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This is to certify that the

dissertation entitled

DESIGN, CONSTRUCTION, AND USE OF THE
NEUTRON WALL ARRAY IN MEASURING THE
8Li(n,gamma)9Li ASTROPHYSICS REACTION

presented by
PHILIP DAVID ZECHER

has been accepted towards fulfillment
of the requirements for

Ph . D . degree in Physics

 

 

4am

Major’professor
Professor Aaron Galons

   

 

Date November 7, 1996

 

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1/93 WMpflS—nu

DESIGN, CONSTRUCTION, AND USE OF THE
NEUTRON WALL ARRAY IN MEASURING THE
8LI(n,y)9LI ASTROPHYSICS REACTION

By

Philip David Zecher

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

November, 1996

Abstract

PART I: A MEASUREMENT OF THE 8LI(n,y)9LI CROSS SECTION AT
ASTROPHYSICAL ENERGIES BY REVERSE KINEMATICS.

We have made the first attempt to determine the 3Li(n, )9Li cross section at
astrophysical energies. This reaction competes with the 3Li(a,n)llB reaction in
the inhomogeneous big bang model, and its reaction rate may affect the
primordial abundance of A > 12 nuclei. Using a radioactive beam of 9Li and the
coulomb dissociation method it is possible to measure the cross section of the
inverse reaction 9Li('y,n)8Li. The cross section of interest, 8Li(n, )9Li, can then be
determined with the detailed balance theorem. With a neutron separation
energy of 4 MeV for 9Li, nuclear dissociation into n+3Li competes strongly with
coulomb dissociation. Therefore, to separate the nuclear from the coulomb
contribution we attempted to measure the cross-section for 6 different targets
with Z ranging from 6 to 92.

PART II: A LARGE-AREA, POSITION-SENSITIVE NEUTRON DETECTOR
WITH NEUTRON/y—RAY DISCRIMINATION CAPABILITIES.

To further study neutron-rich halo nuclei, we have constructed a neutron
detector array. The array consists of two separate banks of detectors, each 2x2
meters2 and containing 250 liters of liquid scintillator. Each bank is position
sensitive to better than 10 cm. For neutron time-of—flight measurements, the time
resolution of the detector has been demonstrated to be about 1 us. By using the
scintillator NE-213, we are able distinguish between neutron and y-ray Signals
above 1 MeV electron equivalent energy. Although the detector array was
constructed for a particular experiment, it has been used in a number of other

experiments and will be a part of future experiments.

To my parents.

I fear from the experiences of the last twenty-five years that morals do not
of necessity advance hand in hand with the sciences.

Thomas Jefferson

m

I fear that machines are ahead of morals by some centuries and when

morals catch up perhaps there’ll be no reason for it.

Harry S. Truman

iii

Table of Contents

 

 

 

 

 

Abstract ii

List of Tables vi

List of Figures vii

Part I 1
A Measurement of the 8Li(n, )9Li Reaction Cross Section at Astrophysical

Energies by Reverse Kinematics. ............................................................................. 1

1 Introduction ........................................................................................................ 1

1.1 Purpose ..................................................................................................... . 1

1.2 Procedure ................................................................................................... 4

1.3 Design ......................................................................................................... 6

2 Experimental Setup ........................................................................................... 7

2.1 Fragment Telescope ............................................................................... 10

2.2 Neutron Wall Array ............................................................................... 17

2.3 Data AcquISItion .................. 24

3 Data Analysis ................................................................................................... 26

3.1 8Li Particle Identification ....................................................................... 26

3.2 Relative Energy Distribution ................................................................ 37

3.3 Geometry and Efficiency Considerations ........................................... 37

3.4 Virtual Photon Spectrum ....................................................................... 42

4 Results and Conclusions ................................................................................. 44

5 Discussion for Future Experiments .............................................................. 49

Part II 53

A Large-Area, Position-Sensitive Neutron Detector with Neutron / 'y-ray

Discrimination Abilities ........................................................................................... 53

6 Motivation ........................................................................................................ 53

6.1 Radioactive Nuclear Beam Advances ................................................. 53

6.2 The 11Li(y,2n)9Li Experiment ................................................................. 54

7 Neutron Detection at Intermediate Energies .............................................. 60

7.1 Scintillation Detection ............................................................................ 61

7.2 Efficiency .................................................................................................. 62

7.3 Tirne-Of—Flight Resolution .................................................................... 66

7.4 Pulse Shape Discrimination .................................................................. 69

iv

8 Neutron Wall Array Characteristics ............................................................... 72

8.1 Size, Configuration and Construction ................................................. 73

8.2 Time-Of-Flight Energy Resolution ....................................................... 81

8.3 Position Sensitivity ................................................................................. 81

8.4 Pulse Shape Discrimination .................................................................. 85

9 Operation .......................................................................................................... 89

9.1 Electronics ................................................................................................ 89

9.2 Calibrations ............................................................................................. 99

9.2.1 Pulse—Height Calibration .............................................................. 99

9.2.2 Time Calibrations ........................................................................ 100

9.2.3 Cross-Talk ..................................................................................... 102

10 The Neutron Wall Array in Recent Experiments ...................................... 105
10.1 Measuring the 8Li(n,'y)9Li Neutron Capture Cross Section (Zecher

et al.) ............................................................................................................. 106
10.2 Two-Neutron Correlations from Heavy Ion Collisions (Gaff et a1.)

....................................................................................................................... 106

10.3 Further Halo-Nuclei Structure Experiments (J. Kruse et al., J.
Wang et al., Y. Iwata et a1.) ........................................................................ 107
Bibliography 109

 

List of Tables

Table 1 - A list of the targets, their thickness’, and the energy loss for 28.53
MeV/nucleon 9Lipart1cles ...................................... 8

Table 2 - Particle Identification (PID) values and variances (6pm) for 9Li and for
its heavier dissociation products ...................................................................... 30

Table 3 - The radiative capture cross section calculated from the detailed balance
theorem. ............................................................................................................... 46

Table 4 - Reaction rates for the four theoretical estimates and our data. ............... 47

Table 5 - Listing of the predominant interactions for a neutron in the scintillator
NE-213. ............................................................................................................... 103

vi

List of Figures

Figure 1 - The primary reaction chain for A > 12 nucleosynthesis in a neutron
rich region. The solid lines indicate the primary production chain, and the
dashed line indicates a destructive chain leading to 4He production. ......... 3

Figure 2 - Schematic drawing of the coulomb dissociation method. ........................ 5

Figure 3 - A Schematic drawing Showing the arrangement of the equipment. The
Si-CSI telescope is 11.5 cm downstream from the target, and the center of
the Neutron Wall Array is 5 meters downstream from the target. ................. 9

Figure 4 - An exploded view of the fragment telescope. The beam is incident
from the right, and passes through a 300 um Si strip detector, followed by
a 300 um Si PIN diode detector and stops in a 0.5 cm thick CSI crystal. The
CSI crystal is viewed by 4 independent Si PIN diode detectors optically -
coupled to the backside of the CSI crystal. ..................................................... 11

Figure 5 Energy Spectra from the Si-Csl telescope for a 24.43 MeV / nucleon 9Li
calibration beam. The beam had an energy spread of 1/5 %. ........................ 14

Figure 6 - The particle-identification Spectrum for one Pb-target run. This
Spectrum Shows all events in the Si-CSI telescope that were in coincidence
with an event in the Neutron Wall Array, without a neutron gate. ........... 15

Figure 7 - The particle-identification spectrum for one Pb-target run. This
Spectrum is the same as for Figure 6, except the signal in the Neutron Wall
Array is passed through a neutron filter. ....................................................... 16

