (Q1—Q12), and four sextupoles (SXI— SX4). The first section with a configuration of Q-Q-Q-SX-D-SX-Q gives a dispersive focus at F 1 allowing measurement of the magnetic rigidity of the particles. The second section with a configuration Of Q-Q-SX-D—SX-Q-Q-Q compensates the dispersion of the first section and gives a doubly achromatic focus at F2. Quadrupole triplet of the last section gives the third focus at F3, where special experimental setups can be installed [35]. For our fragmentation experiment the main particle identification detection setup was placed in F3 focal plane (Figure 2.15). Table 2.6: Fundamental parameters of the RIPS fragment separator [35]. [Parameter | RIPS ] Max. rigidity (Tm) 5.76 Solid angle (msr) 5 Momentum acceptance (%) 6 Dispersion (mm/%) 24 Resolving power 1600 2.4.3 Detectors at the RIPS fragment separator Charged-particle detectors installed at the RIPS fragment separator were used for particle identification and primary beam intensity monitoring. Figure 2.15 shows the RIPS fragment separator with all the detector setups used in the 86Kr fragmentation experiment in ovals next to focal planes F0, F2 and F3. Particle identification detectors In section 2.2, we introduced the particle identification method used. The charge state distributions of 86Kr primary beam at 64 MeV/u are much broader as compared to 36 Primary Beam 86Kr :DFB-gfifiqfi“? D1 Production Target (Be, Ta) [953-] = 0.2% ’ Momentum Slit I Beam intensity monitor Fragments F2 plastic (TOF start) Fragments F3 plastic (TOF stop) Si stack dE+E 1+E2+E3+E4 Figure 2.15: RIPS setup used for the fragmentation experiment [59]. Beam intensity monitor MOMOTA was placed at F0, particle identification setup was placed at F2 and F3 focal planes. Momentum slit at F1 defined the momentum acceptance. primary beams measured at the NSCL. Hence, a significant contamination also from the charge states of individual fragments is expected (more than 10% in hydrogen-like charge state for the primary beam after passing through the 9Be target). To identify the momentum, mass and nuclear charge numbers along with the charge state of each fragment, the full Bp—ToF -AE—TK E particle identification technique must be used (as explained in Section 2.2). The magnetic rigidity, Bp is given by selecting different magnetic settings of the 37 fragment separator. The time of flight, ToF, is measured between two 500 pm—thick plastic scintillators on the flight path of 6 m mounted at the F2 and F3 planes, respec- tively. This is displayed in the ovals next to F2 and F3 focal planes in Figure 2.15. A stack of five silicon detectors was used to measure energy loss, AE, and total kinetic energy, TK E. The silicon stack is comprised of one 300 pm-thick detector and four 500 [rm-thick detectors. The stack was installed at the very end of the RIPS beamline, as shown in the oval next to F3 in Figure 2.15. Energy 1033 simulations showed that all particles heavier than argon will be stopped in this arrangement of silicon detectors. Primary beam intensity monitoring The primary beam intensity is an essential quantity to calculate the absolute frag- mentation cross sections. We monitored the primary beam intensity using a telescope called MOMOTA. It consists of three plastic scintillators and detects the light par- ticles produced in nuclear reactions in the production target. We required triple co- incidence rates to reduce the detector background. Figure 2.16 shows a schematic drawing of the beam monitor telescope at the target position. MOMOTA :1 Primary beam Production target Figure 2.16: A schematic view of the MOMOTA beam intensity monitor with respect to the target and the downstream Faraday Cup (F C). As MOMOTA beam monitor was located further downstream from the target position as the scattered particles off the FC influenced the MOMOTA reading during the beam intensity calibration. Unlike the N SCL experiments, we could not use the 38 Faraday Cup (F C) to calibrate the MOMOTA beam monitor. Instead, the absolute calibration of the MOMOTA telescope was provided by the direct rate measurement of primary beam charge state 8°Kr33+ and 86Kr31+ for 9Be and 181Ta targets, respectively, at the F2 focal plane by the plastic scintillator (detailed discussion in Section 3.2.2). 2.4.4 Magnetic rigidity settings Measurement of the momentum distributions for a wide range of fragments was carried out by changing the magnetic rigidity settings of the RIPS fragment separator. Figure 2.17 shows the magnetic rigidity settings used for 9Be and 181Ta targets. Settings for the 9Be target covered the range in Bp from 1.79 to 2.93 Tm in 45 steps. Settings for the 181Ta target covered much smaller range in Bp from 1.79 to 2.35 Trn in 29 steps, because the charge state distribution Of 86Kr at 64 MeV/ u is much broader than in the case of the 9Be target, resulting in significant contamination of the PID spectrum. . I . . 4 , . . . I . . . , . . . , . . r r . Be—IIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIII II III II II II— TaI—IIIIIIIIIII III-IIIIIIIIIIIII -—] 1 L; 1 1 l 1 1 1 l 1 1 1 l 1 1 1 l 1 L 1 l 1 1.8 2 2.2 2.4 2.6 2.8 39 (Tm) Figure 2.17: Magnetic rigidity coverage for the 8°Kr runs. The error bars in horizontal direction indicate the 0.2% in dp/ p momentum acceptance in the Bp space. The step size we used for the overall scanning was 0.03 Tm. To avoid the charge states Of the primary beam from coming to the focal plane detectors (at F2 and F3) the corresponding Bp settings were not taken, forming the gaps in the magnetic rigidity, Bp coverage in Figure 2.17. At each magnetic rigidity setting we Optimized the beam intensity such that the counting rate of the silicon stack detector was 5 1, 000 counts per second. 39 2.4.5 Data acquisition system Figure 2.18 shows a schematic diagram of the electronic circuit of 86Kr fragmentation experiment. The event trigger was generated by the F2 plastic scintillator. The tim- ing and analog signals were digitized by standard CAMAC (Computer Automated Measurement and Control) Analog to Digital Converters (ADC), Charge to Digital Converters (QDC), and Time to Digital Converters (TDC). The data were collected using the standard data acquisition software at RIKEN [60]. The online data analysis has been done using the ANAPAW [61], a tailored version of the Physics Analysis Workstation (PAW) [62] for the online analysis. All the data were archived to 120m Digital Audio Tapes (DAT). 40 ‘n—--—--——-——---—--——---—------—‘ E4 Delay PPAC —[>« m [m corners [ Delay QDC {gim- ~—[>—‘H E Ii-——-———> Trigger 5 > Sealer i Delay QDC F3 Scint. E TDC L&R . E > Scaler Bit I > register dB, E1, p i rs-h-ae E2, E3, : E > Trigger PPAC Im‘rm m... we anode , - ; Sealer Scaler Okuno %:—E & ——> Scaler \ --—-_----—--———--—-—----_—-----o Figure 2.18: Block schematic of the electronic circuit used for the 86Kr fragmentation experiment at RIKEN. 41 Chapter 3 Data analysis The primary beam charge state measurements are presented in the beginning of the chapter. It is followed by description of the analysis procedures for the beam inten- sity calibration, particle identification, differential cross section evaluation, angular transmission correction and momentum distribution. The method of fragmentation production cross section calculation and the uncertainty evaluation are discussed at the end of the chapter. 3.1 Primary beam charge state distributions In our experiment, we want to avoid detecting charge states of the primary beam —— as discussed in Section 2.1. Therefore, it is important to know the positions and widths of these distributions in the magnetic rigidity space. Different charge states, Q, of one ion with mass number, A, traveling at velocity, 6, are spatially separated in a magnetic field according to Equation (2.1). In our case the two neighboring charge states were spatially separated in the dispersive plane of the fragment separator due to different bending radii in the magnetic field. The charge states of the primary beam were measured by tuning the fragment separator to a Bp between the magnetic rigidities of two neighboring charge states, 42 allowing us to Observe their relative Bp distributions. For each measured beam-target combination the results were normalized with respect to each other and the probabil- ities for different charge states were calculated. A series of the primary beam charge state distribution measurements have been done for 58'“Ni and 86Kr, for which two neighboring charge state distributions fit within the acceptance of the fragment sep- arator. I I I 1 _ I f T I m . I 4 102;- uBe target a 10’? 036 target 1 g 10;? ........... ATa target 1. § 10:; 4T3 target 1 s 15 5 g ’5 i ‘- E i t: I; I "Sufi: A a €10? - E3 “§ 5 58Ni : “@102; a 3.104;: a: a. 103:- 0 1 10-35? ‘ E . I 31045111-. .1 0 1 2 0 2 4 6 Z-Q Z-Q : l l I l 2 10’E DBe target ‘3’ ’8 10_ ATa target Q s . f E I. 14 lgm’E 1, '8 -2 ‘ £10 3 103: a . A 2 .4 10 E- l l l l E 0 1 2 3 Z-Q Figure 3.1: Primary beam charge state probability distributions for 58'“Ni and 86Kr primary beams plus 9Be (open squares) and 181Ta (Open triangles) targets plotted as a function of number of electrons Z — Q. Solid and dashed lines show calculation by GLOBAL parameterization [63] as implemented in LISE++ [56] for 9Be and 181Ta target, respectively. 43 The separation of two neighboring charge states in Bp for ions of Ca is larger (2 5 % in dp/ p) than the momentum acceptance of the A1900 fragment separator (z 5 % in dp/ p) so the charge state distributions were not measured for 40"‘8Ca beams. The charge state distributions measured for 58’“Ni and 86Kr beams at 140 MeV/ u and 64 MeV/ u, respectively, are presented in Figure 3.1 in terms of probability as a function of number of electrons, Z — Q. The distributions obtained on 9Be and 181Ta target are presented as open squares and triangles, respectively. The experimental data are compared to a leading parameterization of the charge state distribution —— GLOBAL [63], shown as solid (9Be) and dashed (181Ta) lines. The target thicknesses used in the experiments were much larger than the equilibration thickness so all the GLOBAL calculations were done for the equilibration thickness of the target mate- rial. The charge state probability distributions are wider for 181Ta for all projectiles shown in Figure 3.1. All probability distributions are reproduced rather well in spite of the fact the parameterization GLOBAL, was derived for Z > 28 ions. Some dis- crepancies are observed in the case of 58Ni and 86Kr projectiles. The GLOBAL code has no isotope dependences; the calculated distributions for 58Ni and “Ni are iden- tical. However, the experimental data show broader probability distributions in the case of the 58Ni projectile (especially for 9Be). The experimental data and GLOBAL predictions for 8°Kr33+ differ by a factor of 10, while the experimental data for 181Ta target are reproduced well up to Z — Q = 6. 3.2 Calibration of beam intensity monitors 3.2.1 N SCL experiments Precise knowledge of the primary beam intensity is essential in determination of the absolute fragmentation production cross sections. In the fragmentation experiments carried out at the NSCL we used two primary beam intensity monitors referred to as N aI(Tl) and Ban (Section 2.3.3). Since the experiments were carried out at different 44 times over the course of almost two and half years there are small differences in the experimental configuration and analysis procedures including the primary beam intensity monitoring. The primary beam intensity monitors were calibrated with respect to the Fara- day Cup (FC) beam current measurement. For the 58Ni fragmentation experiment only one FC was available (Z001); for the other systems (40'48Ca and “Ni), another FC was located in the target box (Z014). Figure 3.2 shows the Z001 FC (left) in the extraction channel and Z014 FC (right) in the target box, in front of the A1900 fragment separator (right). Since both F C’s are installed upstream from the target position (which is Z015) it is not possible to measure the beam current and monitor the counts of the secondary particles from the target simultaneously. We assume the beam intensity variations over a short period of time (2—3 minutes) are negligible. The beam current is taken as an average from multiple instantaneous measurements 5 seconds apart in duration of up to approximately 1 minute. Immediately after the Figure 3.2: Faraday cups Z001 and Z014 in the extraction beam line and the target box, shown relative to the K1200 cyclotron and the A1900 fragment separator. The Z001 and 2014 Faraday Cups are magnified by a factor of 4 in the presented drawing. 45 .8 $01le I I FIVIWI V I lllllil l I I In"! I V IlYlI‘] I I II .1th Y I I 11"" I l I IIlll' l I l IIIII'I T l I FFFYTI I 10 5 Ca /( a 10“; Ca ’ E Z 0 Be target / : : 0 Be target 1 " -9- 4 " 10'9=- 4 31 10 A Ta target a S 5 A Ta target / a Q. E q 1:. : / 1 3 510-10 r ; @1040 :- 1, >. : 1 I 4 4s "S . . -u _ L k 10 1: 12m“- e 3 e 1042? a 10-[2 l:- 3 1 ': ~ E1 U l 1 1111111 1 l lllllll l 1 1111111 I l lllllll l l 1 i-lllll L 1 1 Ill ll 1 1 1 llllll l l l llllll l l 1 111111 1“ 10 102 103 104 1 10 102 103 104 105 NaI(Tl) count rate (cps) NaI(Tl) count rate (cps) .- 5'8' VUHWI U I I'll"! I I IUITII'I Y I IIIHI' I I IIIIII' 1 IE _I& I I I IUIYII I T T111111 I I I an'r U U ITITTII .1 : Ni 3 10:8 Ni ‘2 : 0 Be target f/ : 4 0 Be target 4 A I I A ' ‘ 3, 10'95 A Ta target “a S, 109: A Ta target “a s 5 a s E i o 1 o ~ . >10 0 .4 5 4,1010 / 4. g 5 s x 10-“ E— 3 B I 1 a : c: -11 .— 5 L1. -12 4 LL. [0 : g 1: 10 F E: -13; :_ I l [0‘12 : 5: a 10 h l l llllfllV I I 111111] 1 l lllflll l l llllll‘ L 1 1111111 1 1E“ : 111 l l I 111111 1 1 l Lllul 1 l l llllll l l 1 111111 : 1 10 102 103 104 105 10 102 10’ 104 105 NaI(Tl) count rate (cps) NaI(Tl) count rate (cps) Figure 3.3: NaI(Tl) beam monitor calibration for all primary beams. Beam current measured by the Faraday Cup is plotted as a function of the NaI(Tl) background subtracted counting rate. Calibration data for 9Be target are shown as red Open squares and 181Ta target as blue open triangles. 181Ta target FC reading is offset by a factor of 0.1 in PC current reading for clarity. Solid lines through the calibration data show the fit by a second order polynomial in logarithmic representation. Horizontal arrows indicate the range of the NaI(Tl) counting rate used in our fragmentation experiments for both 9Be and 181Ta targets. 46 I I I IIUIII I l IIIIIII I I Y Illlll _‘ III I I I Ulllll I l IIUIUII I 1 {VIII —I §4°Ca E“Ni 2/ 3 10,7: 0 Be target __ 107; 0 Be target 1, A Ta target A Ta target / ] _ . ,4! 4 / L a N Q. ‘0 l I 11111 Faraday cup (A) N N O o ‘0 Go I l x \ 1] N Faraday cup (A) Q S '5 co l ‘ l ‘ 10-10 5— El 1 LJLJJLLLL l I 111111! L llllllll" [.11 I 1111 I I 1111111 I 1 ll I I 10 102 103 104 10 102 103 Ban count rate (cps) BaFZ count rate (cps) lllll Figure 3.4: BaF2 beam monitor calibration for primary beams mentioned in the text. Beam current measured by the Faraday Cup is plotted as a function Of the Ban background subtracted counting rate. Calibration data for 9Be target are shown as red open squares and 181Ta target as blue open triangles. 