Figure 8 - A low energy pulse-Shape-discrimination spectrum for a cell in the
Neutron Wall Array. The square root of the product of the integrated
charge from the two PMT signals is plotted on the abscissa and the square
root of the product of the PSD Signals on the ordinate. ............................... 20

Figure 9 - A low energy pulse-shape-discrimination Spectrum for a cell in the
Neutron Wall Array. The integrated charge of the PMT signal is plotted
on the abscissa and PSD Signal is plotted on the ordinate. .......................... 21

Figure 10 - A high energy pulse-shape-discrimination spectrum for a cell in the
Neutron Wall Array. The attenuated integrated charge of PMT Signals is
plotted on the abscissa and the PSD Signal is plotted on the ordinate. ...... 22

vii

Figure 11 - Tirne—Of-Flight (TOF) Spectra for the Neutron Wall Array. The upper
pane shows the TOP without a neutron PSD filter applied to the data, the
lower pane Shows the TOP with a neutron PSD filter. ................................. 23

Figure 12 - Schematic drawing of the primary trigger logic. The boxes with the
dark outlines indicate the electronics set-up for each of the two primary
detectors. The modules labeled with fractions are down-scale units; they
produce a logic output for every 1/n or 1-(1 / n) logic input signal. ........... 27

Figure 13 - The PID spectra for a U target. The upper pane Shows the target
in/ target out data; the lower pane Shows the difference of the two. ......... 31

Figure 14 - The PID Spectra for a Pb target. The upper pane shows the target
in/ target out data; the lower pane shows the difference of the two. ......... 32

Figure 15 - The PID Spectra for a Sn target. The upper pane shows the target
in/ target out data; the lower pane shows the difference of the two. ......... 33

Figure 16 - The PID spectra for a Cu target. The upper pane shows the target
in/ target out data; the lower pane shows the difference of the two. ......... 34

Figure 17 - The PID spectra for an Al target. The upper pane shows the target
in/ target out data; the lower pane shows the difference of the two. ......... 35

Figure 18 - The PID Spectra for a C target. The upper pane Shows the target
in/ target out data; the lower pane Shows the difference of thetwo. ......... 36

Figure 19 - Relative energy distributions for the four largest-Z targets. The solid
circles represent the target-in data and the open circles represent the
target-out data ..................................................................................................... 38

Figure 20 - Neutron energy distribution for the Pb target. The dashed curve
represents the detection efficiency for neutrons in a detector with the
approximate characteristics of a cell in the Neutron Wall Array. ................. 40

Figure 21 - The detector acceptance and efficiency. The dip at 0.5 MeV is from
the asymmetric distribution of active cells in the Neutron Wall Array ........ 41

Figure 22 - The virtual photon number 11131, as calculated by the commercial
software package, Mathematica. ........................................................................ 43

Figure 23 - Radiative capture cross sections for 3Li(n,'y)9Li. The data points are
upper limits measured using the coulomb dissociation method with a U
target. All curves are for direct capture to the ground state, except

viii

Malaney & Fowler, which is for direct capture to the ground state and first
excited state. ........................................................................................................ 48

Figure 24 - The neutron energy distribution for neutrons in coincidence with 7Li
fragments from the C and Al targets. This shows the results from a target-
in/target-out backgron subtraction. The narrowenergy distribution
near the beam velocity is indicative of a peripheral collision. ..................... 52

Figure 25 - Experimental setup to measure the complete kinematics of »
11Li-)9Li+2n events (Sackett et al., 1991) .......................................................... 56

Figure 26 - A cutaway diagram of a neutron detector used by Sackett et al.
Neutrons are incident from the left. The scintillator is 5 inches in diameter
and 3 inches in depth for a volume of 59 in3 (0.97 liters) .............................. 57

Figure 27 - The two basic contributions to the light output distribution for a .
monoenergetic neutron beam incident on a detector (from Knoll, ref. 27).64

Figure 28 - Neutron detection efficiencies. The solid curve represents a simple
estimate of the efficiency based on equations 12 and 15. The points
represent the results of the Simulation code TOTEFF. The triangle data
points represent the efficiency from (n,p) scattering, and the square data
points represent the total efficiency for the same detector. ......................... 65

Figure 29 - A schematic drawing of a neutron being detected in a long scintillator
cell, perpendicular to the neutron’s direction. .................. 68

Figure 30 - A representation of the scintillation light output as a function of time
for different radiations in organic scintillator NE-213. ................................. 71

Figure 31 - The integrated charge in the tail of the pulse for neutron and y—ray
Signals, when the total integrated charge for both signals is the same. The
FOM for this Spectrum is 1.35. .......................................................................... 72

Figure 32 - A drawing of an individual detector cell from the Neutron Wall Array.
............................................................................................................................... 75

Figure 33 - A photograph looking lengthwise through a cell. The NSCL’S logo .
was placed at the opposite end of the cell. Only the center image is real,
the others are all reflections from the Sides of the cell. ................................. 75

Figure 34 - Schematic drawing of the passive voltage divider used on the
XP4312/04 PMT. ................................................................................................. 76

Figure 35 - A mechanical cutaway drawing of one of the Neutron Wall Array ...... 78

ix

Figure 36 - A schematic drawing of the bubbling procedure for a completed cell.
For about 1 hour, dry nitrogen gas is blown through the small Teflon tube
that is fed through the reservoir into the cell. ................................................ 80

Figure 37 - Top and Side view of the cell—mounting bracket. .................................. 81

Figure 38 - Neutron and y—ray TOF spectrum from Ar on H0 at 25 MeV / nucleon.
The prompt y-ray peak from the reaction is an indication of the overall
time resolution of the Neutron Wall Array ....................................................... 83

Figure 39 - The relative energy resolution for the Neutron Wall Array. The upper
pane shows the resolution as a function of neutron energy, for various
flight paths; the lower pane shows the resolution as a function of flight
path for various neutron energies. ................................................................... 84

Figure 40 - Time difference measurement of a collimated 5°Co source at three
different positions. The positions are separated by 1 ft (30.48 cm). ........... 85

Figure 41 - A PSD Spectrum from a cell in the Neutron Wall Array. The neutrons

and y-rays are from a PuBe source placed a few feet perpendicularly from
the center of the cell. .......................................................................................... 87

Figure 42 - Individual QFAST spectra for various QTOTAL values. The
QTOTAL values expressed in terms of equivalent electron energy. .......... 88

Figure 43 - The figure-of-merit from Figure 42 as a function of the equivalent
electron energy. .................................................................................................. 89

Figure 44 - Basic outline of the processing of the signals from the Neutron Wall
Array. Q1 through Q4 represent the charge signals: anode, attenuated ,
anode, PSD, and attenuated PSD. .................................................................... 91

Figure 45 - Pulse shape discrimination methods. ...................................................... 94
Figure 46 - A Simple schematic of the method of producing the PSD signal. ....... 96

Figure 47 - The circuit used to produce the PSD pulse. The circuit also acts like
an active splitter for the anode pulse. The QFAST output is gain matched

to the 50 (2 input of the Lecroy 2249W QDC. ................................................. 96
Figure 48 - The master trigger logic. The circled numbers reference timing points
whose relationship is Shown in ........................................................................ 97
Figure 49 - Timing diagram for the master trigger logic. ......................................... 98

Figure 50 - Sample pulse-height Spectra for three different y—ray sources. The
compton edge is used to obtain a calibration of pulse-height to electron
equivalent energy. The energies Shown are the compton electron energies.