181Ta target FC reading is offset by a factor of 0.1 in F C current reading for clarity. Solid lines through the calibration data show the fit by a second order polynomial in logarithmic represen- tation. Horizontal arrows indicate the range of the BaF2 counting rate used in our fragmentation experiments for both 9Be and 181Ta targets. beam current measurement is taken the FC is retracted from the primary beam and the counts of the beam intensity monitors are recorded in sealers for 1—2 minutes. By repeating this procedure and varying the beam intensity by a series of meshes (attenuators), we obtained a dependence of the primary beam current and the beam monitor count rate. Since the beam monitors used are sensitive to the reactions of beam with target the calibration depends on the target material. The beam monitor calibrations were measured three times during each of the 4 fragmentation experi- ments (the beginning, middle and end). No time dependence of the calibration was Observed during our experiments. For each set Of beam intensity calibration a mea- surement of the background radiation was performed by stopping the primary beam at the exit of the ECR ion source. Figure 3.3 shows all the N aI(Tl) monitor calibrations for 4(”480a and 58’“N i beams, in terms of correlation of the FC current reading with the background subtracted 47 count rate of the N aI(Tl) monitor. The calibrations with 9Be target are shown as red squares and with 181Ta as blue triangles. The fit by a second order polynomial in logarithmic representation is displayed as red (QBe) and blue (181Ta) solid line. The horizontal arrows indicate the range of applicability of our primary beam intensity calibration with the NaI(Tl) monitor. The calibration data are described by our fit function within 7—10% over the range of beam intensity we measured. In the case of the more neutron-rich projectiles 48Ca and 64Ni, when the beam current exceeded beyond 10“7 A, we used the BaF2 monitor. The lower part was calibrated using the NaI(Tl) monitor and the upper part using the Ban monitor. Figure 3.4 displays the calibration curves of the BaF2 beam intensity monitor. The calibration curves shown in Figure 3.3 and 3.4 are used in the analysis to determine the total number of beam particles traversing the target in each run. 3.2.2 RIKEN experiment Primary beam intensity monitoring for 86Kr fragmentation experiment was provided by the MOMOTA telescope placed at forward angle outside the target chamber as described in Section 2.4.3. The FC was located after the target. This arrangement, in principle, allowed us to simultaneously measure the beam current (FC) and the secondary particle count rate (MOMOTA). Unfortunately, the primary beam reacts with the FC material and the back-scattered radiation affects the count rate of the MOMOTA monitor. This introduced an uncertainty upward of 15-20%. Thus we had to use a different method of beam intensity monitor calibration. For the 86Kr+gBe system we measured the absolute beam intensity by counting 86Kr33+ ions directly using the F2 plastic scintillator. From the measured charge state distribution as shown in Figure 3.1, the 86Kr33+ charge state is 0.028% of the 86Kr primary beam. During a 16 minute interval we recorded 1,054,792 events of 86Kr33+ in the F2 scintillator, 10,607 counts were recorded in the MOMOTA telescope at the 48 same time. Background measurement yielded a background count rate of 0.008 cps in the F2 scintillator and 0 cps in the MOMOTA telescope. The background subtracted count rate of the MOMOTA telescope is directly proportional to the primary beam intensity. In the case of 181Ta we measured the absolute beam intensity by counting 86Kr31+ ions directly in the F2 plastic scintillator. The 86Kr31+ charge state probability is 0.016% of the primary beam 86Kr. During a 4 minute measurement we recorded 214,525 events of 86Kr31+ in the F2 scintillator and 479 counts were recorded in the MOMOTA beam intensity monitor. 3.3 Particle identification The general particle identification method utilized in our fragmentation experiments is described in detail in Section 2.2. Since there are underlying differences in detector setups and details in the method of measurement of particle identification observables, we describe the particle identification analysis of the NSCL and RIKEN experiments separately. 3.3.1 N SCL experiments The particle identification detector setup was located in the focal plane box of the A1900 fragment separator as shown in Figure 2.9. As discussed in Section 2.3.3 we used the Bp—ToF-AE particle identification technique for all beam-target combina- tions measured at the NSCL, because we have Q = Z for all fragments of interest. This simplifies the whole particle identification analysis because now by plotting the energy loss, AB, in the PIN detector versus the time of flight, ToF, for a fixed Bp, we can separate all fragments traveling through the fragment separator. A typical raw experimental particle identification pattern is shown in the left panel of Figure 3.5. The example provided is for the 48Ca+9Be reaction at Bp = 3.2 Tm. We can 49 ' ' I\l-z'='2' ' ' ' '* 20." ..'..,'. I I 7 - d - '"u‘ ”L. N... 4 “ N - M fill an - - i ' < . a, - I“; l h" .4. A _ 2:16 ,_ " .g a 915- '43-. - fig . -e~- _ $2000 \ - fl. * 1 ‘ (U . a. an C \1- " .-' ‘ c - _. .. ~F . c ,. * a. o - an. E .. . «s .. *5 .. ta * r~ g: ..... - s .. 34. .. 9w: :; 2,... :.-. Emce— .— .4, _ a» — g l '2.- _~,. g. .,..--> a '. :2 5L _ -. — . *‘I: . E889 —_’-~ I ‘ - l l l 0 1 500 2000 2500 3000 -1 1 2 ToF (channels) 0 Neutron excess N-Z Figure 3.5: Uncalibrated (left panel) and calibrated (right panel) experimental particle identification (PID) spectra. The PID plots are shown for the 48Ca+QBe reaction at Bp = 3.2 Tm. see a clear separation of individual groups of events corresponding to different frag- ments. By recognizing the typical features of the PID spectrum and locating the hole corresponding to particle unbound 8Be nucleus, as described in Section 2.3.3, we can identify all nuclides in the uncalibrated PID spectrum shown in the left panel of Fig- ure 3.5. (The time of flight measurement was set up such that the more neutron—rich fragments are located to the left of the N — Z = 0 band.) The values of AE and TOF in channels, were calibrated (Section 2.3.3) to the calculated energy loss and time of flight using the LISE++ code [56]. Then, param- eters K1 and K2 in Equation (2.4) were fitted to the calculated values of energy loss and time of flight to reproduce observed nuclear charges, Z. Mass numbers were cal— culated using Equation (2.1). By performing the calibration for one Bp setting for each primary beam we can extract a set of calibration coefficients which allowed us to present all particle identification spectra in nuclear charge versus neutron excess (N — Z) as shown in the right panel of Figure 3.5. Fragment yields were extracted from calibrated particle identification spectra (ex- ample shown in the right panel of Figure 3.5) by counting events corresponding to 50 3000c- r f a * E’ A — g 1,2000— 5’ “c’ a C _ (U c _ ' 'b 0 '11111-.___-....-.-_. V _ 700 750 UJ ToF(channeB) <1 _ 1000— _ / .i' “ _ lgzo ' 1 1 1 l 1 i 200 800 400 600 ToF (channels) Figure 3.6: Uncalibrated particle identification spectrum of fragments created in re- action 86Kr+181Ta at Bp=2.07 Tm. The inset shows zoomed area around the missing 8Be. individual isotopes. This procedure was applied to all magnetic rigidity settings and all beam-target combinations. 3.3.2 RIKEN experiment At 64 MeV/ u significant fractions of 86Kr are not fully stripped of electrons after traversing the target. The charge state distribution of 86Kr showed that approxi- 51 mately 10% of the primary beam intensity of 86Kr is in 86Kr35+ charge state after passing through the 9Be target. The fraction is even larger in the case of 181Ta target because the charge state distribution for 86Kr primary beam is broader as discussed in Section 3.1. In this case we had to use the general Bp-ToF-AE—TK E particle identification method. This technique allowed us to identify the momentum, p, mass number, A, nuclear charge, Z, and charge state, Q, for every fragment of interest by measuring four quantities the magnetic rigidity, Bp, time of flight, ToF, energy loss, AE, and total kinetic energy, TK E. Basic particle identification plot and calibration procedure is the same as in the case of the N SCL experiments. Figure 3.6 shows an uncalibrated particle identification plot, AE versus ToF, for a 2.07 Tm magnetic rigidity setting using 86Kr+181Ta reaction. The inset shows the zoomed area around the 8Be hole. (The time of flight measurement was set up such that the more neutron-rich fragments are located to the right of the N — Z = 0 band.) From that reference point we can identify all isotopes in the PID spectrum. The T 0F and AE detector calibrations were done following the same procedure used for the NSCL reaction systems. In the case of 86Kr beam isotopes of all elements in the analysis (Z 2 25) come in multiple charge states. The measurement of the total kinetic energy, T K E , must be used to provide clean separation. The analysis procedure is demonstrated in Figure 3.7 for the 86Kr+9Be reaction run with 2.07 Tm magnetic rigidity setting. The left panel of the Figure 3.7 shows the energy loss, AE, versus time of flight, ToF, PID spectrum with gates around elements with Z = 28, 31 and 34. The right panel of Figure 3.7 plots the charge state, Q versus A/ Q ratio plane for fragments with Z = 28, 31, and 34 gates from bottom to top, respectively. We see isotope charge states clearly separated for all elements showed. The counts of individual isotopes in different charge states for the differential cross section calculation were extracted from the charge state, Q, versus A /Q ratio plots. 52 8 8 A O 1’ CD a) -o-I (U c g ‘63 a) .1: 8 9’ (u 'él . 5 2000 ~ "I“ H. ' n '1 1 - ' 2h2 A Ratio NO 70 750 800 Sl'oF (channels) Figure 3.7: Particle identification spectrum for a 2.07 Tm magnetic rigidity setting for the 86Kr+9Be reaction. Left panel shows the PID with three gates around elements with Z = 28, 31 and 34. Right panel shows projections to charge state, Q, versus A/ Q ratio plane of events within the corresponding nuclear charge gates Z = 28, 31, 34 from bottom to top, respectively. 3.4 Cross section analysis 3.4.1 Differential cross sections Yield for a given fragment and the momentum acceptance of a magnetic rigidity setting are combined together with primary beam intensity determined by the cali- bration described in Section 3.2 to obtain the differential cross section, da/dp, using the following formula: do N(A, Z) - M, ——A,Z =—, ( ) I‘t'NA'dt'Ap'TLIVE dp (3.1) 53 where N (A, Z) Number of events for a fragment (A, Z), I Primary beam intensity during the run (s‘l), t Time duration of the run (8), N A Avogadro’s number (mol‘l), Mt Atomic number of target material (gmol‘l), d, Target thickness (gem—2), Ap Momentum acceptance (0.2% in dp/p), TLIVE Live time correction. By plotting the differential cross section, da/dp, as a function of momentum, p, for a given fragment a momentum distribution is obtained. The area of the momen- tum distribution, in this representation, is the fragment production cross section. The momentum distributions are discussed in more details in the following chapter. Here we focus our attention to the transmission correction factor evaluation and the momentum distribution fitting procedure. 3.4.2 Momentum distribution fitting procedure Momentum distributions of fragmentation products provide insight into the fragmen- tation reaction mechanism. Momentum distributions of fragments created in rela- tivistic heavy-ion fragmentation are well described by a Gaussian function [64]. It has been shown that the momentum distributions created in the intermediate energy (20—200 MeV/ u) fragmentation are asymmetric [65]. We can separate the momentum distribution into two half Gaussians joined at maximum, p0, the high momentum side created by a “pure” fragmentation mechanism characterized by GR and the low momentum side created by a “mixed” reaction mechanism described by UL [59]: da S - exp (-(p - ref/(20 )) for P S P0: f. _ _ (3.2) dp S - exp (-(p - IMP/(20%)) for P > P0, 54 where S is the normalization parameter. The experimental width parameters at, and 03, were extracted from the asym- metric experimental momentum distributions using fits with a modified Gaussian function of Equation (3.2). Such procedure, however, results in correlations among the three parameters pg, 01,, and JR characterizing the shape of the fragment mo- mentum distributions. A minute change in the centroid, 190, will result in different division of the total width of the momentum distribution between the UL and 03. These complications, however, do not affect the determination of the fragmentation production cross sections. 3.4.3 Evaluation of fragmentation production cross section The fragmentation production cross section is obtained by integrating the area of the momentum distribution and divided by the transmission coefficient, 5. This trans- mission correction factor, 5, expresses the efficiency correction of our measurement is addressed in more details in the following section. The momentum distributions for fragments in our analysis were obtained by a systematic scan of the magnetic rigidity space as described in Sections 2.3.4 and 2.4.4 for the A1900 and RIPS fragment sep- arator experiments. Most of the experimental momentum differential cross sections extend over 1—4 orders of magnitude (typical examples shown in Figure 4.1). Since the range in the Bp space covered in our experiments is limited, there are fragments in our data with parts of the momentum distribution (either low or high momentum side) not measured. Figure 3.8 shows examples of complete and incomplete momen- tum distributions of 50V and 50Mn fragments, respectively, produced in 64Ni-l-S’Be fragmentation reactions. The complete momentum distributions were fitted by a modified Gaussian func— tion of Equation (3.2) by minimization of X2 method. Most of the experimental mo- mentum distributions were described extremely well over almost three orders of mag- nitude (e. g., see 50V in Figure 3.8). Overall approximately 60—80% (depending on 55 E , f . - r a fi , ......... , ...... 2: 104.- a 10 g” ‘ : ’7: ‘ ”79105:— 1 133103? ‘ .03 E E (D : : E 0) ‘ 2104 210'“? a V [5— ‘E V E 5 b'8 E i b|g ’ 50 i U 105: 50V —: D 107;F Mn 1 C l 8;. 1 106—111 IIIIIIIII 1 IIIII 10:1111 IIIIIIIII 1IIIIL+‘ 24000 26000 24000 26000 p|| (MeV/c) pll (MeV/c) Figure 3.8: Momentum distributions of 50V and 50Mn created in fragmentation of 64Ni on 9Be target. Shown fragments demonstrate a typical complete (50V) and incomplete (5°Mn) momentum distributions. Experimental data are depicted by open squares and the fit by modified Gaussian function of Equation (3.2) is shown as a solid line. the reaction system) of the fragments have complete momentum distributions. The above described fitting procedure was not suitable for the distributions with very few experimental points (< 5), or for distributions with missing left or right sides. In order to extract fragmentation production cross sections for these fragments and to minimize the uncertainties we took advantage of the larger set of completely measured momentum distributions. This set of data provided a data base for empirical systematics of the three parameters of the modified Gaussian function (Equation (3.2)): the matching point 190, the variances of the left, 01,, and the right, 03, sides. The analysis procedure for extraction of the production fragmentation cross sections for the fragments with incompletely measured momentum distributions is demonstrated in the case of fragmentation of 64Ni on a 9Be target. In the top left panel of Figure 3.9, the fitting parameter p0 is plotted as a function of fragment mass number. All p0 determined by fitting of the experimental data with the modified Gaussian function were within 1% of the polynomial systematics shown as a solid line. This parameterization assumes that pa for a given mass number are identical, but the experimental data show deviations. In order to get an even 56 ‘li ' l j l l I 30000? _ 25800} -: a . l G 3 1 S _ S 25700 f j § 20000 e — g : : Q0 I (1025600 j j r - _ _ . 1 1 1 . 1 . 1 . 1 ‘ : 1 1 I 4 41 1 1 ‘ 10 20 30 40 50 60 2 4 6 8 1o Fragment mass A Neutron excess N-Z 1. 20 Asa-Pr 0R (MeV/c) l C) O 1 20 Ala-A: ‘40 Figure 3.9: Systematics of the centroids and variances of the momentum distributions for the reaction 64Ni+9Be. Upper row shows the centroids, p0; the variances, UL and 03, are shown in the bottom row. 57 more precise prescription for the centroids we fitted individual isobar chains as a function of neutron excess to get a “local” parameterization. The top right panel of Figure 3.9 shows a linear fit for the centroids of isobars A = 50. In this local systematics for the centroids it was shown that the experimental data were within 0.5% of the parameterization line. The variances 0L and on are modeled by the Goldhaber model [64] UL 2: 0’0 AP—l , (3 3) a A(Ap—A) ° where Ap and A are atomic mass of the projectile and fragment respectively and 06‘, 062 are used as fitting parameters. Best fits to the experimentally determined 0;, and 03 are shown in bottom panels of. Figure 3.9. To describe the experimental data 06’ = 140 and 06* = 94 MeV/c were used. The incomplete momentum distributions were fitted with Equation (3.2) by having the centroid p0 and variances 0L and an to range over values centered at values determined by the parameterization. The size of the interval over which the parameters were allowed to vary was :|:1% for the centroid p0 and :l:13% for the variances 0L and 03. Figure 3.8 shows examples of complete and incomplete momentum distributions (open squares) along with the fitted modified Gaussian function of Equation (3.2) (solid line) for fragments 50V and 50Mn respectively. All fragmentation cross sections Ufrag were calculated using Ufrag=S'\/Z2T:'(0'L+0’R), (3.4) where S, 0L and 03 are parameters of the modified Gaussian function (3.2). The calcu- lation of fragmentation production cross section, afrag, in Equation (3.4) is equivalent to integrating Equation (3.2) over all momenta, p. In the case of 86Kr primary beam the total fragment yield is distributed over different charge states. For 86Kr+9Be reaction system we analyzed fragments with 58 Z — Q = 0 charge states. Since the measured charge state distribution for the pri- mary beam is reproduced well for the 86Kr+gBe reaction, we use the GLOBAL code to correct the final cross sections for all fragments in the analysis. The calculated corrections vary between 1—9% for isotopes of 25 S Z S 36 elements. For the 86Kr+181Ta reaction we analyzed the three most abundant charge states (Z -— Q = 0, 1, 2) in order to harvest most of the total cross section. The sum of these three charge states for the 86Kr beam on 181Ta target is reproduced very well by the GLOBAL calculation. The corrections for fragment cross sections in the 86Kr+181Ta analysis, calculated by GLOBAL, vary between 0.1—3% for isotopes of 25 S Z S 36 elements. 3.4.4 Transmission correction evaluation The transmission correction, 5, is essentially a correction factor for the experimen- tal acceptance. It accounts for finite spatial and angular acceptance and transport efliciency of the fragment separator used in the experimental measurement. The dif- ficulties in the evaluation of this correction arise from our inability to measure the essential ingredients. The transmission correction can be factored into momentum (longitudinal direction) and angular (transverse direction) transmission. The coor— dinate system used in our calculations and in further discussion is shown in Figure 3.10, where ‘z’ is in beam direction and ‘x’, ‘y’ are in plane perpendicular to the beam direction. Figure 3.10: Coordinate system used in ion optical calculations of transmission cor- rection. Where ‘Z’ is in the primary beam direction and ‘X’ and ‘Y’ are in the per- pendicular plane. 59 In Section 2.1 we discussed that the narrow momentum acceptance implies sim- pler evaluation of the transmission. To evaluate the momentum acceptance (0.2% in dp/p) effect we used a universal Monte Carlo code for the transport of heavy ions through matter within ion-optical systems MOCADI [66]. All MOCADI simulations were carried out in the first order. The momentum acceptance was defined as a ratio of the number of particles transported to the focal plane of the fragment separator and the number of particles within i0.1% of the central momentum in the target plane. This ratio is calculated only for particle transmitted in angular space with 100% probability, in order to ensure the separation of the angular and momentum transmission corrections. The momentum acceptance in our definition is independent of the magnetic rigidity setting and the fragment mass and charge. The momentum transmission in our case expresses the “effectiveness” of the momentum cut made by the slit system at the intermediate image of the fragment separator. The calculated momentum transmission is 962t2% and 98:1:2% for the A1900 and RIPS fragment sep- arators, respectively. The uncertainties in the momentum slit opening and angular cut contribute to the final error of the momentum transmission. Calculated angular acceptance areas, used in the evaluation of the momentum transmission, of the two fragment separators are presented in Figure 3.11. Calculations were carried out using the primary beam (“Ni and 86Kr for the A1900 and RIPS respectively) with unreal- istically large uniform angular distribution (i80 mrad) in both the 0 (‘x’ direction) and o (‘y’ direction) angles. Simulated plots in Figure 3.11 show the projection of particles transported to the focal plane of the fragment separator to the 45 versus 0 plane at the target position, thus defining the angular acceptance of the A1900 (left panel of Figure 3.11) and RIPS (right panel of Figure 3.11) fragment separators in our MOCADI simulations. For evaluation of the angular transmission we used LISE++ [56]. This code uses analytical methods to calculate the transport of particles through the fragment sepa- rator, which enable it to execute the calculations much faster than Monte Carlo based 60 50 IjfiTj—TrIIII -1 (p (mrad) 50 I I rj I ‘ 100 Figure 3.11: Angular acceptance of the A1900 (left panel) and RIPS (right panel) fragment separators as calculated by MOCADI in 43 versus 0 plane. Dashed-dotted line denotes the input angular distribution (:l:80 mrad) for the MOCADI calculation. MOCADI. A benchmark calculation was performed for the 58Ni+QBe system using identical parameterization for the angular distributions of fragments and the primary beam emittance. In Figure 3.12 we show the calculated angular transmission curves using LISE++ and MOCADI codes. Since the overall agreement is better than 4%, we use LISE++ for all angular transmission calculations. The angular transmission was defined as a portion of the angular distribution, for a given fragment, transported to the focal plane of the fragment separator. Results of the angular transmission calculations depend on the primary beam emittance and the fragment angular distribution assumptions. Since the experimental data for these two components are scarce, we had to rely on parameterizations and estimates. The emittance of the primary beam is given by its spatial and angular distribu- tions. The primary beam spot size in the target plane is approximately 2 x 2mm2, which gives us an estimate of the spatial component. The direct measurement of the angular distribution of the primary beam in the target plane is not available. We did get an estimate of the primary beam angular distribution for the 58Ni primary beam by measuring the position and angular distributions at the intermediate image 61 (I2) of the A1900 fragment separator. In order to reproduce the measured distribu- tions using MOCADI Gaussian profiles of the primary beam with variances in spatial ox 2 ay 2 1mm and angular 0'9 2 0,5 = 7mrad coordinates were used. Another estimate of the primary beam emittance can be inferred from the calcu- lations for all beams delivered by the K1200 cyclotron in stand alone mode during the 1990S as shown in Figure 3.13. Most of the beam emittances fall within approx- imately 147r - mm - mrad in both ‘x’ and ‘y’ directions. If we assume the normalized emittance of the K1200 and the CCF is the same we can take the K1200 stand alone mode emittance as an upper limit of the CCF primary beam emittance. This result is consistent with the above mentioned conclusions of MOCADI calculation. In the case of the RIKEN fragmentation experiment we had to rely solely on ion optical calculations [68]. The primary beam spot size in the target plane for the RIKEN experiment was approximately 1 x 1mm2. The ion optics calculations suggest that the emittance of the primary beam is roughly half of that at the CCF, resulting in 0'9 = 043 = 3mrad [68]. C .9 *6 “5 0 0 " .1 0-4 ,- —LISE++ 4 o. ' ----- MOCADI 1 l 1 1 l L d 20 40 Fragment mass number A Figure 3.12: Comparison of angular transmission calculated using the LISE++ and MOCADI codes. Parameterization of Equation (3.5) for the variance of the transverse momentum distribution is used in both cases with 00 = 100 MeV/c and Up = 200 MeV/c. Primary beam emittance parameters: a_.c = 0,, = 1 mm and 09 = 0,), = 7 mrad. 62 on O) IIIILLIAII A I theta (mrad) phi (mrad) ‘3. .. 1?. . . . I N llllll Figure 3.13: Calculations of the primary beam emittance ellipses in 0 versus a: phase space (left panel) and d) versus y phase space (right panel) for all primary beams delivered by the K1200 cyclotron in stand alone mode during the 19903 [67]. 63 I I I I I 1 I I TI 1 l T fl T I I I I I—l j I —1 — f j - 86Kr+Be 08* ~ ' c c l— .9 : ‘ .9 °"’. *6 I ‘6' m a) t0.6— _ t o - 006 _ 0 c.9453 mrad 0 ‘ i, . ........ *°e¢=1mrad . 0 4 __ ce¢=7 mrad j g7 __ cs9 1,=3 mrad ' _ . ..... cew=10mrad . ~ ..... °e¢=7 mrad ~ —..09¢=15 mrad + 0.4K _..c”=10 mrad - “11111111111111.1‘ .IIL1IIIIIIPII1II‘ 20 30 40 50 60 70 80 Fragment mass number A Fragment mass number A Figure 3.14: Angular transmission correction as a function of fragment mass number for the A1900 (48Ca+9Be) and RIPS (86Kr+9Be) fragment separators. The calcula- tions were done for different values of variances of the Gaussian profile of the primary beam angular distributions, aw. All calculations used Equation (3.5) to evaluate the variance of the transverse momentum distribution 01 with 00 = 100 MeV/c, (ID = 200 MeV/c. Due to uncertainties associated with the determination of the primary beam an- gular distributions we carried out a series of angular correction calculations using the LISE++ code. The primary beam angular distributions were simulated with Gaussian profiles, in all cases, varying the variances, “M: in ‘x’ and ‘y’ directions respectively. The LISE++ calculations in Figure 3.14 for the A1900 fragment separator were done using the 48Ca+gBe reaction with 09",, = 3, 7, 10 and 15 mrad, and for the RIPS fragment separator using the 86Kr+9Be reaction with a“, = l, 3, 7 and 10 mrad. We see that the uncertainty of the 09,4, is within i3 mrad of the above estimated values of 7 and 3 mrad for the A1900 and RIPS, respectively, resulting in the angular trans- mission correction uncertainty at the level of approximately 3—4%. For all angular transmission calculations we used 0'9 = ad, = 7mrad, 0x = a3, = 1mm for the NSCL primary beams and 09 = 04, = 3mrad, 0x 2 0y = 0.75 mm for the RIKEN primary beam as explained above. Experimentally the longitudinal and the transverse components of momentum 64 distributions after fragmentation are equal (within 10%) in relativistic energy regime [69]. However, this is not the case at intermediate energies [70,71]. Since there are no published experimental data available for the angular distributions of the fragments measured in our fragmentation experiments, we relied on the parameterization [70] of the variance of the transverse momentum distributions, 0 13 Amp —— A) A(A — 1) 2 = 2 2 “i ”0 Ap —— 1 + ”DAP(AP -— 1) (3.5) where A p and A are mass numbers of the projectile and fragment, respectively. The first term in Equation (3.5) comes from the Goldhaber model for the width of a longitudinal momentum distribution and the second term results from the deflection of the projectile by the target nucleus [71]. 00 can be determined by our measurement of the longitudinal momentum distributions and an, the orbital dispersion, is taken from Ref. [70], where it is shown to describe the fragmentation data of 16O at 92.5 and 117.5 MeV/ u. From the published data [70] we estimated the orbital dispersion parameter an to be 185 i 15 MeV/c for the NSCL experiments and 225 :l: 25 MeV/c for the RIKEN experiment. We were able to confirm the parameterization of Equation (3.5) with the fragment angular distributions measured in a separate experiment at the N SCL using the S800 spectrograph [73]. Figure 3.15 shows the angular distributions for 44Ca and 59Co fragments produced in the 64Ni+QBe reaction at 95 MeV/u. The angular distributions were reconstructed with the inverse mapping technique using the S800 spectrograph for the reaction 64Ni+9Be [72]. Based on the measured angular distributions in the ‘x’ direction, we calculated the variance of the transverse angular distribution al(0) = 33 :l: 2 and 16 :l: 1 mrad for 44Ca and 59Co fragments, respectively. These transverse angular distributions translate to transverse momentum widths a; = 482 i 29 and 319 :i: 20 MeV/c for 44Ca and 59Co, while the parameterization (Equation (3.5)) predicts ai = 446 :l: 12 and 301 :l: 12 MeV/c. There is a good agreement between the 65 1oq'rlr 1 1OOA-fi‘fifiTT'l""l“— * 44C ‘ . 59 . a Co A A ~ I =5 =2 L 53 £3 a) i a) *" _ _ ~500— - S 50 S 0 ~ 0 f o o I I L I I I I I l I I I I l I I l I I m l I 0 -100 0 100 0 -100 0 100 0 (mrad) 9 (mrad) Figure 3.15: Angular distributions in the target plane for 4"‘Ca and 59Co fragments from the 64Ni+9Be reaction, measured at the NSCL using the S800 spectrograph from Ref. [72]. See text for details. measured width and predictions from Equation (3.5). As was discussed in the previous sections, the experimental momentum distri- bution for our fragmentation data are described by a modified Gaussian function of Equation (3.2) characterized by two width parameters 0;, and 03. For the purposes of the angular transmission calculations we approximated the momentum distributions with equivalent Gaussian shape. The actual ao parameter was determined by fitting (01, + 03) / 2 from the experimental data. Values of co and 01) parameters used in our transmission correction calculations are listed in Table 3.1. Final transmission corrections were obtained as a product of the momentum and the angular transmission corrections. The calculated transmission correction, s, is plotted as a function of fragment mass number in Figures 3.16 and 3.17 for all reac- tion systems. Figure 3.16 displays the mass dependence of the transmission correction for the 40Ca (t0p panels), 48Ca (bottom panels) beams with 9Be (left panels), 181Ta (right panels) targets. The transmission correction for the 58Ni (top panels), 6“Ni (middle panels), and 86Kr (bottom panels) beams with 9Be (left panels), 181Ta (right panels) targets are showed in Figure 3.17. The shaded region denotes the estimated 66 ion t ISSIOn COFTGC o '00 .0 a: 9 .5 .0 Transm N ‘ ‘ ' It... ,,r:.; I . . I 1. 1-4... 1 l I I I l 1 A; ...... 11:15] ' 14 ills .C: llnggLJ_1_lIl l l 1 I l 1 1 1 I L I l l l 20 30 4o Fragment mass number A 1 r r r I r 771' I l i I I I j—fi I’m _ ,‘3‘12‘17? _, C r~~ ' _ 48 9 ...::;;~;»~ . o a+ e o— . .Lxfi‘I-U‘ +- * size” ‘ o 3 — LL — .3367” (D ' _ « h '- WW ‘1 o a” .255? 0 6~ - — C - .rfsféii‘ )- ‘.:‘el:i" * o I n: o- "‘ 41%;?” -—1 a) e" I. ”gr-4 .4 J '3 -- 0 4— a e ' i111? f as ‘ U) *' riff”! ‘ C l- .25 ‘ ‘ '1 cu a a h 0.2 if? F “ _, )- '4 1 1 1 l 1 1 1 1 l 1 1 1 1 L L a 20 30 4o Fragment mass number A .0 on .o .o O) , a .0 Transmission correction N . . r w, -1 [A L l l I l 1 1 l 30 Fragment mass number I. 40 A 1 l l ' l ’ _.-:—::-:-:-' C. I _._.-:-:':'::::::. lllll g :“Ca-i-‘a’Ta ,- 0 _ _. G) 0.8_ : d t _ - C 0.6: .. “ .9 - . -- 0.41- a? a E r .5" 1 (D r j C L ~ 1 g 0.2:? a _II.1II..111111111: 20 30 40 Fragment mass number A Figure 3.16: Final transmission correction, c, as a function of fragment mass number for 40"“BCa primary beams with 9Be and 181Ta obtained for 0.2% momentum accep- tance. 67 .0 on .0 O: l 5%, .0 .p 1 , x'r' :fr’ Transmission correction .0 AL 1 l I I l l I I I I I _ .' "1T 1111 1 I l l + | IIL+1J_14_1111 .T o 'ILJLI 20 30 40 50 Fragment mass number A 1 I l T l l I I T» ‘ 1:??? ."I:15?i:.:~::“_.-,-,II. 4 8 I “Ni 936 i ._ _ + a g 0.82 g _1 L- P ‘l ., h ’- _;Cv.;+' 4 o j F iii... 4 U 0 6* . fl C . T w ‘4 .9 . g 4 ~ift‘v“ o— 0.4—- 9:325" a E ” .515 A fir; U) - $.43 ‘ C F :1, as i .5 A h 0 1— ' 5 4 i I , l l l 20 40 Fragment mass number A 60 .0 b "1. ”it .° N rrth‘ L l L I r r v r t r 1 T T r l I m l l l c b 86 9 ,.v:?'. ””1" I I _Q . Kr+ Be — 8 0 8V 5” _] - F -5 g I. “$.53” -< — .4” -t 5 8 f 55‘ I 0.6” ”3‘5"; 1 c ._ d1? .1 .9 r if ‘ .vz-‘i” (‘2 I 15‘”?