............................................................................................................................. 101
Figure 51 - A laser time calibration for a cell in the Neutron Wall Array. The laser

pulses are separated by 20 ns. ........................................................................ 102
Figure 52 - A typical example of a cross-talk event. ................................................ 104

Figure 53 - The results of a monte carlo Simulation that models the effects on
relative momentum resolution caused by position resblution and out-
scattering ............................................................................................................ 105

xi

Part I

A Measurement of the 8Li(n,'y)9Li Reaction Cross Section at
Astrophysical Energies by Reverse Kinematics.

1 Introduction

A persistent question that arose from Edwin Hubble’s 1929 discovery of
the expanding universe is whether or not the universe contains enough mass to
halt its expansion. Given current cosmological theories, if the universe is to halt
its expansion (or is closed), one must conclude that most of the matter in the
universe is non-baryonic. This conclusion has lead many to search for exotic
forms of matter or so called ”dark matter.” While most of these attempts have
turned up empty, some recent enhancements to the standard big bang theory
suggest it is possible to close the universe with baryonic matter. An
observational test of these new theories might lie in observable quantities of
primordial A > 12 isotopes. To calculate these quantities, additional reactions
must be included in the nucleosynthesis chain. Many of these reactions have
never been studied. One reaction in particular, 3Li(n,'y)9Li, is the subject of our
study.

1.1 Purpose

The nucleosynthesis associated with the standard big bang (SBB) model
begins with a homogeneous neutron-to-proton ratio, and it constrains the
baryon-to-photon ratio, 11. (If one believes that the universe is closed, then the
constraints on 1] indicate that most of the matter in the universe must be non-

baryonicfl) A study by Witten2 of the quark-gluon plasma to hadron phase

2

transition in the early universe has brought into question the assumption of a
homogeneous neutron-to-proton ratio. In this study, an inhomogeneous density
distribution leads to the formation of neutron-rich regions. These regions
develop because the neutrons’ long scattering length alloWS them to diffuse more

easily than the protons from the higher to the lower-density regions.

Nucleosynthesis in these neutron rich regions would be very different
than in the SBB model, and has been modeled by Malaney & Fowler.3 These
models can successfully reproduce the observed primordial isotope abundanCe
up to 7Li--where the SBB ends—with a baryon density sufficient to close the
universe. As an observational test, Applegate, Hogan, and Scherrer have
suggested that nucleosynthesis in these neutron rich regions could lead to

observable amounts of A > 12 isotopes.4

The primary reaction chain leading to A > 12 isotopes, is
1H(n, )2H(n, )3H(d,n)4He(t, )7Li(n, )8Li,
8Li(or,n)11B(n, )IZB(B)12C(n, )13C(n, )14C,
with a weaker chain passing through the 7Li(oc, )11B channel.5 A reduction of up
to 50 percent in heavy element production could come from two leak reactions,
8Li(n, )9Li and 8Li(d,n)9Be.5 Figure 1 shows a schematic view of this reaction

sequence.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1 - The primary reaction chain for A > 12 nucleosynthesis in a neutron.
rich region. The solid lines indicate the primary production chain, and the
dashed line indicates a destructive chain leading to 4He production.

4

Malaney and Fowler comment ”that the predictions concerning the level
of A > 12 production remains uncertain due to the presence in the reaction chain
of the radioactive isotope 8Li which has a half-life of 0.9 S.”3 Of particular
difficulty is the radiative, neutron capture reaction, 3Li(n, )9Li , where both the
target and the projectile are radioactive. The theoretical estimates of the
8Li(n,'y)9Li cross section vary by more than an order of magnitude. We present
the first attempt at determining this cross section by making a kinematically

complete measurement of the inverse reaction 9Li(y,n)8Li.
1.2 Procedure

To study the radiative capture process of neutrons, such as 8Li + n —> 9Li +
7, one would typically bombard a target of 8Li with neutrons and measure the
energy of the emitted y—ray. Unfortunately, 3Li is unstable and has a half-life of
850 ms, much too short to make a useful target. Although it is possible, with the
advent of radioactive beam facilities, to produce a beam of 8Li, one would then
be required to produce a target of neutrons. Since these two methods are

extremely difficult or impossible, a new approach is needed to study this process.

A similar Situation exists for the radiative capture of protons with
radioactive beams; although proton (H) targets are possible, these targets are
very difficult to construct and add complications to the analysis of the reaction.
To study such difficult radiative capture reactions, Baur, Bertulani and Rebel6
suggested the method of coulomb dissociation. In this process, one measures the
photodisintegration cross section of the time reversed reaction 9Li + y —> 3Li + n,
by passing the 9Li through the virtual photon field of a large-Z nucleus. This
cross section can then be related to the radiative capture cross section by the

detailed balance theorem7

 

 

 

 

target

Figure 2 - Schematic drawing of the coulomb dissociation method.
2(2 j 9 u + 1) k:
(211,” + 1)(2j,, +1) k2

 

00.”)(8Li + n—>9Li + y) = 0(rln)(9Li + 7—98Li + n), (1)
where k is the wave number for the 8Li+n channel, k1' is the photon wave number,
and jsu, jau, and jn are the appropriate Spins. Figure 2 Shows a schematic

drawing of the coulomb dissociation process.

The nuclear coulomb field produces a continuum of virtual photon
energies. For each event it is necessary to measure the center of mass decay

energy Ed of the reaction products to determine the excitation energy

E =15 =E,+S,,, (2)

where Sn, the neutron separation energy, is the binding energy of the least bound

neutron in 9Li. Once the decay energy is known, the measured differential

coulomb excitation cross section do/dE, can be related to the

photodisintegration cross section a (M) by

6

E do
a ,.=—’-—. (a)
(7.) "£1 dB, ‘

where nm is the virtual photon number for the electric dipole transition, which is
assumed to dominate and is calculablefir8 The virtual photon spectrum is

discussed and calculated in section 3.4 on page 42.
1.3 Design

To measure the coulomb dissociation cross section, we used a 9Li beam of
28.53 MeV / nucleon, incident on a high-Z target. In previous experiments using
the coulomb dissociation method, the neutron separation energy of the projectile
was sufficiently small that coulomb dissociation was the dominant
reaction,9r1°rllr12 and nuclear effects were ignored. However, the neutron
separation energy for 9Li is 4.063 MeV. This reduces the available number of
virtual photons with sufficient energy to dissociate 9Li, thus reducing the
coulomb dissociation cross section. Consequently, we expect the measured 9Li

dissociation cross section to contain a non-negligible nuclear component.

Our interest in the coulomb component lead us to attempt to estimate the
nuclear component of the cross section. The coulomb dissociation cross section
depends, among other things, on Z2 of the target, and for very low-Z targets we
expect the coulomb dissociation to be negligible compared to the nuclear
dissociation. The nuclear dissociation Should scale with the square of the target-
plus-9Li radius. Therefore, if we normalize the nuclear dissociation to the
observed cross section for a low-Z target, we can then scale the nuclear cross
section for larger-Z targets where coulomb dissociation is expected to be
Significant. To measure the deviation from a pure nuclear dissociation in a
systematic way, we have measured the dissociation cross section for six different

targets with Z ranging from 6 to 92; see Table 1 for a complete list of the targets.