- 4 (I) C (O p . l 1 l I I 1 I14 1 1 I I l I l l I o I 70 80 Fragment mass number A d ‘I r- .o .o .o O) on r 1 T I T IT Transmission correction 0 ix: L l k - 5’Ni+’°’Ta T—I—II—II—I—TIIIITIIIIF' ..... IIII liIllllIllllIJlllILll O 20 30 40 50 Fragment mass number A .° p 1 Transmission correction O in P l l o L I l l l I 1 4O 60 I A I -q n I. Transmission correction I I I I I I I I I I Kr-I-1 "Ta 60 7O 80 Fragment mass number A Figure 3.17: Final transmission correction, e, as a function of fragment mass number for 5854Ni and 86Kr primary beams with 9Be and 181Ta targets obtained for 0.2% momentum acceptance. 68 Table 3.1: Values of 00 and 00 parameters in Equation (3.5) used in the angular transmission correction calculations for different primary beam target combinations. Beam Target 00 OD (MeV/c) (MeV/c) “oCa jBe 106 :l: 4 185 :l: 15 181Ta 101 j: 5 185 :l: 15 48Ca 9Be 109 :t 5 185 :l: 15 181Ta 106 :l: 4 185 :t 15 58Ni 7"Be 113 :l: 3 185 :l: 15 181Ta 110 i 3 185 :t 15 64Ni ”Be 117 :l: 5 185 :t 15 181Ta 114 :t 3 185 :l: 15 86Kr 9Be 147 :l: 5 225 :l: 25 181Ta 153 i 5 225 :l: 25 uncertainty in the transmission correction factors which are dominated by the un- certainty of the co parameter for light fragments and the uncertainty of momentum transmission for heavy fragments. 3.5 Error analysis The uncertainties in the final fragmentation cross section are calculated based on the statistical uncertainty, the beam intensity calibration (7—10%), the error calculated by the fitting procedure and the transmission uncertainty (2—8%). Uncertainty of the target thickness determination and momentum slit opening were estimated to be smaller than 2% and hence they are neglected in our error analysis. Other uncertain- ties like reaction losses in detectors and secondary reactions in the target material are smaller than 1% so their contributions are also neglected. For the fragments mea- sured with incomplete momentum distributions, a systematic error stemming from the extrapolation of the parameterization of pg, 0;, and 0’}; were included in addition to the above mentioned errors. 69 Chapter 4 Experimental results This chapter presents the experimental data extracted from the four projectile frag- mentation experiments carried out at the NSCL and one at RIKEN. The fundamental parameters, such as the shape, maximum, and width of the momentum distributions of fragments are presented. The production fragmentation cross sections are com- pared to the empirical parameterization of the fragmentation cross sections and to published experimental data measured at higher incident energies. The target and projectile dependence of the measured fragmentation cross sections is discussed at the end of the chapter. 4.1 Momentum distributions More quantitative discussion of the properties of the fragment momentum distribu- tions is provided in this section. The obtained fragment momentum distributions are compared to high energy systematics. The basic properties of the momentum distributions of the target or projectile fragmentation products were the subject of many studies [65, 74—76]. In general, the momentum distributions created in the fragmentation of relativistic projectiles are very well represented by Gaussian functions centered at velocities near that of the 70 10'2E 10:3 o o 93 nSWfi £10“i Em : v E MO4E Sl‘1 " 8|Cl : U10-5E U _ i 106: . 10": .5 104%" l l' “g I: : 10-2 A1025 j A o : o g 5 133103 E c0103: 5 E a: 2 i 2 o. 4’ . 0.0.4 Slum a 8'0 _ : 1o~5 10.5? Ea Figure 4.1: Examples of the momentum distributions. Distributions of 33P are shown for the 40"’E’Ca+9Be and 49V for the 58"5“Ni+9Be systems. The solid line shows the modified Gaussian fit of Equation (4.1), and the dashed line shows the Gaussian fit to the high momentum side of the distribution. projectile [75]. Many models and parameterizations have been developed to describe the variance (width) and the maximum of the Gaussian-shaped momentum distribu- tions [64,77]. On the other hand, the fragment momentum distributions at intermediate ener- gies are asymmetric [65]; they can no longer be approximated with a Gaussian func- tion. Different functions have been used to reproduce the asymmetric momentum distributions: Gaussian functions with a different cutoff [76], polynomial exponential 71 functions [65], or Gaussian functions with exponential tails [74]. Our experimental data are very well described by a modified Gaussian functions of Equation (4.1). To demonstrate the asymmetry of the measured momentum distributions, we show the momentum distributions of 33P from the fragmentation of 40"‘8Ca on a 9Be target and 49V from the 58"’4Ni+9Be reaction in Figure 4.1. The solid lines in Figure 4.1 show the fits using the following modified Gaussian function: do _ S - exp (-(p - POP/(20%)) for p S 100, Zp‘ _ (4.1) S - exp (-(P — 1902/9030) for P > no, where S is the normalization factor, p0 is the centroid, 0L and 03 are width parameters of the “left” and “right” halves of the momentum distribution. The modified Gaussian function describes the experimental data over 3—4 orders of magnitude extremely well. On the contrary, the fit to the high momentum side of the momentum distribution by a symmetric Gaussian function (dashed curve) shows a large deficit in the low momentum part of the distribution. 4.1.1 Widths of the momentum distributions At high bombarding energy (> 200 MeV/ u), the momentum distributions were found to be independent of the bombarding energy and the target. The observed widths of the experimental momentum distributions showed a parabolic dependence on frag- ment mass number. The Goldhaber model [64] developed in 1974 considers the pro jec- tile nucleus, as being composed of independent nucleons moving freely in a spherical potential well. The only correlations assumed in the model are those arising from momentum conservation. The momentum distribution of the nucleons results in to- tal momentum zero at any time in the rest frame of the projectile nucleus. When a certain number of nucleons are removed suddenly from the projectile, the remaining residual fragment recoils in the opposite direction with the same momentum magni- 72 tude. Furthermore, assuming the isotropy of the internal momenta of the nucleons in the projectile, the recoil momentum distribution of the fragment, projected on a given axis, has a Gaussian shape. Goldhaber showed that the width 0 of this distribution is related to the the masses of fragment, A, and projectile, A p [64]: Amp — A) AP _1 , (4.2) 0:00 where the 00 parameter, also called reduced width, is the root-mean—squared momen- tum of the individual nucleons, < pf >: (4.3) Assuming the nucleons are distributed in the projectile according to the Fermi gas model we have: 3 < 1)? >2 gpf’ermit (4'4) where ppermi is the Fermi momentum. By inserting this equality into Equation (4.3), we get: 2 03 = 3%5. (4.5) Hence, the Goldhaber model relates the Fermi motion of the nucleons to the reduced widths of the fragment momentum distributions in the projectile fragmentation reac- tions. The Fermi momentum, ppermi, can be experimentally obtained from quasielastic electron scattering. Moniz et al. [78] measured values of the Fermi momentum for tar- get materials varying from 11Li to 208Pb. By interpolating their results, we estimated the pperm, to be approximately 250 MeV/c for 40"‘8Ca and 260 MeV/c for 58MN i and 86Kr nuclei. Calculated reduced width, 00, parameters are listed in Table 4.1 along with the values determined from our experimental data. In 1989 Morrissey [77], assuming that the momentum distribution is a convolution 73 Table 4.1: Goldhaber reduced width parameter, 06‘, for the right side and 06‘, for the left side of the experimental momentum distributions values listed for all investigated reaction systems. Last column shows the reduced width parameter of the Goldhaber model calculated based on the Fermi momentum from Ref. [78]. Beam Target 05 of Fermi mom. material material (MeV/c) (MeV/c) (MeV/c) 40 Ca gBe 125:1:10 851:8 112 181Ta ll7:I:9 84:}:7 112 48Ca 9Be 134i9 893:6 112 181Ta 129:}:9 88i7 112 58M gBe 133i10 9755 116 1"lTa 129i10 95i6 116 “Ni 9Be 1403:? 94:l:5 116 lBlTa. 136i7 93i4 116 86m jBe 175:1:11 12118 116 l81Ta 181:1:9 119i10 116 of the primary (fast) process and the subsequent sequential decay, and introduced a modified systematics for widths of the momentum distribution based on compilation of high energy fragmentation data (projectile and target). The momentum width in his systematics is given by: a = const- Ap — A, (4.6) where the constant is a parameter of the systematics, usually taken as 85—100 MeV/c. In the case of projectile fragmentation, it describes only the projectile-like fragments, while light fragment widths are generally overestimated, which can be already seen in the original paper. Asymmetric momentum distributions in the intermediate energy regime are ex- plained by existence of different, competing reaction mechanisms. Apart from the “pure” fragmentation component, which completely dominates at relativistic ener- gies, both the low momentum tail typical for more dissipative processes [79] and the broadening associated with nucleon pick-up reactions in the prefragment formation 74 phase [80] will influence the shape of the final momentum distribution. There are two width parameters 0'}; and (IL in the description of our experimental momentum distributions for the left and right halves of the momentum distribution. Guided by the previous works [59,80,81] we assume the right side of the momentum distribution, 03, is completely dominated by the “pure” fragmentation reaction mechanism and the left side, 01,, has more significant contributions from different reaction processes so it is not expected to behave according to the fragmentation parameterizations. The extracted width parameters of the right side of the momentum distribution, 03, are shown as a function of fragment mass loss with respect to that of the pro- jectile, Ap — A, in Figure 4.2 for 40*""Ca, 58""‘Ni projectiles and in the top panels of Figure 4.4 for the 86Kr beam. The experimental data do not follow monotonically- increasing trend with the number of removed nucleons from the projectile as suggested by the Morrissey systematics plotted as dashed curves in Figure 4.2, with const = 85 MeV/c. The disagreement with the experimental data is not surprising as this sys- tematics based mainly on target residues from light-ion induced reactions was not meant for extrapolation towards large mass differences between the projectile and reaction products (Ap — A > Ap / 2). The solid line shows the fit by Goldhaber for- mula (Equation (4.2)), where the reduced width, 00, is taken as a fitting parameter. The values of the Goldhaber reduced width parameter, 062, for the right side of the experimental momentum distributions, are listed in Table 4.1. For a given projectile the of,” does not depend on the target material, but increases slightly with the mass of the projectile. The values of the of extracted in our fragmentation experiment should be compared to the values calculated from the Fermi momenta measured with quasielastic electron scattering, listed also in Table 4.1. We see that the experimental values from this study are lower than the Fermi momentum values for all investigated beam-target combinations. This is in line with other intermediate and relativistic en- ergy fragmentation experiments when similar discrepancies have been reported [75]. To explain the difference, Bertsch [82] in his model used Pauli correlations. Weber et 75 — I T. 1" I I fi'fF—‘F ' I I 400 — .25 __ : ' O o o '9¢ . . .I... . O o . r '6 O. 0 O 98-.«1: GOO . O 01 - Q {6. 0;.’£:.. '0'.‘ --- 9 .. -l 3°07 0 .2?" ' ° °o 1 : 43'. 000 _g°:;= ' : 200 L :03 21;? _‘ I ~-' 40 9 c 48 9 i ’5 3 -- Ca+ Be ~» Ca+ Be 3 S 100 .. .5 1 a: .' ~ 2 I . I v __ 'x o, S m 400 7 .0" o'A é @ d b : '0' Q) 0 .0 (30...; j I. 0' o O 0.554.: O 1 300 _ O f O .' _:l' I" O .. '_' O :7?" O : . , 00 0:; " j 200 f O 333’ I, - 4O 18 48 18 - 0 Ca+ ’Ta } Ca+ ’Ta 3 100 : t‘ - L I I I l I I I I I I I I I l u 1 1 1 1 I 1 1 1 1 I 1 14 L I 1 1_1 1" O 30 I I ‘ ‘ ,. é o .1 .. «’9‘- . f; 0 400 .“'- _>‘4.(“.°D. “ d A . " _ (\3 200 > I d) t 2 v (I b 400 - 200 ‘ 0 Figure 4.2: Width of the right side of the momentum distribution 03 for the 40"’E’Ca and 58""‘N i primary beams on 9Be and 181Ta targets, plotted as a function of number of removed nucleons, Ap — A. The solid line shows the fit by Goldhaber formula with reduced width, of, listed in Table 4.1 and the dashed line depicts the Morrissey systematics with const=85 MeV/c. 76 al. [75], alternatively, attributed the discrepancy to the mass loss in the evaporation stage which is not included in the Goldhaber model. The values of the of parameters for fragments created in fragmentation of 86Kr at 64 MeV/ u are larger (z 120 MeV/c) than the ones obtained for the NSCL reaction systems (% 90 MeV/c). However, no correlation with target material used in the projectile fragmentation reactions at different beam energies (64 and 140 MeV/ u) has been observed, which is consistent with the conclusions of other similar investigations [59]. The width parameters of the left side of the momentum distribution, 01,, are shown as a function of the mass loss of the fragment with respect to that of the projectile, Ap — A, in Figure 4.3 for the 40"“‘Ca, 58"‘4Ni projectiles and in the bottom panels of Figure 4.4 for the 86Kr beam. The solid line through the data points represents a fit by the Goldhaber formula (Equation (4.2)), however this fit is just formal, because we do not expect any fragmentation formula to apply for the left side of the momentum distribution, 01, (as discussed above). Nevertheless, it is clear that the experimental widths, 01,, can be roughly described by the Goldhaber model using larger values for the reduced width parameter 03‘, as listed in Table 4.1. 4.1.2 Centroids of the momentum distributions Fragment velocity after the reaction is a rather important experimental observable bearing information about the first stage of the reaction — abrasion. The observed momentum distributions peak near the beam velocity. Small deviations in fragment mean velocities, Up, from the projectile velocity, ’01), can be seen in a simple picture as the friction phenomenon during nuclear reaction. Nuclear bonds are broken during the nucleon removal process, causing the projectile to slow down. In a model introduced 77 600—- —— o — L o I Cb. _ O .J ¢¢ 00 o O 400— 00630 —- - I I o r _ G 200— 40Ca+9Be -- 48Ca+9Be - \ - . a . r 2600—4 'rkkr'A‘Akfij Tf+1jltr .,::‘fid b" b 0 O " __ 9830(1) ¢ (:3 q 400 _ Q30 ‘ 200 -- 48Ca+"3'Ta j 1 l I 1 " I l I l l 4 0 1o 20 30 1o 20 30 AP'AF P I l J v I r f '7 r 600- it _ 00 -. q, 4 : “”295 QDOCSJQO :: f) ‘ 400— j- ‘ a r 58Ni+9t3e . 64Ni+9I3e , S 200 - , g e § : f s % . g g I} ‘ _: : r é q.) I o 35’ r- o r -— 64Ni+“"Ta Id 14L 1 I l 20 4o AP'AF Figure 4.3: Width of the left side of the momentum distribution 0;, for the 40"‘8Ca and 58MM primary beams on 9Be and 181Ta targets. The solid line shows the fit by Goldhaber formula with reduced width, 06’, listed in Table 4.1. 78 400 (IR (MeV/c) 200 llirrlliilt .4 un- ‘- I1 I"4J1iil;1114‘ ' I "I 'l' I O’L (MeV/c) Figure 4.4: Width of the left 0;, and right 0’}; sides of the momentum distribution for the 86Kr primary beam on 9Be and 181Ta reaction targets. The solid line shows the fit by the Goldhaber formula with the reduced width listed in Table 4.1 and the dashed line depicts the Morrissey systematics with const=85 MeV/c. 79 by Borrel et al. [83] the fragment velocity deviation, vp/vp, is expressed as: It depends on the mass of the projectile Ap and fragment A plus the energy of the beam Ep in units of MeV/ u. The underlying idea is that the energy cost is 8 MeV per removed nucleon. Other models describe the change of velocity in terms of the momentum trans- fer. A number of models have been proposed to treat the small momentum transfer from the bombarding particle. They all assume a two-step mechanism and treat the result as a quasi two—body system. Although differing in details of the initial interac- tion, Cumming et al. [84] realized they all predict the same functional form for the momentum transfer in the projectile reference frame p“ as PM = A—fz' (1+k(1-/32)”2) (4.8) where 8 is the pro jectile’s velocity, AET is the energy transferred to the prefragment, and the parameter It sets the rate at which p” decreases as ,8 increases. Different - authors arrived at different values of the AET and 1: parameters studying various systems. Kaufman [85] compiled high energy fragmentation data (0.4—2.1 GeV/u) and described the data with parameters k = O and AET = 13 MeV/AA. On the other hand, Morrissey analyzed heavy residues from target spallation reactions [77] and obtained k = 1 and AET = 8 MeV/AA. Experimental fragment velocities, Up, are calculated by translating the po param- eter of the modified Gaussian fitting function (Equation (4.1)) for every fragment in our analysis into the velocity space. In order to make the cross system comparisons clear, we plot them as relative deviations from the projectile velocity (Up/Up — 1) in %, in Figure 4.5, 4.6 and 4.7 for the 40"180a, 58"MM and 86Kr beams, respectively. 80 Ifi I I r I I I I T I f I I I I I I l I I I I r I l I I I I I 0 ———————————— —] o———————————— .. .] L. o O - O . - A O A .\° - ¢ 0 < s ~ 0 v v 0000 000 £1 ' o 1 g: 0 ° 9 f ‘9. O O > ’ ‘ > ’ ° 40Ca+""'l'a * - ~ - “Barrel ] 4— --—-Kaufman — -4r --°-Kaufman J - , — Morrissey . ~ — Morrissey ] L 1 1:! L l l I I 1 L4 1 I l l 1 L141] 14 l L l J_1 1 1 1o 20 30 4o 10 20 30 4o Fragment A Fragment A T T I I I r I Ifi I I I I I I IT I I I I I I I I I I I I I I I fiI I I I Iii I o ————————————— 0————~—*——————fi . . . 4 .. . I. .J A O A o\° [ ¢ (f ¢ o\° - 0 é ‘- -2— — 1- -2— ~ . o , - é >‘L ' (pf ' ] >‘L ' “3% i :a - Mali ~ I :1 -<> ,go New. * “Barrel ~ - -4~ ""Kaufman — -4F “Kaufman - - — Morrissey . — Morrissey l l l l 1 J I 1 l 1 l l l l l l l l 1 l l l l l l l l 1 1+1 P 10 20 30 4o 50 1o 20 30 4o 50 Fragment A Fragment A Figure 4.5: Relative deviations from the projectile velocity (vp/vp — 1) for all frag- ments with complete momentum distributions identified in the fragmentation of 40"“‘Ca isotopes on 9Be and 181Ta targets plotted as a function of fragment mass number, A. Parameterizations of Borrel, Kaufman and Morrissey are shown as dot— ted, dashed and solid lines respectively. 81 I I T— I— I T I I I I I I I Ifi—I —I—I I I I I I I I I I T I I I rj I I I I I T I I o _____________ 0 _____________ ’0‘ _ ’o‘ _ E 90¢? o E o ‘TCL -2 0 o i To. -269 200%6 r > ' ‘ > " :0. _ _ :0- Q o - - Borrel ‘ ~ ‘ -4- ""Kaufman -‘ -4- ----Kaufman - - — Morrissey - [ —Morrissey « l 1 1 1 1 l 1 1 1 L l 1 1 1 L l 1 1 1 1 J + 1 1 1 l 1 L 1 L 20 30 4o 50 60 20 30 4o 50 60 Fragment A Fragment A I I I I I I I I I I I I I I I I I I I I T I I I I I I I I I I I I I I I I I I T I I I I I o ___________ o ___________ 4’41, " J 2\: ’- ¢¢¢ é - o .00 .. '7 “”88? '7 fiffoféé is r i * at ‘P <9 ‘ > - > ' ° “NH“"I’a ‘ - ~ ...... Borrel ] 4F -----Kaufman - -4L ----Kaufman a — — Morrissey - — Morrissey L1 J 1 1 l 1 1 111 1 1 1 1 1 L1 1 1 1 1 1:1 1 1 1 l 1 1 1 l l 1 1 1 1 I l 1 20 30 4o 50 6O 20 30 4o 50 60 Fragment A Fragment A Figure 4.6: Relative deviations from the projectile velocity (Up/Up — l) for all frag- ments with complete momentum distributions identified in the fragmentation of 58"5“Ni isotopes on 9Be and 181Ta targets plotted as a function of fragment mass num- ber, A. Parameterizations of Borrel, Kaufman and Morrissey are shown as dotted, dashed and solid lines respectively. 82 _[ o _____________ - , . .. .] h- -2— ~ -2— — A [- -« A - .4 o\0 . . °\° . ‘T 4* -— '7 -4- — >°’ _ >°' _ :a. - ‘cL . «3- — > -6~ — ~ - Borrel 4 ~ ”- Borrel - , Kaufman - Kaufman -8 ” ' — Morrissey ‘ -8 r ' — Morrissey ‘ .- I 1 1 1 I l 1 1_1 L 1 141_L J_L 1 4 1" ’- I l l 1 1 I 1 1 J_L AI_;1 1 1 I 1 1 _ 50 60 7o 80 so 60 7o 80 Fragment A Fragment A Figure 4.7: Relative deviations from the projectile velocity (vp/vp —— 1) for all frag- ments with complete momentum distributions identified in the fragmentation of 86Kr primary beam on 9Be and 181Ta targets plotted as a function of fragment mass num- ber, A. Parameterizations of Borrel, Kaufman and Morrissey are shown as dotted, dashed and solid lines respectively. The experimental data are shown as open circles and three calculations by models of Borrel, Kaufman and Morrissey are shown as dotted, dashed, and solid lines, respec- tively. The experimental fragment velocity deviations exhibit a considerable scatter making the comparisons with the above discussed parameterizations difficult. The studied projectile fragmentation data show no dependence of the fragment velocities on the projectile or the target material (within the scatter of the experimental data). The overall profile of the relative experimental velocity is similar for all the NSCL reaction systems (Figure 4.5 and 4.6) in that the velocities gradually decrease with the fragment mass number and saturate around -2%. All presented models predict a further decrease of the velocity with the mass number which is not experimentally observed. A clear deviation from the experimental data starts around 10 removed nucleons for the 40"“BCa beams and around 15 removed nucleons for the 58"MM beams. The Borrel parameterization predicts overall larger fragment velocities as compared to the systematics of Kaufman and Morrissey (see Figure 4.5 and 4.6). The Kauf- man and Morrissey systematics, on the other hand, produce similar predictions for 83 velocities of projectile-like fragments. However, it must be noted that all presented parameterizations fail to reproduce the overall shape of the data. For the 86Kr reaction systems, only the fragments with A > 50 have been ana- lyzed, so we only see an overall trend of the fragment velocities as a function of the fragment mass number, A in the vicinity of the projectile (Figure 4.7). In the case of 86Kr+9Be reaction the experimental data are scattered much more than the data of the NSCL reaction systems. This effect is most probably caused by a combination of much lower beam energy (64 MeV/ u) and rather thick target (z 100 mg/cm2). The Borrel systematics predicts larger fragment velocities than the data for most of the fragments (left panel of Figure 4.7). The Kaufman and Morrissey calculations inter— sect the experimental data points close to the projectile, but deviate significantly for fragments with mass numbers 55—60. In the case of the 86Kr+181Ta reaction (right panel of Figure 4.7) the number of complete momentum distributions extracted is much lower than for the 86Kr+9Be system, making the conclusions more difficult. Due to broader charge state distributions of the 86Kr beam on 181Ta target a smaller range in the magnetic rigidity was covered for the 86Kr+181Ta reaction. The momen- tum distributions of fragments in the vicinity of the projectile were not measured (or measured completely). This limitation does not allow us to compare the experi- mental fragment velocities in the vicinity of the projectile to the parameterizations. The experimental velocities for lighter fragments A a: 52—65 are, again, larger than predicted by Kuafman and Morrissey parameterizations. 4.2 Fragmentation production cross sections Fragmentation production cross sections were extracted according to the analysis procedure described in detail in Section 3.4.2. This section summarizes the results of the reaction cross section analysis for all studied systems. The vast majority of measured nuclides are classified as fragmentation reaction products. However, nucleon 84 exchange and pick-up isotopes have been also identified in our study. Table 4.2 lists the Table 4.2: Number of fragments and pick-up (including exchange) cross sections mea- sured for all reaction systems. Beam Target Number of fragments ] pick-up 9Be 100 11 4003' 181Ta 101 15 9Be 176 26 4803 181Ta 167 32 58 . 9Be 184 12 N1 181Ta 179 10 64 . 9Be 240 3 N1 ISITa 232 2 9Be 180 0 861“ T8%. 70 0 | Total: | 1629 ] 111 J number of measured fragmentation products and pick-up products (including nucleon exchange). The presented table should be understood as a summary of the nuclides identified in our study. It is by no means an exact summary of reaction mechanism for the various isotopes measured in our data. Because the experimental data were taken in separate experiments, scanning different parts of the magnetic rigidity spectrum, Table 4.2 only lists the number of cross sections we extracted. Figure 4.8—4.12 present an overall view of the fragment cross sections in the style of the nuclear chart for all investigated reaction systems. The range of the measured cross sections spans over 8 orders of magnitude, from nb to hundreds of mb. Figure 4.8 and 4.9 present the isotope cross sections measured in the fragmentation of 40Ca and 48Ca, Figure 4.10 and 4.11 display the fragment cross sections of 58Ni and 64Ni and Figure 4.12 shows the 86Kr beam fragmentation product cross sections. For reference, the projectile is represented by the symbol of box with a cross inside and the stable isotopes are highlighted with the black framed boxes. The number of nuclides and their position in the chart of nuclides (Figure 4.8—4.12) using 9Be and 181Ta target is very similar for all investigated systems except for the 86Kr projectile where measurements 85 of fragments with 181Ta target were incomplete. Even though more neutron-rich fragments are expected to be produced by the neutron-rich projectile 48Ca, the number of measured isotopes in 48Ca (202) fragmen- tation is almost twice that of 400a (111). On the other hand, for the 58Ni and 64Ni projectiles the cross section of 196 and 243 isotopes were measured, respectively. For the reactions of the 86Kr beam, we did not have nucleon pick-up isotopes with full momentum distributions identified because of the rather limited range in magnetic rigidity covered. 4.3 Cross section comparisons Before a more quantitative presentation of the measured fragmentation cross sections, we introduce the empirical parameterization of fragmentation cross sections (EPAX). Then the measured cross sections are compared to EPAX and previously published experimental data (where available). 4.3.1 EPAX parameterization A large amounts of early spallation and subsequent projectile fragmentation data have stimulated an interest to understand the underlying dependences and correlations and to systematize them. By studying the numerous cross sections measured in different experiments in the relativistic energy regime, two very important facts have been learned [47]: 1. Fragment isotopic distributions become approximately energy independent above beam energy z 200 MeV/ u; 2. Target spallation cross sections for fragments close to the target nucleus pro- duced by proton or heavy ion beams differ only by a constant factor close to the ratio of the total reaction cross section. 86 The terms “limiting fragmentation” and “factorization” [86] have been introduced to refer to the latter two observations. They formed the basis of many empirical parameterizations valid for specific projectile and target combinations [87]. In 1990 Siimmerer et. at [47] combined the efforts of many different groups developing em- pirical parameterizations into a universal empirical parameterization of cross sections (EPAX) version 1. This version has been refined and improved with inclusion of rel- ativistic projectile fragmentation data available in 2000 resulting in EPAX version 2 [48]. In the EPAX2 parameterization [48] the fragmentation cross section of a fragment with mass, A, and nuclear charge, Z, created from projectile (AP, 2,) colliding with a target (At, 2,) is given by 0(A, Z) = YAn exp (—R|Z,,,.ob — Z|Un
). (4.9) The first term YA describes the sum of the isobaric cross sections with mass number, A, and the second term exp (—R|Zp,ob - Z [Un
) is the so called charge dispersion,
i. e. the distribution of the elemental cross sections around maximum value, meb,
for a given mass. The shape of the charge distribution is controlled by the width
parameter, R, and the exponents, UIn and Up, describing, the neutron-rich (n) and
proton-rich (p) side, respectively. Where the neutron-rich fragments are defined with
meb —— Z > 0 all others are considered proton-rich. The factor n = W normalizes
the integral of the charge dispersion to unity.
The mass yield, YA, is parameterized as an exponential function of the number of
removed nucleons, AP — A,
YA = SP exp [—P(A,, — A)]. (4.10)
87
S is the overall scaling factor which accounts for the peripheral nature of the frag-
mentation reaction and proportional to the sum of the projectile and the target radii:
S: 82(A],/3+A,1/3+Sl). (4.11)
With SI = —2.38 and $2 = 0.270 being EPAX2 parameters. The slope of the expo-
nential function in Equation (4.10), P, is taken as a function of the projectile mass,
Ap, with P1 = —2.584 and P2 = —7.57 x 10"3 as EPAX parameters:
P = exp (PgAp + P1). (4.12)
The charge dispersion in Equation (4.9) is described by three parameters R, Zprob,
and Un(p)- These parameters are strongly correlated [48] and are very difficult to obtain
by a fitting technique. To account for the asymmetric nature of the shape of isobaric
distributions, the exponents, UI, and Up, for the neutron-rich and proton-rich sides are
different. The maximum of the isobar distribution, meb, lies in the valley of stability
and it is parameterized as:
2....(A) = 277A) + A, (4.13)
where Z3(A) is approximated by a smooth function of the mass number, A:
A
2‘3“) ‘ 1.98 + 0.0155A2/3’ (4'14)
and the A parameter is found to be a linear function of the fragment mass, A, for
heavy fragments and a quadratic function of A for lower masses:
A3A2 ifA < A4,
A = (4.15)
A2A+A1 ifA 2 A4.
88
Where A1 = —1.09, A2 = 3.05 x 10”, A3 = 2.14 x 10‘“, and A4 = 71.35 are
EPAX2 parameters. Similarly, the width parameter, R, of the charge distribution is
parameterized as a function of the fragment mass, A, with R1 = 0.885 and R2 =
—9.82 x 10“3 EPAX2 parameters:
R = exp (R2A + R1). (4.16)
This description is sufficient to predict the cross sections of fragments located close
to the line of stability and far from the projectile nucleus, also referred to as the
“residue corridor”. For fragments with masses close to the projectile, corrections to
the parameters A, R, and YA are introduced, according to the following equations:
A = A [1+ d1(A/A,, — d2)2] , (4.17)
R = R [1 + r1(A/A,, — m2] , (4.18)
YA = YA [1 + y1(A/AP “ 3M2] - (4-19)
The corrections to A, R, and YA parameters are applied only for fragments with
mass number, A, fulfilling (A/Ap — d2) > 0, (A/Ap — m) > 0, and (A/Ap — 312) >
0, respectively. With EPAX2 parameters d1 = —25.0, d2 = 0.80, 1‘; = 20.0, r2 =
0.82, and y1 = 200.0, y2 = 0.90, characterizing the corrections to the A, R, and
YA parameters. A final correction is applied in the case of projectile nuclei far from
the line of ,B-stability, ZB(A,,). In this case it has been shown that the fragment
distributions keep some memory of the A /Z ratio of the projectile nucleus resulting
in a correction to the maximum, meb, of the charge distribution:
zpmbm) = 23m) + A + Am, (420)
89
where Am is expressed separately for neutron-rich ((Zp — Zg(Ap)) < 0) and proton-
rich ((Zp — Z3(Ap)) > 0) projectiles:
Am : (Zp — Zg(Ap)) [711(A/Ap)2 + n2(A/Ap)4] for neutron rich, (4.21)
(2,, — Zg(A,,)) exp [p1 + p2(A/Ap)] for proton rich,
where m = 0.40, 712 = 0.60 and p1 = —10.25, [)2 = 10.10 are parameters of EPAX2.
The EPAX2 parameterization altogether contains 24 parameters (SI, 52, P1, P2,
R1, R2, A1, A2, A3, A4, Un, U1, U2, U3, m, 712,121,122, d1, d2, 7'1, 7'2, yl, and y2), many
of which are strongly inter-correlated. These parameters have been determined by a
fit of the fragmentation data of 40Ar [88], 48Ca [42], 58Ni [89], 8f’Kr [75], 129Xe [90],
and 208Pb [91] projectiles measured at 600, 212, 650, 500, 790, and 1000 MeV/u,
respectively.
The main goal of the EPAX2 parameterization is to obtain a smooth analytic de-
scription of the fragmentation data within a factor of two [48]. The empirical param-
eterization assumes the limiting fragmentation regime when the fragmentation cross
sections are no longer beam energy and target dependent. Since it is based on analytic
formula, the cross section calculations are fast. Because of its simplicity, the EPAX2
parameterization has been implemented in many codes (MOCADI [66], LISE++ [56]).