7
To determine do / dB, we used a measurement of cross section versus E,

Then we get a (m) from equation 3 and the radiative capture cross section from

equation 1. For each event 157 is determined by measuring Ed and using equation
2 with 5,. = 4.063 MeV. If we make a complete kinematic measurement of the
two reaction products, the 3Li fragment and the neutron, in the laboratory frame,
we can deduce Ed as follows: By knowing the masses of the 8Li and neutron, the
decay energy can be calculated from the relative velocity Vrel of the 3Li and

neutron, E, = 31qu

nail

where It is the reduced mass.” Vrel can be expressed as the

difference between the center of mass velocities in the 9Li frame of each product,
V", = V" -V.u. In the laboratory frame, VI” = V” +V,u and Vf‘; = V.“ +V91'J.

Therefore, the relative velocity of the two products is simply the difference
between their laboratory velocities, V", = Vf‘b - V313. The laboratory velocities

can be measured if we know the energy and direction from the target of each

reaction product.

Since big-bang nucleosynthesis begins when the temperature has dropped
to kT z 100 keV 1, the 8Li(n, )9Li reaction is of astrophysical interest for only low
energy neutrons. Consequently, we are primarily interested in photo-
disintegrations with photon energies of approximately 4.0 to 4.5 MeV. With a 9Li
beam energy of 28.53 MeV / nucleon, the beam velocity is much greater than the
velocities of the decay products; as a result, the decay products are forward

focused in the laboratory.

2 Experimental Setup

The measurements were performed at the National Superconducting
Cyclotron Laboratory (NSCL). The laboratory’s A1200 Spectrograph was used to

produce an isotopically pure beam of 9Li beam by a fragmentation reaction of 60

MeV/ nucleon 15Ni4+ on a thick 9Be target. The 9Li beam was sent into an
experimental hall where, after passing through a target, the charged fragments
were detected in a Si-Csl telescope and the neutrons were detected in the Neutron

Wall Array.

The strong forward focusing of the reaction products mandates that we
place both the fragment and neutron detectors at zero degrees with respect to the
beam. We can let the telescope act as the beam stop because the beam current for
a secondary radioactive beam is typically much less than a primary beam; for our
9Li beam the rate was about 5000 particles per second. Unfortunately, the zero
degree telescope provided more material than the target for 9Li in which it could
interact. To overcome this difficulty, we must make target-in and target-out
measurements and subtract the target-out from the target-in to observe the

reactions from the target.

Figure 3 shows a schematic diagram of the experimental setup. Not
Shown in Figure 3 are the A1200 Spectrograph and a small, thin plastic scintillator
detector placed after the telescope and subtending the same solid angle as the
Neutron Wall Array. This plastic scintillator was used to veto protons that made. it
to the Neutron Wall Array.

Table 1 - A list of the targets, their
thickness’, and the energy loss for 28.53
MeV/nucleon 9Li particles.

 

 

Target Thickness AE

(mg/cmz) (MeV)

238U 339.2 27.27
208Pb 495.0 40.77
120Sn 356.8 34.29
“Cu 282.1 33.12
27A1 261.9 34.38
12C 125.2 15.84

 

 

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2.1 Fragment Telescope

The fragment telescope must perform four functions: distinguish the 8Li
fragments from the unreacted 9Li beam particles and other fragments, provide
the energy and position information of the fragment and a timing signal for the
Neutron Wall Array. The particle identification is accomplished by using the
technique of a AE-E telescope. The energy loss AB of a particle passing through a
thin absorber is directly proportional to its mass and charge squared and
inversely proportional to its energy. When an E (stopping) detector is used to
measure the residual energy of the particle after it passes through the AB

detector, both the mass and charge can be determined for low-Z isotopes.14

The fragment telescope is composed of three individual detectors: a Si-
strip detector for position, AE and timing information; a Si PIN diode detector fOr
additional AE information; and a CSI stopping detector for residual energy
information. Figure 4 shows an exploded view of the fragment telescope. Both
of the Si detectors have an active area of 5x5 cmz. The Si-Strip detector was 309
um thick and the Si PIN diode was 322 pm thick. The Si-strip detector has 16
vertical strips on one side and 16 horizontal strips on the other side; when a Li
particle passes through the strip detector, most of the charge is collected on a
Single vertical strip and a Single horizontal strip, thereby identifying the location
of the event. The CSI detector is a 0.5-cm-thick square crystal, 6 cm on a side; it is
viewed by four Si-PIN diode detectors optically coupled to the back side of the
crystal. By summing the AE Signals from the Si detectors and comparing the sum
to the energy in the CsI detector, we can identify the different fragment isotopes

entering the telescope.

11

Fragment Telescope

 

 

 

 

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SItPlN dlogeldetecttqrs Si PIN diode
o View S CI'VS a 300 um Si strip detector
16 vertical by
16 horizontal
300 um

Figure 4 - An exploded view of the fragment telescope. The beam is incident
from the right, and passes through a 300 um Si strip detector, followed by a 300
um Si PIN diode detector and stops in a 0.5 cm thick Csl crystal. The CS]
crystal is viewed by 4 independent Si PIN diode detectors optically coupled to
the backside of the CSI crystal.

12

The Si detectors were calibrated with both an a-particle source and a
series of calibration beams. The energy deposited in the Si detectors by the oc-
particles is independent of the thickness of the Si detectors because the or-
particles’ range is sufficiently smaller than the thickness. Conversely, each
calibration beam passes through the Si detectors, and the energy deposited
depends on the thickness of the detectors. By comparing the two calibrations, we
were able to determine the thickness of each detector. The calibrated energy
signals were summed to produce the AE value for the particle identification and
then added to the CsI to obtain the fragments’ total energy. The lower pane of
Figure 5 Shows the AE Spectra for a 24.43 MeV/ nucleon 9Li calibration beam.
Very narrow momentum slits 04%) were used in the A1200 Spectrograph to
produce this beam; therefore the observed width represents the intrinsic
resolution of the Si detectors and not the energy spread of the beam. The

measured energy width is about 6% FWHM, or 0 A5 = 0.61 MeV. The asymmetric

distribution is a natural result of statistical nature of the energy loss processes.

The light output of the CSI crystal, as a function of energy deposited,
varies throughout the volume of the crystal. This produces very poor energy
resolution when the crystal is viewed as a whole by the PIN diode detectors. To
optimize the resolution, we use the pixel information from the Si strip detectors
to calibrate 256 discrete regions of the CSI detector. Although the resolution still
varies among regions, the mean resolution is about 2.6% FWHM; the best region
has a resolution of 1.5% FWHM and no region used in the analysis has a
resolution worse than 3.0% FWHM. The upper pane of Figure 5 Shows the
calibrated energy spectrum from the CsI detector for the 24.43 MeV/ nucleon 9Li

calibration beam. The measured width of the peak is 2.1% FWHM, or
0G;I = 1.71 MeV. The error in the Si detectors and the CSI detector combine to

13

give an error in the total fragment energy of o m, = 1.8MeV .

Figure 6 and Figure 7 Show the AE-E particle identification Spectra for the
Pb target runs. Figure 6 Shows the telescope events in coincidence with an event
in the Neutron Wall Array; Figure 7 Shows the same data when the Neutron Wall
Array event was required to be a neutron. In Figure 7 you can see the 1 / E energy
loss curves for 7Li, for 8Li, and very clearly for 4He. The bar running to the left of

the 9Li are mainly events that take place in the CSI crystal.

14

 

UUUUIUUUUIUUIIIUUleUUUU'UUIU

 

 

1
A 1500 1 CSI Detector j
> - .
é) .. u
E 1000 ~j -j
C
3 " .
C ‘ ..
3 u .
2 500 j j
g d \ '
O u U U U U l U U U U l U U j U l U U LU I T U U U l U U U U -

 

 

 

0 50 100 150 200 250 300

Energy (MeV)
1000 .+.. .. .. .. 1' .. ., .. ..