It is not only used to calculate the yields of rare isotopes for the existing (CCF-
NSCL [52], GSI [44], RIKEN [35]), but also fragment yields for the next-generation
radioactive beam facilities (RIA [92], FAIR [93], RI beam factory at RIKEN [94]). The
EPAX2 is also available at http: / /www-aix.gsi .de/ 'sue/epax/epax_v2.htm1.
Since the EPAX2 parameterization is reproducing an “average” behavior of the
fragmentation cross sections for a variety of beam—target combinations, it produces
reliable results while interpolating between experimentally measured data points. The
extrapolation to very exotic (neutron-rich or proton-rich) fragments may be unreli-
able, because the slopes of the isotopic distributions are only measured in the vicinity
of their maxima. A minute change of the slope parameter may cause differences in
90
orders of magnitude in the extreme tails of the fragment isotopic distributions. Hence,
the EPAX2 should be used with extreme caution when calculating yields for fragments
very far from the experimentally measured regions.
4.3.2 Comparison to EPAX
The experimentally determined fragmentation production cross sections for the 10
reaction systems are plotted in Figure 4.13 (4°Ca+9Be), Figure 4.14 (4°Ca+181Ta),
Figure 4.15 (48Ca+gBe), Figure 4.16 (“8Ca+181Ta), Figure 4.17 (58Ni+9Be), Fig-
ure 4.18 (58Ni+181Ta), Figure 4.19 (64Ni+gBe), Figure 4.20 (64Ni+181Ta), Figure 4.21
(86Kr+9Be) and Figure 4.22 (86Kr+181Ta). Each panel represents isotope cross section
data (filled squares) for one element, plotted as a function of neutron excess, N — Z, of
each isotope. The nucleon pick-up and exchange cross sections are shown as filled tri-
angles. All comparisons shown in Figure 4.13—4.22 were calculated using EPAX2 [48].
EPAX does not predict pick-up cross sections and the description of the light frag-
ments (A < Ap / 2) is generally not as good as the predictions for fragments closer to
the projectile, because light fragments may be produced in more central collisions in
other reaction mechanism such as multifragmentation. The maxima of very light ele-
ments with Z < 9 (for the NSCL reaction systems), which may be produced in more
central reactions, do not agree with EPAX calculation. In general, isotope distribu-
tions from EPAX are wider than the measured ones, resulting in over-predictions for
the very neutron-rich and proton-rich fragments for all investigated reaction systems
measured at 140 and 64 MeV/u. This behavior is especially pronounced in the case
of 40Ca projectile.
For all investigated systems, we also observed differences between the EPAX cal-
culated maximum of the isotopic distribution and the experimental data for elements .
close to the projectile. The differences between the EPAX maximum and the exper—
imental data are noticeable for almost all projectiles measured at 140 MeV/u but
are really pronounced for fragments of 86Kr (Ebenm = 64 MeV/u), which suggests
91
stronger sensitivity to the incident energy. This systematic discrepancy between the
intermediate energy fragmentation data and EPAX parameterization has been re-
ported by Siimmerer [97]. The Fermi spheres of the target and projectile nuclei have
larger overlap at intermediate energies than at relativistic energies. There may be
increasing contributions to the prefragments with charge numbers greater than that
of the projectile from the transfer-type reactions. Subsequent decay of these primary
fragments feeds the neutron-deficient isotopes close to the projectile.
The production of 50Ca in the reaction of 58Ni+9Be represents the lowest cross sec-
tion measured from among the NSCL reaction systems. To achieve the measurement
of such low cross section (0.41 :t 0.16 nb), we used a thick 9Be target (578 mg/cm2).
In addition, an Al degrader (240 mg/cmz) was placed at the dispersive image (image
2 in Figure 2.4) position to further separate the fragments with the same A/ Q ratios.
The wedge is especially effective in deflecting the light fragments from reaching the
focal plane detector setup, thus reducing the counting rate of the PIN detector, allow-
ing us to use the maximum beam intensity. All other cross sections (including those
in RIKEN) were measured without the use of the degrader. The lowest cross section
(15 :f: 7 pb) of the study was measured for a fragment 79Cu produced in 86Kr+9Be
reactions at 64 MeV/u.
4.3.3 Comparison to other data
In addition to our 40Ca+9Be data, Figure 4.13 also shows the fragmentation data of
40Ca on a hydrogen target at 356 MeV/u [95]. The latter data are shown as open
squares. Compared to our data, the isotope distributions obtained from the hydrogen
target tend to be narrower than our data and the cross-sections are smaller. The
narrower isotopic distributions generally indicate lower excitation energy reflecting
lower center of mass energy for the system with hydrogen target.
Our fragmentation data of 48Ca on 9Be target are compared to the data measured
at 212 MeV/u at Berkeley [42] shown as open squares in Figure 4.15. There are
92
differences between the two experimental data sets. Even though the fragmentation
data of Ref. [42] were included in extracting the EPAX parameters, the deviation of
this data set from the overall EPAX fit (by a factor of 3) was noted in the original
paper [48]. On the other hand, our fragmentation data seem to agree pretty well with
EPAX predictions except for a small shift of the maximum for elements (S—Ca) close
to the projectile (discussed at the end of the previous section).
More recently, cross sections of 46"‘F’Ca, 44’45'46'47K, 41’42'44145Ar and 39"‘0'41’42Cl iso-
topes have been measured in the projectile fragmentation of 48Ca on a deuterium
target at 104 MeV/ u [96]. These data are shown in Figure 4.15 as open triangles. As
mentioned in Ref. [96], the cross sections for fragments with few nucleons removed
obtained with the 2H target are higher than our data with the 9Be target as expected
from EPAX. In the case of Ca and K isotOpes, they are even higher.
The experimental cross sections for the reaction of 58Ni+9Be are plotted along with
the relativistic energy fragmentation of 58Ni on a 9Be target at 650 MeV/ u by Blank
et al. [89] measured at GSI shown as open squares in Figure 4.17. The experiment of
Blank et al. focused on the proton-rich side of the fragment spectrum for fragments of
elements 21 g Z s 28. Where there are overlapping data points, an interesting trend
is observed. Where there are overlapping data points, an interesting trend is observed.
The fragment cross-sections from the GSI experiment are consistently higher by 70%
than our data for the proton-rich (Z > N) isotopes. In the slightly neutron-rich the
GSI data are slightly lower by 30%. Unfortunately there are not enough overlaps
between the two sets of data to determine if the widths of the distributions are wider
or narrower.
In Figure 4.21 our cross section data are compared to an early experiment with
86Kr projectile and 9Be target at 650 MeV/u by Weber et al. [75], which were used
to fit the EPAX parameterization [48]. For light fragmets the latter data show wider
isotope distributions as compared to our fragmentation data analysis. The cross sec-
tions on top of the isotopic distributions for Co—Zn elements agree rather well with
93
the measurements done at very different energies. The isotopic distributions measured V
at 650 MeV/ u generally appear wider than our measurement at 64 MeV/ u. It must
be noted that there is considerable scatter (Figure 4.21) in Weber et al. data, making
detailed comparison of the distributions width very difficult.
94
Protons
E projectile
5 7 D stable \
Neutrons,
Figure 4.8: Nuclides identified in the fragmentation of 40Ca projectile on two targets.
There are 111 and 116 isotopes shown for 9Be and 181Ta targets respectively.
95
Protons .
E prolectlle
U stable
\/
Neutrons
Figure 4.9: Nuclides identified in the fragmentation of 48Ca projectile on two targets.
There are 202 and 200 isotopes shown for 9Be and 181Ta targets respectively.
96
Protons \
a projectile
El stable
Neutrons '
Figure 4.10: N uclides identified in the fragmentation of 58Ni projectile on two targets.
There are 197 and 189 isotopes shown for 9Be and 181Ta targets respectively.
97
\
I
Protons
“Ni+'8'TO
E projectile
D stable
\
Neutrons fl
Figure 4.11: N uclides identified in the fragmentation of 64Ni projectile on two targets.
There are 246 and 234 isotopes shown for 9Be and 181Ta targets respectively.
98
Protons
a projectile
El stable
86Kr+ ””Ta
Mn
27 29 31
Neutrons '
Figure 4.12: Nuclides identified in the fragmentation of 86Kr projectile on two targets.
There are 180 and 70 fragments shown for 9Be and 181Ta targets respectively.
99
t I—f f I I I V I ' I * f I ' ' T T ' ' ' v
I I
I
10 ~ ' 7* ' r ' ‘r J
[- \ o 0 If ]
]— . Ju— IiP . . up 4
it .
. I
102 “r
10. 1:
A _.
Q a
E 1r
c -2 d-
.Q10 1
“ un-
8 1
0:102 1:
3 10. 1: .
e _[- a au- 0 .1
1h - 0
0 db I db d
1 0
I
- .p q ‘ -1
102 b b 1L-
.. P 'f S .
1L ‘ I
s .L I : ‘ %+ 1r 4r c r j T: s - If
102 . 'r ‘r
l- .11- up 41- f
0 P f
a 1 A
I: :i- ..:. di- {
1 [ I r
1- -Il- ‘ u]- d
D 4’ ‘ h
102? , 1] ‘r I 1: i
:- cu- J- 11- d
. Cl 0 Ar . K ' Ca
A I 1 L4 I A l 1 L 1 I 1 1 A 1 I A A A LL 1 1 1 I 1 L L I 1 1 A 1 I L
O 5 0 5 0 5 0 5
Neutron excess N-Z
Figure 4.13: Isotopic distributions of measured cross sections plotted as a function
of neutron excess for 5 5 Z s 20 elements detected in the 40Ca+9Be reaction at
140 MeV/u. The experimental fragmentation data are shown as filled squares, cross
sections of pick-up reactions are depicted with filled triangles, the solid line shows the
calculation by EPAX and open squares show 40Ca+1H data at 356 MeV/u [95].
100
V V r V V V V V V V V ' V V V V I V V V ' V—Tfif V ' V fi 1 r V7 V V V I V
102 ’ " T “’ ‘
A I ' qr . . 1 I I 4; . 1
I I
- -I— . .fi. .1- . I
I 0 {p 1b 1
- —1l- -1- di- -1
I- db fly 1» 1
10.1 n -- .1!- d- d
I- «p 1: 1D 1
hi —Ir- 1’- ‘h . .1
- B 0 C r I I 1' O 4
A A A A L . A4 A A A I A A A A l A A L4 L A A A l A A J 4 A 1 4A
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102 l- —u- q- A:- q
,. < 1: o - I 1
l I I '
10 l— -i- 4:- dp- -I
b up 1' 1h 1
I I
A - —u- ‘l' cm- I
D b 1. u 1i 1
E I— «I- «b ... d
v 0 0 I 1
' I
c -2 .. A- ' .. .. ..
o :- 1p 0 1r 1
I- . .
«o-a .. -- e _ a a. g .
o I 4} b {I
A A A A A A A A A L A 1 A #A A l A A l A A A A l A A_ A J A+A A l
102 u— —:y- dh «l- . -1
w p I AL - . - 1r - 1p . - 1
w 10 :— —p- d All- 1
m I- «L 1F 1h 1
h L— —1- -— uh- . d
p . 4. . ID - ’ T
o I
L —u— d- AL- I!
p I fit up I 1» 1
I A
10°2 - —u- ur- -- q
I 4! 1D I 1
I I I A
- —u— I t Au- ‘ A1
I- 0 h J
A A I A A A A J k A A l A A A A l A A A L A A A A l A A A l A A l
v v I v v v V f f ff' 1 v Y 1' I v I Y I v v v v I I v v I I I t r Y
D ‘F 4. 1b 1
102 D - 1b . . lb t .
10 I- 1L- 1. 1|- 4
h 0 r ‘ b
‘ A
i- -|- .- dh d
b 0 h . I
. ‘ A l
b ‘r db h
b 1 I
A
A
-2 i— -|- ‘ du- :-
1 p 1! h D
: Cl A 1' Ar 1' K J: Ca
A_A 1 A A A A l A A A l A A A A l A A J l A J A A A A A l l
01.
C
01
o 5 o 5 o
Neutron excess N-Z
Figure 4.14: Isotopic distributions of measured cross sections plotted as a function
of neutron excess for 5 3 Z s 20 elements detected in the 40Ca+181Ta reaction at
140 MeV/u. The experimental fragmentation data are shown as filled squares, cross
sections of pick-up reactions are depicted with filled triangles and the solid line shows
the calculation by EPAX.
101
' v v v v ' fi1—v Via—r7 . l - l v v v I v
10 ' ' ‘
F ' ‘
— \ I D a -1
> 1; b D
O‘ I- I- h- u-
10 D - 1D 4. D 1
I— I- ul- -1
I 1» 1p 1
'3 I— u- .- cl
p 1b > 1
I— u- u-
h 1L- I 1
‘5 r- h- v- 0 ..
1 b 1 1»
A4 A A _ l A A A l A A A l l A A
10 I— up -I- cli- i
b p 1p 1»
D D
u up .1. ul- .1
p db 1? db J
A '1 i- -I- ' un- ' D -L . .1
D b ~I 1- 1r J
r- -Ih- -- up -l
h It 1» 1b 1
v
'3 7- -II- . ul- db
c 10 b 1r 1} 1| 1
o h- —I u)- up
a: ’ ‘r Ne ' “ Na M ' i
o ’5 h- -1I- u!- -r- u:
m T ._v.,----, ,. .0-.,.. .,.- .r...0.-,.v-ft44.-.-,%r.‘
m I- - di- up .1
10 h 1' 1» 1» D 1
m L- —l— at .1:- a .4
m y- db 1 1p 1
o #- —1I- uh- r- d
‘ F . 1p 1} . 1D 1
010-2 - -- -. 1. ‘
In D 1
F ’ Jr .
p— —n- i- d- -1
h It I. a. I 1
L- -4- - J . +- J
h 0 1 I 1
10.5 - Al " S' “r P '1' s “
I It I 1 b 1
' C
:- c-u- .1- l-
. A44 A A A A LA A A A A A A AAA A A_AJ ALA A l A A A l A A A A l A l A A A A
' I ' ' ' ' I V V V I V V V ' lTTj’ I ' U ' V ' f1] ' ' ‘TTT I I V
I- c1- :1:- 1i-
1 D 1D A {D F
I- D db .- 1. '- qr
b a 1 1» .
u up I!» db
D 1» 1 ‘ r
-‘ I- .1 qt- n-
10 . » r »
l- . uh I ‘ I- i‘ '4
. 1» 1: I f
p ‘ at ‘ - db
D D D
I ‘ A
4 I- d. 1!- db
10 ’ A" l ’
- Cl .. Ar . K 1r
D
’ A A LA A A A l A A A A l A A A ‘ A A LA A LAI A A A A l A A A 0 A A i A A A A l A A A LJALA A A 11 A A A A l A A A A lA_A A
O 5 10 O 5 10 O 5 10 O 5 10
Neutron excess N-Z
Figure 4.15: Isotopic distributions of measured cross sections plotted as a function
of neutron excess for 5 3 Z S 20 elements detected in the 48Ca+QBe reaction at
140 MeV/ u. The experimental fragmentation data are shown as filled squares, cross
sections of pick-up reactions are depicted with filled triangles, the solid line shows the
calculation by EPAX and open squares show 48Ca+9Be data at 212 MeV/u [42]. Open
triangles depict the fragmentation data of 48Ca on a 2H target at 104 MeV/u [96].
102
1o2 *
10 ~
—L
O.