 

.
800 -3 Si AE Detector

600 5

400 -'-

.1

J
200 j

lllllLLllll'lllllll

 

 

Yield (counts/0.25 MeV)

 

O I I r U I I I I Uj I U U U I FT U U

0 1 0 20 30 40
Energy (MeV)

Figure 5 Energy spectra from the Si-CSI telescope for a 24.43 MeV/nucleon 9Li
calibration beam. The beam had an energy spread of 1A %.

15

 

 

E
V 20 '-
LLl
1O - :Ifi',i?;:':‘.. _ unreacted
,. ' 9Li peak

 

 

 

 

I I . U l I r U l U

0 60 120 180 240 300 360
Total Energy (MeV)

Figure 6 - The particle-identification spectrum for one Pb-target run. This
spectrum shows all events in the Si-Csl telescope that were in coincidence with
an event in the Neutron Wall Array, without a neutron gate.

16

 

 

 

 

40 I .l I
. j 1' ;
,I
30 - ..
g 'l .I- '
é
DJ 20 "‘ T
10 - -
int-5 '3-
. :4"- -
O .5. I r 1 I l I I I r U

 

 

0 60 120 180 240 300 360
Total Energy (MeV)

Figure 7 - The particle-identification spectrum for one Pb-target run. This
spectrum is the same as for Figure 6, except the signal in the Neutron Wall
Array is passed through a neutron filter.

17

2.2 Neutron Wall Array

We detect the neutrons with the NSCL’S Neutron Wall Array, a large, 2xZ
m2, position sensitive neutron detector. The Neutron Wall Array is discussed in
detail in Part H. Although the detector consists of two 2x2 m2 planes, only one
half of one plane was available at the time of the experiment. Each of the planes
consists of 25 2-m-long glass cells filled with a liquid scintillator. Each cell is
viewed by two photomultiplier tubes (PMT) optically coupled to the ends of the

cell.

The primary mechanism for detecting neutrons with energies of between 5
and 30 MeV is by observing the scintillation caused by a recoiling proton that has
elastically scattered a neutron. The proton scintillation process cannot provide
unique information about the incident energy of the neutron; therefore, it is
necessary to use the time-of-flight (TOF) method to determine the energy of the
incident neutron. When using the TOP method, the energy resolution is
dependent on two factors: the intrinsic time resolution of the detector and the
length of the flight path. The flight path was 5.07 meters from the target to the
center of the array; this value was chosen as a balance between the acceptable

solid angle subtended by the detector and the required energy resolution.

The Neutron Wall Array has a scintillator volume of about 500 liters, spread
through two thin detecting planes with a very large surface area. This
configuration makes it very sensitive to cosmic-ray and y—ray backgrounds, since
cosmic-ray efficiency is primarily dependent on the detector’s surface area. To
eliminate this background, the Neutron Wall Array was built using the liquid
scintillator NE-213, which has the well known property of pulse-shape-
discrimination (PSD), allowing us to distinguish between neutron events and 'y-

ray/cosmic-ray background events.

18

Although there are a number of different methods of performing this
pulse-shape-discrimination, most are not suitable to a detector as large as the
Neutron Wall Array. To overcome the difficulties of these existing methods, we
developed a new method which is discussed in Part 3. Our method produces a
signal called QFAST, which is proportional to the charge in the early portion of a
pulse from a PMT. If QTOTAL is the total integrated charge from a PMT, then
for an equal amount QTOTAL from a 'y-ray and a neutron, the value of QFAST is

different for y—rays and neutrons.

The scintillation light inside a cell is attenuated as it makes its way to each
PMT, so that the luminosity measured at the PMT is L pm = L0 e""/’1 , where Lo is
the initial luminosity, x is the distance to the PMT, and A is the attenuation
length. If the other PMT is a distance I (the length of the cell) away from the first

-(l—x)/1

PMT, then its measured luminosity is L PM“ = L0 e . If we take the square

root of the product of these two measured luminosities,

4.1
Lmeasured = LPMTLPMTZ =Loe / I

then Lmeasmd is independent of the position of the event within the cell. We use
the Lmeasured value to set a lower limit threshold in software. The limit we have

used is 2 MeV y-ray equivalent energy, or a 5.0-5.5 MeV neutron energy.

Figure 8, Figure 9, and Figure 10 Show samples of the PSD spectra. Figure
8 shows the PSD for the square root of the product of the total charge signals and
the QFAST signals. No software threshold has been applied to these data; as can
be seen, we have very good pulse shape discrimination for low energy neutrons.
Unfortunately, as the neutron energies increase, the PMT closer to the event
saturates, and the square root of the product method fails. For these events, we

use a spectrum such as that in Figure 9 to determine if the event was a neutron.

19

In Figure 9, we compare only one PMT signal to its corresponding QFAST signal.
If the pulse is large enough to saturate both PMT signals, then we use an
attenuated PMT signal; this is Shown in Figure 10.

The upper pane of Figure 11 Shows the TOF Spectrum for one cell in the
neutron wall, including all y—ray and cosmic-ray events. The TOF is measured
between the Si-Strip detector and the mean-time of two PMTS on a cell in the
Neutron Wall Array. It is not possible to know an exact zero for the clock from the
electronics; therefore, we use the observed prompt y—ray peak in the TOF
spectrum to determine the time-equalS-zero channel. The prompt y-ray peak 'is
produced when the Neutron Wall Array detects the 'y-rays from nuclear
interactions in the target and telescope. Since their time is independent of their
energy, and we know the flight path, we are able to determine when the clock
started. The inset to the upper pane of Figure 11 shows the y—ray peak in more
detail. The width of this peak is a measure of the intrinsic time resolution of the
neutron detector and allows us to know the neutron energy resolution. For a 24-
MeV neutron, over a 5-meter flight path, the FWHM energy resolution will be
3%. The lower pane of Figure 11 shows the neutron only TOF Spectrum for the

cells used during the experiment.

20

 

IIII'IIIIIIIII'IIIT'IIIT.

    
    

QFAST (arbitrary units)

neutrons

 

IllllllllllJllllJlllllll

IIIIIIILIIIIIIIrn-ni'o

 

 

 

I I I I I I I I I I I I I I I I I I I I I Ififi

0.0 0.5 1.0 1.5 2.0 2.5
E) (MeV)

Figure 8 - A low energy pulse-shape-discrimination spectrum for a cell in the
Neutron Wall Array. The square root of the product of the integrated charge
from the two PMT signals is plotted on the abscissa and the square root of the
product of the PSD signals on the ordinate.

21

 

    

y-rays / cosmics

QFAST (arbitrary units)
llllllllllllllllLlllllll
"3
IllLlllLlllllllLlJlLLill

 

 

 

cl
J
T
-1
J
1

Figure 9 - A low energy pulse-shape-discrimination spectrum for a cell in the
Neutron Wall Array. The integrated charge of the PMT signal is plotted on the
abscissa and PSD signal is plotted on the ordinate.

 

      

'y-rays/
cosmics

neutrons

 

QFAST (arbitrary units)
ILLIIIIIIIIJIIIJLIIIJIII

Allll'llelllllJllllLlllL

IIII'rIII'

0 1 0 20 30 40 50
E) (MeV)

  

 

 

 

Figure 10 - A high energy pulse-shape-discrimination spectrum for a cell in the
Neutron Wall Array. The attenuated integrated charge of PMT signals is
plotted on the abscissa and the PSD signal is plotted on the ordinate.