M
A
C
M
Cross section (mb)
O
_. L
1 0""
10'4
1o2
1 0'2
1 o4
V I 7* 7 f' V v V V ' V V V V I V I V V ' V V V V I V V V V V l V V‘f" r' I V V V V I V V V V ' V V V V I—fifi
. .1!- d- I1»-
» 1» .. 1» 1»
I q». un . . un- . -1
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» 1» 1» 1» 1
I
»— -1»- -1»- -1»- .4
p 1» 1+ 1» 1
»— —1— -1»- d»- -1
5 1» 1» 1» 1
» - N -- O 1
b 1» 1» 1
A A A l A A A A I A A A A l A A A l A A LA A l A A A
V I V V V V I V V V I ‘7' V Y Y I V V V Y I V fv V ' V Y ff
#- l- q- 'l
b 1» 1» . 1
p— ». di- -1
b b 1» 1
p- »- -II- 1
b 1» - 1» . 1
»— »- q- -1
» » 1» 1
»— db -1
r D {I l
p- I- Jn 11
b 1» 1» 1
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» 1» 1» 1
A A l A A A A l A A A A l A A A A A A LA A A A l A l A A A
v w v I v v v v I v v v v I v v v v v I v v 7 rr v v v v ‘ v v v v
»- I— I- -1
I r 1» . 1
— I- 43 '1
» b 1» 1
v— 1»- «4
b 1 b 1
- t- I '1- . "
p 1 1» 1
»— »- -1- q
» 1» 1» 1
I- I- uh I 11
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A A A A A l A A A l A A l A A A A l A A A A l A A A A l A A I A A A
,va-vvv' v v'vv V,vavvvv'vvvv' vvv“vr]vvvaYT—vv'vv » IVTvr
7- 1-1 1»- 1»-
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Neutron excess N-Z
O
5
10
Figure 4.16: Isotopic distributions of measured cross sections plotted as a function
of neutron excess for 5 3 Z S 20 elements detected in the 48Ca+181Ta reaction at
140 MeV/u. The experimental fragmentation data are shown as filled squares, cross
sections of pick-up reactions are depicted with filled triangles and the solid line shows
the calculation by EPAX.
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Neutron excess N-Z
Figure 4.17: Isotopic distributions of measured cross sections plotted as a function
of neutron excess for 5 3 Z S 28 elements detected in the 58Ni+gBe reaction at
140 MeV/ u. The experimental fragmentation data are shown as filled squares, cross
sections of pick-up reactions are depicted with filled triangles, the solid line shows the
calculation by EPAX and open squares show 58Ni+QBe data at 650 MeV/u [89].
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Neutron excess N-Z
Figure 4.18: Isotopic distributions of measured croSs sections plotted as a function
of neutron excess for 5 3 Z s 28 elements detected in the 58Ni+181Ta reaction at
140 MeV/ u. The experimental fragmentation are data shown as filled squares, cross
sections of pick-up reactions are depicted with filled triangles and the solid line shows
the calculation by EPAX.
105
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Neutron excess N-Z
Figure 4.19: Isotopic distributions of measured cross sections plotted as a function
of neutron excess for 5 3 Z S 28 elements detected in the 64Ni+9Be reaction at
140 MeV/ u. The experimental fragmentation data are shown as filled squares, cross
sections of pick-up reactions are depicted with filled triangles and the solid line shows
the calculation by EPAX.
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Neutron excess N-Z
Figure 4.20: Isotopic distributions of measured cross sections plotted as a function
of neutron excess for 5 3 Z S 28 elements detected in the 64Ni+181Ta reaction at
140 MeV/u. The experimental fragmentation data are shown as filled squares and the
solid line shows the calculation by EPAX.
107
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Neutron excess N-Z
Figure 4.21: Isotopic distributions of measured cross sections plotted as a function of
neutron excess for 25 3 Z S 36 elements detected in the 86Kr+9Be reaction at '64
MeV/u. The experimental fragmentation data are shown as filled squares, the solid
line shows the calculation by EPAX and open squares show 86Kr+gBe fragmentation
data at 500 MeV/u [75], the nucleon pick-up cross sections from this data set are
shown as open triangles.
l 14—L
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Neutron excess N-Z
Figure 4.22: Isotopic distributions of measured cross sections plotted as a function
of neutron excess for 25 3 Z S 36 elements detected in the 86Kr+181Ta reaction at
64 MeV/ u. The experimental fragmentation data are shown as filled squares and the
EPAX calculation is shown as a solid line.
109
4.3.4 Target dependence
In order to explore the target dependence of the fragmentation cross sections for a
given projectile, we constructed target isotOpe ratios, Rtgt(A, Z), for isotopes with
mass, A, and proton number, Z. R¢9t(A, Z) E aTa(A, Z)/aBe(A, Z), where 0T8(A, Z)
and 038(A, Z) are the fragmentation cross sections measured for reactions with 181Ta
and 9Be target, respectively. The RAMA, Z) ratios were calculated for all projectiles
in our study. By convention, fragment cross sections created in interactions with the
more neutron-rich target (181Ta) are used as numerators.
Rtgt(A, Z) for elements 8 5 Z 3 18 are plotted as a function of the fragment mass
number, A, in Figure 4.23 for 40Ca (left panel) and 48Ca (right panel) beam. Figure
4.24 shows the Rtgt(A, Z) as a function of fragment mass number, A, for elements
10 _<_ Z s 26 for 58Ni (left panel) and 64Ni (right panel) beam. The target isotope
ratios, Rtgt(A, Z) for elements 25 3 Z S 36 for the 86Kr primary beam are plotted as
a function of fragment mass number, A, in Figure 4.25.
The isotopes with even proton number, Z, are plotted with filled symbols and the
isotopes with odd Z are plotted with open symbols in Figure 4.23—4.25. The lines
connecting the isotopes of one element are drawn to guide the eye. Only ratios with
errors smaller than 20% are plotted; for clarity we do not show the error bars of the
Rt9t(Ai Z)-
In the geometrical limit, the target isotope ratios, Rtgt(A,Z), are given by the
differences of the total reaction cross sections which are proportional to the sum of
radii squared [98]. This leads to the expression:
(Ayn/1w
Rm: 13 13 2’
(AP/ +AB/e)
(4.22)
where Ap is the mass number of the projectile, ATE and Age are mass numbers
of 181Ta and 9Be. The horizontal dotted lines in Figure 4.23—4.25 indicate the Rtgt
110
values calculated by Equation (4.22). The target isotope ratios calculated using EPAX
parameterization are shown as horizontal dashed lines in Figure 4.23-4.25. EPAX
systematics assumes only peripheral interactions parameterizing the cross sections as
a linear function of the sum of projectile and target radii. The experimental data,
however, exhibit a complex dependence of the target isotope ratios as a function of
the fragment mass number, A, for all investigated projectiles.
In the case of the 40Ca primary beam, left panel of Figure 4.23, we can see the U-
shaped curves for different elements suggesting enhanced production of neutron-rich
as well as neutron-deficient isotopes of a given element with 181Ta target. This is a
direct consequence of the wider isotope distributions for reactions with the 181Ta tar-
get compared to the ones with the 9Be target. In the case of the 48Ca projectile, right
panel of Figure 4.23, the U-shaped curves are not as pronounced and smooth as those
seen in the left panel of Figure 4.23 for the 40Ca beam. The magnitude of the effect is
small but larger than EPAX and geometrical limit predictions for light neutron-rich
isotopes. Such differences may be interpreted as an effect of isospin transfer from the
target to the projectile. The relatively steep increase of the production of neutron-rich
fragments in the fragmentation of 40Ca (Z /A = 0.50) on a 181Ta (Z /A = 0.40) target
may be due to the relatively large difference of the isospin asymmetry, whereas in the
case of 480a (Z /A = 0.42) projectile the asymmetry is similar to that of the 181Ta
target.
For the 58Ni projectile, left panel of Figure 4.24, the enhanced production of
neutron-rich as well as neutron-deficient isotopes is not as pronounced as in the case
of 400a, but the overall increase of the fragmentation cross sections with decreasing
fragment number can be observed. The more neutron-rich 64N i projectile, right panel
of Figure 4.24, exhibits less pronounced U-shaped curves. A clear enhancement, in-
versely proportional to the fragment mass number, A, of the production cross section
using the 181Ta target, can be observed for light elements (10 S Z 5 16). Again, this
behavior may be interpreted as an effect of the isospin transfer from the target to
111
I r *rTY l' '
5- 4°Ca 5F
FE‘4" fil‘4t
< <
.33 33
>3 .- >3
Nz- N_2-
S, :5
as a
o'" 6‘
1L111111LLLAIAA114+LJAAA 1brr114411.1.r1..4_4
15 2o 25 30 35 20 30 40
Mass number A Mass number A
Figure 4.23: Target ratios of the fragmentation cross sections U'ra(A, Z )/aBe(A, Z),
of fragments 8 3 Z S 18 for two projectiles 40Ca (left panel) and 48Ca (right panel).
The horizontal dashed and dotted lines indicate the ratio calculated by the EPAX
formula and Equation (4.22), respectively.
6r 6-
A5 Kr5
N4 -4
s S,
83- 33-
>? >3
N2 N2
3 S.
Q as
of" o"
1 1
20" "30 '140‘ 50“ "60
Mass number A Mass number A
Figure 4.24: Target ratios of the fragmentation cross sections 0T8(A, Z) /oBe(A, Z) of
fragments 10 3 Z s 26 for two projectiles 58Ni (left panel) and 64Ni (right panel).
The horizontal dashed and dotted lines indicate the ratio calculated by the EPAX
formula and Equation (4.22), respectively.
the projectile. For the 58Ni (Z /A = 0.48) projectile, however, the isospin asymmetry
is larger than in the case of the 40Ca beam, which may account for the less pro-
nounced enhancement of cross sections for neutron-rich fragments. In the case of the
64Ni (Z /A = 0.44) beam, we do not observe strong enhancement of the production
cross sections for the neutron-rich fragments because of a rather similar asymmetry
between the target and the projectile.
The overall trend of the target isotope ratios, R¢gt(A, Z), for the 86Kr projectile
measured at 64 MeV/u is similar to the projectiles measured at 140 MeV/ u, but with
112
—L
O
T 177 171—11]
1
oTa(A,Z)/oBe(A,Z)
I I TTITI
N
g
111
A 1 1 l 1 1 l 1 l A 1 1 1 L
I-
60 70
Mass number A
Figure 4.25: Target ratios of the fragmentation cross sections 0T8(A, Z) /0’Be(A, Z) of
fragments 25 3 Z s 36 for 86Kr projectile. The horizontal dashed and dotted lines
indicate the ratio calculated by the EPAX formula and Equation (4.22), respectively.
steeper overall SIOpe. The experimental cross section data set for the 86Kr+181Ta
reaction is much smaller than the one for the 86Kr+QBe reaction, not allowing us to
investigate the enhancement of the production cross section of neutron-rich isotopes
for individual elements.
There are many differences in the shape of the target isotope ratios for different
projectiles. One common feature is that the overall slope of the target isotope ratios,
Rtgt(A, Z), seems to get steeper with increasing mass of the projectile.
Unfortunately, the observed small enhancement of the fragmentation cross sections
using 181Ta target is offset by the large difference in atomic mass of the two target
materials (approximately a factor of 20). The 181Ta target must be more than 20
times thicker in order to have the same number of target nucleons per cm2 as 9Be.
Phrthermore, effects like the energy loss and angular straggling must be considered
when using thick targets for production of rare isotopes. In the case of the 86Kr
projectile at 64 MeV/u a relatively broad charge state distribution for 181Ta target
makes it undesirable to us. However, if the rising trend of the Rtgt for the 86Kr primary
beam in Figure 4.25 continues for fragments of lighter elements, it may overcome the
above mentioned handicaps of the 181Ta target material. Unfortunately, our limited
set of the fragmentation cross sections does not allow us to make this conclusion.
113
4.3.5 Projectile dependence
The ratios of the fragmentation cross sections for a given fragment with neutron,
N, and proton number, Z, Rpm-(N, Z), from different projectiles were calculated to
address the projectile dependence. Rpm-(N, Z) E 043(N, Z) /a40(N, Z) in the case of
the Ca primary beams and Rpm-(N, Z) E 064(N, Z) /058(N, Z) for the Ni projectiles.
By convention, fragment cross sections from the more neutron-rich projectile (“Ca
or 64Ni) are used as numerators.
Figure 4.26 displays the ratios, RpmfiN, Z), for the reactions of the 48Ca and 40Ca
with 9Be and 181Ta targets. The Rpm-(N, Z) ratios calculated for even and odd Z
elements are shown as filled and open symbols, respectively. Similarly, Figure 4.27
shows the Rpm-(N, Z), for the reactions of the 64N i and 58N i primary beams with 9Be
and 181Ta targets. Ratios obtained with the 9Be and 181Ta targets are plotted in the
top and bottom panels, respectively, of Figure 4.26 and 4.27 as a function of fragment
neutron number, N.
Peripheral projectile fragmentation reactions are mostly responsible for produc-
tion of fragments with mass A > A p / 2. Most likely, lighter fragments are produced by
more central collisions with different reaction mechanisms including multifragmenta-
tion. Indeed, light fragments (Z S 9 and Z S 12 for Ca and Ni systems, respectively)
exhibit the linear isoscaling behavior as observed in multifragmentation [99]
Rpmj(N, Z) = R21(N, Z) = Cexp(aN + BZ), (4.23)
where the isoscaling ratio R21(N, Z) is factored into two fugacity terms a and ,8,
which contain the differences of the chemical potentials for neutrons and protons of
the two reaction systems. C is a normalization factor of the isoscaling ratio. The lines
in Figure 4.26 and 4.27 for elements Z S 9 and Z S 12, respectively, are the best
fits of the data to Equation (4.23). The best fit values of a and B parameters for all
investigated reaction systems are listed in Table 4.3.
114
For heaviest fragments (Z 2 15 and Z 2 22 for Ca and Ni systems, respectively),
longer chains of isotopes were measured. The data no longer exhibit the linear isoscal-
ing behavior of Equation (4.23). Instead, the data are better represented by a function
which contains second order terms in neutron, N, and proton number, Z,
BMW-(N, Z) = Cexp(aN+alN2+fiZ+filZ2), (4.24)
where C is the normalization factor, a, m, ,8, and 61 are parameters of the proposed
function. The curves in Figure 4.26 for elements 15 S Z S 19 are the best fits of the
data to Equation (4.24). In the case of the Ni reaction systems we fitted the Rpm,-
ratios for elements 22 S Z S 26 by function of Equation (4.24).
It may seem that this second order function may be in contradiction with the
findings of the deeply inelastic scattering of Kr isotopes on Ni isotopes. However,
close examinations of previous data [100] show that the observed isoscaling is more
similar to Equation (4.24) than to Equation (4.23). Furthermore, closer examination
of the 10 S Z S 14 and 13 S Z S 21 regions for the Ca and Ni reaction systems,
respectively, revealed that they cannot be described by a single set of parameters of
Equation (4.23) or (4.24). Instead we noticed a gradual change of the a, a1, 6, and
61 parameters with every isotope chain of 10 S Z S 14 and 13 S Z S 21 elements
for Ca and Ni systems, respectively. This observation suggests that the isotopes of
these elements contain contributions from both projectile fragmentation as well as
multifragmentation. Unlike multifragmentation which can be assumed to be statistical
and thermal the large prefragment residues are most likely formed in non-equilibrium
processes and there is no a priori reason to assume that the fragments would observe
isoscaling. Impact parameter event selection detector in future experiments should
allow better distinction between the above described processes.
115
QB 5
103:— e o:
E o 3
@102;— f ' In?
a 5 a
$105— “ 3:
b : ‘ :
>\ 15’ * _i
N E . E
210‘; ° 5
"6o 5 . E
V — _
010""? I? a
10'3 : 1: 2:195
:L 1 l 1 I l l I l L l 1 L:
10 15 20
Neutron number N
103g“— 9?:
Q1025 9%
g E E
911055 E
\b : :
’NT ‘5 5
E1045 5
3 E 3
01025 a
10's; 3
_1 1 1 1 l 1 1 1 1 l 1 1 1 1 I
10 15 20
Neutron number N
Figure 4.26: Ratios of cross sections 043(N, Z)/a40(N, Z) of fragments created in
48Ca and 40Ca reactions on 9Be (top panel) and 181Ta (bottom panel) targets. The
projectile ratios for even and odd Z elements are denoted by filled and open symbols,
respectively. Solid and dashed lines show the fit by Equation (4.23) for 5 S Z S 9.
Curved solid and dashed lines denote fits by Equation (4.24) for 15 S Z S 19.
116
5..
3..
064(N,Z)/658(N,Z)
A
O.
N
664(N,Z)/658(N,Z)
_. 3 ES. 5.
_L
0.
_L
O.