23

 

   
  

 

60 I I I I I I I I I I T T I I I ' I ' I .
.. 12 I FWHM

A 4 prompt 10 1.1 ns

2 - tray ""T 3 . _

g 40 .. peak 4 i i .

Q - . q I ‘ l l 1'

5g - / g h” I“ ' “IL LI"

:3 - -4.8 -2.4 0.0 2.4 4.8‘

g 20 - -

E . .

.93 " '

>. .

0 _ WI.
8 300 - ‘
w - i
<1: .. ..
Q . ..
.22 200 - '
c _ .
3
o q ..
3 ‘ .
'2 1 00 " -
a) . .
'>‘- i i
O - u I I I I I I I I I

 

 

 

 

O 48 96 144 1 92
Time-Of-Flight (ns)
Figure 11 - Time-Of-Flight (T OF) spectra for the Neutron Wall Array. The

upper pane shows the TOF without a neutron PSD filter applied to the data,
the lower pane shows the TOF with a neutron PSD filter.

24
2.3 Data Acquisition
There are three primary sub-systems in the data acquisition system. The
first sub-system acquires data from the Neutron Wall Array, and is described in
more detail in Part II. The next sub-system acquires data from the Si-CSI
telescope, and the final sub-system contains the master-trigger logic for

controlling the data acquisition computer.

The Neutron Wall Array acquisition system provides direct and attenuated
pulse-height information and a QFAST signal for each of the PMTS in the array.
In order make the time-of-flight (TOF) measurement, the acquisition system also
provides a time value for each PMT, measured against some common step
Signal. For this experiment, the common stop Signal comes from the Si-CSI

telescope.

The stop signal from the Si-CSI telescope comes from a logical OR of the
timing Signals from the 16 vertical strips of the Si-strip detector. Each of the
vertical strips is connected to a pre—amplifier that provides a slow Signal
proportional to the energy deposited in the strip, and a fast timing signal. Only
the energy Signal is used for the horizontal strips. A 16-channel programmable
constant-fraction-discriminator is used to produce a logic signal from each of the
16 fast signals. Since the TOF stop Signal passed to the Neutron Wall Array is
created by a logical OR of 16 different channels of electronics, the TOF must be
corrected for the time variations between the channels. When a signal is
produced in a strip by a charged particle passing through the strip, the
neighboring strips also create small Signals. The signals in the neighboring strips
are about two orders of magnitude less than in the strip that was hit. These
smaller signals will not pass the threshold of the constant fraction discriminator

and will not contribute to the logical OR signal. Therefore, we calibrate the

25

Neutron Wall Array TOF with each of the 16 vertical strips, and for each event We
use the calibration from the strip with the maximum pulse height to calculate the
TOF.

A logical AND is formed from the logical OR signal of the Si—strips and
the computer-not-busy Signal; this AND Signal is then used to create the gate
signal for ORTEC AD811, peak-sensing, analog-to-digital—converters (ADC) that
measure the pulse-height signals from the all the strips, vertical and horizontal.
This same gate Signal is also used for AD8115 that read out the Si PIN diode

detector and the four CSI light detectors.

For this experiment, a valid event is one where signals from the Neutron
Wall Array and the Si-CsI telescope fall within some specified coincidence period.
Registering this coincidence and instructing the computer to read out the data
from the appropriate modules is the primary purpose of the master-trigger sub-

system, Shown schematically in

Figure 12. The coincidence period is defined as the 250 nanoseconds following
an event in the Si-CSI telescope; a time sufficiently large to allow the lowest

energy (slowest) neutrons that we can detect to make it to the Neutron Wall Array.

To calculate the dissociation cross section, we must know the number of

9Li particles incident on the target. We determine the number of incident

26

particles by counting the number of 9Li’s that enter the telescope, Since the vast
majority of the 9Li particles do not interact in the target and pass into the
telescope. Counting all of these events would create an unacceptable dead-time
in the electronics, and many coincidence events would be missed while the
computer is busy counting the beam particles. Therefore, we only record
1 / 200th of events in the Si-CSI telescope that are not in coincidence with an event
in the Neutron Wall Array. To provide information during analysis on the
performance of the Neutron Wall Array, we also record 1 /1000 of the Neutron Wall

Array events that are not in coincidence with an event in the Si—CSI telescope.

3 Data Analysis

The primary difficulty in the data analysis is the identification of the 8Li
particles in the telescope. The procedure we used is discussed in section 3.1.
Once the 8Li-neutron coincidences are identified in the data, the 8Li+n system’s
decay energy is calculated from the measured kinematics (see section 3.2). Then
decay energy spectra for each target are folded with a decay energy dependent
detection efficiency to obtain a measured dissociation yield. Using this measured
dissociation yield and the detailed balance theorem, we determine the 8Li(n,y)9Li

radiative capture cross section.
3.1 8Li Particle Identification

To calculate the dissociation cross section for 9Li, we must determine if the
particle entering the telescope was 8Li. Although the coincidence requirement
with a neutron in the Neutron Wall Array eliminates all but a few of the 9Li from
the telescope spectrum, it is necessary to distinguish the 3Li from 7Li, 4He, and

reaction products produced in the telescope itself.

27

 

Neutron Wall Array

 

Primary Trigger Logic

 

 

Fast Clear F J

1-1/1000

 

NWA Trigger

 

 

1/1000F1

 

 

 

 

 

 

TOF Stop F

Si-CSI Telescope

 

 

U 0

Computer
Start

 

 

 

 

 

 

 

ADC Gate “20° —

 

 

 

 

 

Telescope Trigger

 

 

 

jComputer Busy

 

 

 

 

 

 

 

 

Figure 12 - Schematic drawing of the primary trigger logic. The boxes with the
dark outlines indicate the electronics set-up for each of the two primary
detectors. The modules labeled with fractions are down-scale units; they
produce a logic output for every lln or 1-(1/n) logic input signal.

28

The strong forward focusing of the reaction products, when the decay
energy is low, requires us to place the telescope detector at 0° with respect to the
beam. Unfortunately, the unreacted 9Li’s entering the telescope have much more
mass to interact with than they had in the target. If an interaction in the
telescope produces a neutron that is detected, it is difficult to determine if the
event took place in the target or the telescope. To remove this telescope
background, we measure the cross section with the target and without the
target, the so called target-in and target-out measurements, and subtract the
target-out cross section from the target-in cross section. To avoid creating
systematic errors between the target-in and target-out measurements, the beam
energy for the target-out measurement was reduced by an amount equivalent to

the energy loss in the targets.

Although it is possible to identify the 8Li isotopes from the AE-E Spectrum
shown in Figure 7, it is nevertheless difficult. To make the task easier, we take
advantage of the fact that light ions, such as He and Li, obey the empirical
power-law formula R = aE “ , where R is the range of a particle with an energy E
and a and a are constants.15 Using this power-law relationship, we derive a
particle-identification (PID) function that is directly proportional to the mass and

charge squared (A22). The PID function can be expressed as
PID: E“ -(E—AE)" (4)

where E is the total energy of the fragment entering the Si-CSI telescope, AB is the
energy loss in the Si detectors, and (118 selected to provide, for a given fragment
species, a constant value of PID over the energy range of interest. For our range

of energy, values of a from 1.75 to 1.80 are acceptable. We used a = 1.78.

29
The width of the PID peak for a given isotope can be found by using the

two individual detector resolutions. For any function of two parameters,

x = f (u,v) , where the variance of each parameter is known, the variance of x is

._ .3152 .352
ax—au(a‘) ”(3’) (5)

if u and v are uncorrelated.16 If the variances of the Si and CSI detectors

given by

correlated, the error in one must influence the error in the other. Since the
electronics are sufficiently separated, there are no electronic effects to cause a
correlation. Although there is energy straggling in the Si detector, which could
affect the energy deposited in the Si and C51 in a correlated fashion, the expected
energy straggling is about 0.3 MeV, less than half of the measured telescope

resolution of 0.61 MeV. Therefore we will assume that the errors in the CSI and

Si-AE signals represent the intrinsic resolutions of the detectors and thus 0 Eu

and 0 A5 are uncorrelated. Using equations 4 and 5 and E = EC“ +AE , the
variance in PID is given by

aim = 0110:“ (E"“ — E22152 + GZEEZ‘H’]. (6)

Using equations 4 and 6, we get the following values of PID, and 6pm for 9Li and

for its heavier dissociation products.

30

Table 2 - Particle Identification (PID) values and variances (arm) for 9Li and for
its heavier dissociation products.

 

 

Isotope Etotal Ec51 AB PID 691])
(MeV) (MeV) (MeV) +20 +20

9Li 210 185.6 24.4 134.2 4.0
8Li 185 160.4 24.6 ’ 121.6 3.6
7Li 165 140.8 24.2 108.9 3.3
6Li 140 115.6 24.4 95.4 2.9
8He 185 174.1 10.9 55.7 1.6
6He 140 129.2 10.8 44.2 1.3
4He 95 84.3 10.7 31.7 0.9

 

 

The upper pane of Figure 13 shows the PID spectrum for the uranium
target and for the target-out (telescope contribution); the lower pane shows the
difference between the two Spectra. The following five figures Show the same
information for each of the other five targets: Pb, Sn, Cu, Al, and C. The mest

prominent features in each of these spectra are the 7Li and 4He peaks.

The 3Li yield in these figures is very small, but it can be observed in the
four heaviest targets. The 7Li peak in each Spectrum is clear enough that we are
able to fit it to a gaussian distribution. Allowing all the parameters to vary, we
obtain good agreement between the centroid and width values in Table 2 and the
fit results. The fits to the 8Li peaks Shown are the result of fitting the 7Li and 8Li
region of data to a double gaussian distribution, holding fixed the centroids and
widths. (The 4He peaks are half way between the expected 4He and 6He peaks
from Table 2; this is attributed to the non-linear response, and projectile-Z
dependence of the CsI light output.)

31

 

TIrIITjII'IjITleII'IIII'Ijjf

 

150 _ U target - . -
------------------- blank target q; ,,
o ' W? q
E _ ;-§’ 5 _
Q . :n
.23 1 :::§ -:§: 3:“; d
S . . """ I '°
o 50 - '2} ''''' "
o | i
O _ IL .I "3

counts/20 PID
lb m
o o o
l _l—_—] l l I
F“
«F'
“F.—
_&
J;—
fig:
[EL-:—
I ‘:_ l

 

 

 

IIII'IIIIIIIIIIIIIIIIIIIIIIII

0 25 50 75 100 125 150
PID/20

Figure 13 - The PID spectra for a U target. The upper pane shows the target
inltarget out data; the lower pane shows the difference of the two.

32

 

IIII.IIII'IIII'IIII'TIII1IIIT

 

 

 

 

150 .. —— Pb target ' -
------------------- blank target .
e
Q ': '~.
g .2 ”ii:
8 50 - 22:3 -
q If it“ .
....... " ‘
0 _ ' . . _ "”
' 60 ': 4H6 ' 7Li 1
Q 40.. \ V a
D. - .
2°" l l l
‘3 0 ..L_1__:_____!.l_l.!_!..__.!.l_l.!-._!.l .....................
8 .l ' In ‘l'ij I {Wt-E '
0
-20 - .
'40 IIIIIIIIIIIrITUfiII'UIII'TIIT
0 25 50 75 1 00 125 150
PID/20

Figure 14 - The PID spectra for a Pb target. The upper pane shows the target
inltarget out data; the lower pane shows the difference of the two.

33

 

UIUU'IIIf'IIII'IIII'IIIIr‘rII

 

80 - Sn target ' "
- ------------------- blank target 1
60 - -

counts/20 PID

 

 

 

 

 

 

 

counts/20 PID

 

 

 

 

 

-20 IUIU‘W'jTIIIIIIIIIIIIIIIIIUII

O 25 5O 75 100 125 150
PID/20

Figure 15 - The PID spectra for a Sn target. The upper pane shows the target
inltarget out data; the lower pane shows the difference of the two.

 

120-
100%

counts/20 PID

40-1

counts/20 PID

IIII'IITI'IIrj'II

 

II'IIIrIIIII

 

 

 

LL_. _ sun 1.. . 1M4

"I'Hl'lll'l'l'

Cu target .

------------------- blank target -

gs." 3 _

. :- xii. '1
Eli;- - J”: lviu’I-llllLllllll .J....
I I I FI I I I I T I I I I I I I I I r t

4H9 7Li 8Li .

 

 

Ir'finv

 

 

'20 'j"l"'r[""F""lTr"l"T'l
O 25 50 75 100 125 150
PID/20

Figure 16 - The PID spectra for a Cu target. The upper pane shows the target
inltarget out data; the lower pane shows the difference of the two.

counts/20 PID

count/20 PID

Figure 17 - The PID spectra for an Al target. The upper pane shows the target

160

35

 

120-

I I I I I ' 1
- —— Al target ‘ ‘
------------------- blank target -
ss . .3 ‘

N
=:
u I ‘ ' .
3 i k.
.0 I. d
3" l ’3 r i: ' ' '
: :coqnv
, ”a"
_ . . s 1...."1 n I I 1 l

III'IIII'IIIIrjIII'ITII

 

  

 

 

Ls-..” lJull..IL.h lllll '.U.
s '1||I'l|l H W'-

 

 

 

IIII'jIIIIIIrI'IIII'jIII'IIIj

25 50 75 100 125
PID/20

inltarget out data; the lower pane shows the difference of the two.

150

36

 

120 IIII'IIITIIITr‘fIfir'IITjTrTfi

100 _ —— C target ..

- - ------- blank target .

Q .-

n. .

O _
Q

(I) ..

‘E ..
3

o .
O

 

 

 

 

 

counts/20 PID

 

 

 

 

 

rIIIlIIII'IIIIIrIIIlIIII'IIII

O 25 50 75 100 125 150
PID/20

Figure 18 - The PID spectra for a C target. The upper pane shows the target
inltarget out data; the lower pane shows the difference of the two.

37

3.2 Relative Energy Distribution

Using the PID value from the telescope discussed in the previous section,
we identify the 3Li fragments as 1.5 times those within to of the expected 8Li
PID value. This conservative definition of 8Li PID is necessary because of the

large yield of 7Li. For each identified 3Li fragment, we use its total energy and

direction from the target to determine its lab velocity, V312. We do the same for
each neutron, and obtain Via”. Then, the relative decay energy Ed of the excited
9Li is given by

15,:1 v2

2 (7)

where p is the reduced mass and V", = V?" - V1”; . The yields for relative decay

energy bins 0.5 MeV wide are shown in Figure 19 for the four heaviest targets.

The error bars shown are statistical.
3.3 Geometry and Efficiency Considerations

To obtain the dissociation cross section from this measured yield, we must
know the efficiency of our detection system. Two factors constitute the detection
efficiency: the intrinsic efficiency of each detector, and the geometrical
acceptance of the detectors. The geometrical acceptance decreases with
increasing decay energy, therefore, the efficiency must be folded with the decay

energy distribution and cannot be applied to the total yield.