M
T r r r I
'->~1
:_ i —;
E; iii §
.. A ,i t . ..
_ s t . o A
13;; 13'
O Q 1:1 _
z:
1 1 I 1 1 1 1 l 1 1 1 1 l 7
1o 15 20 30
Neutron number N
P I1jg1] I T Ff ' l I Ifl I I‘r I IT I I I I
— Ta
E
F 1
5 HH-
'_'_' A * t o o
_ c t . o I
_ * o o
E o o ' D
E 9 :1 ’ ° ‘
_ + l A
: =22
— 1 l l l l l l 11 l l l L 1 l 1 L1 1 J l g]
10 15 20 25 30
Neutron number N
Figure 4.27: Ratios of cross sections 064(N, Z) /058(N, Z) of fragments created in
64Ni and 58Ni reactions on 9Be (top panel) and 181Ta (bottom panel) targets. The
projectile ratios for even and odd Z elements are denoted by filled and open symbols,
respectively. Solid and dashed lines show the fit by Equation (4.23) for 8 S Z S 12.
Curved solid and dashed lines denote fits by Equation (4.24) for 22 S Z S 26.
117
Table 4.3: Best fit values of the a, 011, ,8, and 31 parameters of the isoscaling function
of Equation (4.23) and the second order function of Equation (4.24) for Ca and Ni
systems. me and me. values indicate the range of elements for which the best fit
values were obtained.
Projectiles Target Range
material me Zmax a 01 fi 61
988 5 9 0.8714 0.0 —0.9971 0.0
Ca 15 19 — 1.2764 0.0776 1.8367 —0.1116
181Ta 5 9 0.7586 0.0 —0.8706 0.0
12 19 — 1.5350 0.0850 2.5532 -0.1340
9Be 8 12 0.5503 0.0 —0.6142 0.0
Ni 22 26 — 1.8047 0.0514 1.4966 —0.0556
181Ta 8 12 0.3961 0.0 —0.4670 0.0
22 26 — 1.4904 0.0444 1.4062 —0.0533
118
Chapter 5
Comparison to models
In order to get a physical understanding of the underlying fragmentation processes
and the reaction mechanism, three reaction models, varying in complexity and so-
phistication, were used to understand the results of our experimental measurements.
Calculations by the macroscopic Abrasion-Ablation (AA) model, the macroscopic-
microscopic Heavy Ion Phase Space Exploration (HIPSE) model and the sophisti-
cated fully microscopic Antisymmetrized Molecular Dynamics (AMD) model were
compared to the experimental data of this study. The AA model used in the present
work is a modified version of the geometrical AA model implemented in LISE++ [56].
In all described theoretical models the nuclear reactions are simulated in two stages:
primary (fast nuclear reaction dynamics) and secondary (slow decay of excited pri-
mary fragments).
First the three reaction models are discussed, introducing the main assumptions
and the theoretical concepts. Then the primary fragments generated by these models
are compared with each other. The discussion continues with the description of evap-
oration codes used to de—excite the residues obtained by the reaction models. At the
end of the chapter the calculated properties of the final fragments are compared to
the experimental data.
119
5. 1 Reaction models
5.1.1 Abrasion Ablation model
In the fully macroscopic Abrasion-Ablation (AA) model, the projectile and target
nuclei are assumed to be spherical. The Coulomb deflection of the projectile trajectory
is neglected. In the case of a peripheral reaction, at a given impact parameter, b, the
projectile and target nucleons which lie in the overlap region (participant nucleons)
are removed from the original nuclei, while the remainder nucleons do not participate
(spectator nucleons). This first stage of the reaction, called abrasion, is very fast; the
timescale is given by the relative velocity of projectile and target (see Figure 1.3).
The AA model does not describe the ultimate fate of the participant nucleons, since
they are not necessary to model the fragmentation of projectile or target nuclei.
Analytic formulas relating the impact parameter, b, to the number of removed
nucleons, AA, have been derived [101]. Unfortunately the expression of impact pa-
rameter, b, as a function of number of removed nucleons must be obtained numerically.
The cross section, ame(A), for producing a primary fragment with mass, A, from a
projectile, A p, is calculated from the obtained b(AA) function:
0mm) = 7r { [b(AA — 0.5)]2 — [b(AA + 0.5)]2}. (5.1)
Assuming there are no proton-neutron correlations in the reaction system, the proba-
bility to form a residue with a given mass number, A, and nuclear charge, Z, is given
by a hypergeometric distribution. Hence the total cross section, 0pm(A, Z), to form a
primary fragment (A, Z) is
0161A, 2) = [(AZZ) (IQ/($451)] 0mm). (5.2)
Where AP, Zp, and Np are mass, charge and neutron numbers of the projectile and AA,
120
AZ, and AN are numbers of nucleons, protons, and neutrons, respectively, removed
in the interaction. The projectile and target dependence is contained in the b(AA)
dependence.
The highly excited and deformed spectators de-excite in the second stage, called
ablation, by evaporating light clusters and nucleons (see Figure 1.3). This evaporation
stage of the reaction is taken to be slower (z 10‘21—-10‘16 3) compared to the abrasion
step (z 10‘22—10‘23 s) and results in the formation of the de—excited nuclei detected
in the experiments. In the original AA model [45] the excitation energy of the primary
fragments is determined from the surface energy (typical value z 1MeV/fm2). This
value of the excitation energy, however, must be regarded as a lower limit, because
the energy of the primary fragment can be altered by other processes (such as transfer
of energetic participant nucleons into the projectile prefragment).
To increase the excitation energy, many different mechanisms have been proposed,
such as scattered nucleons from the participant zone to the spectators [102] or in-
clusion of the frictional energy in the target-projectile interaction [103]. Gaimard
et al. [104] calculated the excitation energy based on vacancies created in the Fermi
distribution of the nucleons in the spectator nucleus. This simple statistical “hole-
energy” model yields an average excitation energy of 13.3 MeV per hole (abraded
nucleon). Another investigation by Schmidt et al. [105] resulted in an average ex-
citation energy of 27 MeV per abraded nucleon for fragments of heavy projectiles
(A > 100) in the most peripheral collisions. The above mentioned estimates of the
mean excitation energy per nucleon illustrate the level of uncertainties in determina-
tion of the excitation energy in the AA model.
In our calculations we used a modified version of geometrical Abrasion-Ablation
model that has been implemented in the LISE++ code [56]. The mean excitation
energy, E*, of the primary fragment is expressed as a linear function of the number
of abraded nucleons, AA:
E* = K . AA. (5.3)
121
Table 5.1: Best fit values for the K and S parameters of the excitation energy in
the Abrasion-Ablation model for all investigated reaction systems. Uncertainties es-
timated from the minima in the two-dimensional K versus S space.
Beam Target K S
(MeV/u) (MeV/fl)
400a 9Be 1112 621:1
181Ta 9 2t 1 4 :l: 1
480a 9Be 1112 6i1
181Ta. 11 :i: 2 6 :l: 1
58Ni 9Be 13 j: 2 7 :l: 1
181Ta 13 i 2 8 :l: 1
“Ni 9Be 13 i 2 7 i: 1
181Ta 15 i 2 8 fl: 1
86Kr 9Be 19 :l: 2 10 :l: 2
181Ta 18 fl: 2 10 d: 2
The fluctuations around this mean value are described by a Gaussian distribution
with a variance, 031, parameterized as:
0E: : S ’ V AA. (5.4)
The K and S are taken as fitting parameters of the model. The excitation energy
is assigned to prefragments according to Equations (5.3) and (5.4). These excited
primary fragments decay as described in the analytical LisFus evaporation code [106].
The K and S parameters are determined by fitting the measured cross sections
of projectile-like fragments (A > Ap/ 2) for a given reaction system. The obtained
best fit values of the K and S parameters for all our reaction systems are listed in
Table 5.1. This approach has been applied in the case of the 78Kr+9Be reactions at
140 MeV/ u by Stolz et al. [107]. The measurement focused on proton—rich fragments
in Ge—Se region and obtained K = 12 MeV/ AA and S = 3 MeV/m.
The obtained values for the K and S parameters in our analysis as listed in Table
5.1, do not depend on the target for all investigated projectiles within the quoted
uncertainties. Both Ni projectiles seem to require a slightly larger values for the K
122
parameter than the Ca beams, but the differences are still within the large uncer—
tainties. The 86Kr reactions, however, require much steeper slopes of the excitation
energy as a function of the number of removed nucleons in order to describe the
fragmentation data.
The LISE++ implementation of the AA model [104] provides fast calculations over
a wide dynamic range of final fragment cross sections. With only two parameters, the
excitation energy and its fluctuations, the predictions reproduce the experimental
data extremely well as will be discussed in Section 5.5. Currently it requires fitting of
principal parameters to the experimental data for each reaction, its predictive power is
rather limited. However, it may be used to extrapolate the production cross sections,
in the case of one reaction system, for very exotic fragment species when a systematic
set of fragmentation cross sections becomes available.
5.1.2 Heavy Ion Phase Space Exploration
The Heavy Ion Phase Space Exploration (HIPSE) model has been implemented to
bridge the gap between the statistical models, which reduce the description of the
reaction to a few important parameters, and fully microscopic models which require
a large amount of CPU time to describe the motion of individual nucleons. Based
on a macroscopic-microscopic “phenomenology,” it accounts for both dynamical and
statistical aspects of nuclear collisions.
The nuclear reaction, as described by HIPSE [108], can be separated into three
stages: approach of projectile and target nuclei, partition formation and the cluster
propagation phase (ending with an in-flight statistical decay). In the entrance channel
at a given beam energy, Eg, a classical two—body dynamics of the center of masses of
the target and the projectile nuclei is assumed. For projectile and target nuclei with
mass numbers, Ap and AT, and positions, rp and rT, respectively, the dynamical
123
evolution associated with the following Hamiltonian is considered:
—AL—-EB = p—2 ‘1' VA A (er — I’pl) (5.5)
AT +1413 211. T P ,
where VATAP is the nucleus-nucleus potential, p is the relative momentum and ,u =
me p / (mT+m p) is the reduced mass with mp and mT being the projectile and target
mass, respectively. The proximity potential [109] is used to express the nucleus-nucleus
potential VATAP at large distances 7" > (RT + Rp), with RT and Rp radii of the target
and the projectile nuclides, respectively. This potential is unambiguously defined when
the two nuclei are well separated. For small relative distances 7' S (RT + Rp), a simple
third order polynomial extrapolation is used, assuming continuity of the derivative of
the potential at each point. The value at r = 0 is expressed as:
VAT/119(0) = Gal/£310 = 0) (5-6)
where the potential hardness parameter, aa, is an adjustable parameter of the model,
and Vfl'fi: (r = 0) is the energy of the system assuming that the two nuclei overlap
completely in the frozen density approximation. This energy corresponds a pm’om’ to
the maximal value of VATAP (0), leading to 01,, S 1. Figure 5.1 shows a dependence of
VATAP on relative distance between target and projectile nuclei for different values of
the an parameter. The frozen density or “sudden” approximation assumes that the
internal degrees of freedom do not have time to reorganize themselves and the system
has a strong memory of the initial conditions. At energies close to the fusion, barrier,
the 010 parameter represents a measure of the degree of reorganization of the internal
degrees of freedom during the reaction. Since the model does not treat the internal
reorganization of nucleons explicitly, an is taken as a free parameter, depending only
on the beam energy, representing the absence of knowledge of the nucleus-nucleus
potential at large overlaps.
The trajectories of the projectile and the target nuclides are determined by the
124
O LALlLLAA[LPAAIAAAAILJLALLJAL
10 20 30
Relative distance (fm)
Figure 5.1: Nucleus-nucleus potential VATAP as a function of the relative distance for
the 129Xe+1208n system with (1,, = 0, 0.01, and 0.2 [108].
Hamiltonian in Equation (5.5). At the minimum distance of approach, the two re-
acting nuclei overlap according to the impact parameter and the potential hardness
parameter, aa. The frozen density approximation is applied again, explicitly, to sam-
ple the positions and the momenta of the nucleons in the center of mass of each of
the reaction partners. The semiclassical Thomas-Fermi theory is used to get realistic
ground state density distributions for each of the reaction partners. The nucleons
are sampled according to a Metropolis algorithm where the Pauli principle is taken
into account in each nucleus by requiring that Ar, - Apr 2 h, where Ar.r and Ap,
are the relative position and momentum of two nucleons with the same isospin, T.
This procedure ensures a uniform arrangement of the nucleons in each of the reaction
partners. At the end of the first stage of the reaction positions (2:, y, z),- and momenta
(p1, py, 132),- for a set of AT + Ap nucleons are obtained.
Before the clusters are defined, the overlap region is determined based on the
following consideration: a nucleon initially in the target at position r,-, is assumed to
125
be in the overlap region if Ir,- — rpl S Rp, where rp is the position of the projectile
in the center of mass frame, and Rp is the radius of the projectile. For the nucleons
initially in the projectile analogous criteria are required Ir,- — rTI S RT, where r7» is
the position of the target in the center of mass frame, and RT is the radius of the
target. This approach corresponds to the so-called “participant-spectator” picture.
However, experimentally it is observed that the quasi-projectile fragments have a
slightly reduced kinetic energy with respect to that of the initial projectile. This
effect is taken into account by introducing the exchange of particles between the two
partners during the reaction. In the HIPSE model it is introduced ad hoc by assuming
that a fraction, 9:”, of the nucleons coming initially from the target (projectile) are
transferred to the projectile (target). It is expected that the number of exchanged
nucleons decreases with the beam energy, and thus mt, depends only on the initial
kinetic energy.
A simplified procedure is applied to treat nucleon-nucleon collisions inside the
overlap region characterized by a mass number, Awer. Only a fraction of the nucleons
in the participant region experiences a collision. Thus the number of collisions is
defined by NCO“ = Am, - 1:60“. The fraction of the nucleon-nucleon collisions, mm",
is the third free parameter of the HIPSE model. It is expected to depend only on
the energy of the beam. A two-body nucleon-nucleon collision is simulated only in
the momentum space. The final spatial positions of the two nucleons are randomly
distributed inside a sphere with radius
Rcoll : 12A1/3 + really (57)
0087‘
where parameter rwu is taken to be 4 fm which reproduces the low energy multifrag—
mentation data [30]. It muSt be noted that due to the Pauli exclusion principle not
all the volume of the sphere with radius, R60", is accessible.
After nucleon-nucleon collisions take place, the clusters are defined according to
126
a straight-forward coalescence algorithm. First, one of the nucleons is chosen at ran-
dom, constituting a starting point from which the fragment is built. Then, another
nucleon, 2', from the participant region is chosen. Since the exact conditions when a
nucleon in the nuclear medium can be absorbed by a cluster are not known, a simple
phenomenological criterion is used. In particular, the nucleon with relative position,
1",, and relative momentum, 19,-, with respect to fragment is assumed to be absorbed
by it if
2 V
—p‘— + a“ < 0, (5.8)
2m 1 + exp [4”? ]
where m is the nucleon mass and the parameters d; and th correspond to limits in
position, 7', and momentum, p, space, respectively. The distance, d f, is expressed as
Rf + rent, where Rf is the radius of the fragment and rent is 7 fm for all nucleons
undergoing a collision and 2.5 fm for all others. Using cht = —p2/2m, pcut=500
MeV/c was obtained and the diffuseness parameter, a, has been fixed to be 0.6 fm.
If the phase space condition of Equation (5.8) is fulfilled the nucleon is absorbed by
the fragment and a new cluster is formed provided that it exists in the mass table. If
the above mentioned requirements are not fulfilled the simulated event is discarded
as unphysical.
The whole procedure is repeated until all nucleons in the participant region are
exhausted. The position, momentum, mass and charge of the clusters are then updated
at each step. If there is more than one possibility for aggregation at a given step, the
nucleon is absorbed by one of the fragments at random.
Once all the residues are defined with mass number, position, momentum and
angular momentum (A,, R.,-, P,-, L,), deduced from the nucleon properties in every
cluster, a clock is started corresponding to t = 0 fm/c for the forthcoming dynamics.
The fragments are propagated according to the Hamiltonian
Pr?
H: E; m, +ZVA.A,(IR.- —R.-I)- (5.9)
i