38

 

150€T"Tj"l'TTTT"I"'
.. U

IIIIIIIIIILIIJI

 

 

N

O

O
l

_L

O

O
11:11::

 

O

 

 

01
O

.U U o

IIIILIIJLLIIIIIIIII

IIIIIIIJIIIIIIII

Yield (counts/0.5 MeV)

 

IJLIIJLIIIIII I I I I I I I I I

_L .3 a
01 o 01
o 0 w

   

'IIII

 

 

O
—L
N
GD
&
U'I

Decay Energy (MeV)

Figure 19 - Relative energy distributions for the four largest-Z targets. The
solid circles represent the target-in data and the open circles represent the
target-out data.

39

The Si-CsI telescope has an intrinsic efficiency for particles that enter it of
8:1. The Neutron Wall Army is a more difficult problem. Since we detect a
neutron by detecting the scintillation from an elastically scattered proton, the
efficiency depends primarily on the (n,p) cross section and the thickness of the
detector. Many computer simulations have been developed to calculate the
efficiency of a neutron detector of a given geometry; we use a code called
TOTEFF." The efficiency can vary significantly, depending on the velocity of the
incident neutron; fortunately, our neutron velocities are narrowly focused about
the beam velocity, see Figure 20. Therefore, we use the average efficiency of

8:0.11 for cells in the Neutron Wall Array.

Once these intrinsic efficiencies are known, we determine the overall
efficiency by using the monte carlo method to simulate various decay energies
and the geometrical acceptance of our apparatus. The curve of efficiency versus
decay energy is shown in Figure 21. The clip around 0.5 MeV decay energy is
attributed to the unsymetrical distribution of active Neutron Wall Array cells

about the 0° beam position.

40

 

 

 

 

 

 

50 IIIIIITIIIIIVI'IIIIIIrit
.—+—Pb target _ 0.14
- ~ .‘ ?\- —<>— - blank target
40- - 0.12
A ‘ - 0.10
E 5’
Q - 0.08 =-
s q s
3 ' rn
o a;
3 204 a 0.06 e.
'0 (D
E - 8
>. . . ~<
- 0.04
10- -
- 0.02
O UéIUlITOIj'IIIIIYUIj'IIIl 0.00

 

 

0 10 20 30 40 50
Neutron Energy (MeV)

Figure 20 - Neutron energy distribution for the Pb target. The dashed curve
represents the detection efficiency for neutrons in a detector with the
approximate characteristics of a cell in the Neutron Wall Array.

41

 

CD

I I I j I IV I I I I l I I I I

01

.h

03
IIIIIJIIIIIIIIJIIIIIIILIJIIII

/. ‘
ILIIIIIIIII/

I l I I I I l I I I I I I I I I

N

Acceptance (percent)

—L

 

 

O

 

. I ' ' I ' ' I '
0.0 0.5 1.0 1.5 2.0
Decay Energy (MeV)

Figure 21 - The detector acceptance and efficiency. The dip at 0.5 MeV is from
the asymmetric distribution of active cells in the Neutron Wall Array.

42

3.4 Virtual Photon Spectrum

To determine the photodisintegration cross section from the measured
dissociation cross section, we must know the virtual photon number for each
target nucleus. The virtual photon spectrum calculation and coulomb
dissociation process are discussed at length in a review paper by Bertulani and
Baur.13 Sackett has shown that their relativistic approximation for the E1 virtual
photon number is valid for 11Li in our velocity range.19 Sackett also
demonstrated that there is little dependence on the projectile radius, and the
relativistic approximation is independent of the mass of the projectile; therefore

we use this relativistic approximation. The virtual photon number 1151 is given

by

nEl (E, /h) = izfiafif {mom - £32 x2(K12 - 1(3)} , (8)

C

where Ko and K1 are modified Bessel functions of the argument 3:, with
x = Erbmin /h7 v . The minimum impact parameter, bmin, is the sum of the target

and projectile radii. Figure 22 shows the virtual photon spectra as a function of

photon energy for the six targets used.

43

 

        

 

 

 

II|I I I 1 -IIIqI I I I -IIIII I I N .-III‘I I I \ -III\.II I I I

I \. . m I

I \. . us I

u . \ .. x .

b n u I . . c. ..
I upscAc . \ t t :8
. . _ u . ~ \ x s. I
j _ u . m u .. \
I _ . m n u .\ I
_ H _ _ u c.

I \ .. N fl

I .\ x u l 6

T .\ .x s. I

I .s. H. I

I .x. n. I

I .x. r 4

I \. \s I

I . K. I

I \. I

I t. I 2

I \. ~\ I

1 . x u

\s

I \\ '

I:II I I 1 -I1II I 1I I -IIIqI q I I -IIII II I I -IIIII I I I O
4 3 2 1 O 4.|
0 0 0 0 O O

E

Figure 22 - The virtual photon number n51, as calculated by the commerci

software package, Mathematica.

4 Results and Conclusions

Originally, the analysis was to follow this procedure:

1. measure the 9Li -—> 8Li + n dissociation cross section for six targets with Z
from 6 to 92 (i.e. C, Al, Cu, Sn, Pb, U),

2. assume the 9Li dissociation cross section for C and A1 are primarily ‘
nuclear and then use the Z-dependence in a nuclear model to calculate the
nuclear cross sections for the four higher-Z targets,

3. subtract the calculated nuclear cross sections from the measured cross
sections for the higher-Z targets to obtain their coulomb cross sections,

4. finally, use these coulomb cross sections in the detailed balance theorem to
obtain the ground state direct capture cross section, 3Li(n, )9Li.

Unfortunately, we did not observe any 8Li in the target-out subtracted PID
spectra for the C and Al targets (see Figure 17 and Figure 18), and are unable to
calculate the corresponding dissociation cross sections. Without a measurement
of these dissociation cross sections, we are not able to make any estimate of the
nuclear dissociation cross sections for the heavier targets. To proceed, we can

only discuss the present data in terms of limits to the coulomb cross section.

If all of the 8Li production is assumed to proceed via coulomb
dissociation, we obtain an upper limit on the coulomb dissociation cross section.
With this extreme assumption, it is wise to consider only the highest-Z targets,
Pb and U, where coulomb would play its largest role. If Y is the 8Li yield for
some range of decay energy where the 8Li PID value was within :l:o ”D of the ’

expected value, then the measured dissociation cross section, 0 mm“, , is given by

_ 2 Y
measured 2 8 NT N}; I

 

0'

(9)

45

where e is the detection efficiency for the measured decay energy, Np is the

number of incident 9Li particles, and N T is the number of target atoms per square

centimeter. The 3/ 2 factor is included to compensate for our limited to m, range

when selecting the 8Li from its PID measurement. We obtain the differential

cross section do/dE, by dividingo by the range of decay energies used to

measured

obtain a From the differential cross section, the coulomb dissociation

measured ’

cross sectlon o (M) 18 found usmg equatlon 3,

E, do

0' = — .
(7. ) "El (157

(3)

Finally, we use the detailed balance theorem, equation 1, to calculate the I
radiative capture cross section, 8Li(n,y)9Li. Table 3 shows the results of these
calculations for the Pb and U targets. For each target, two decay energy bins, 0.5

MeV wide, from 0 to 1.0 MeV were considered.

The radiative capture cross sections shown in Table 3 are only upper
limits. To obtain a lower limit, we could assume that all of the observed 8Li yield
from the intermediate targets (Cu and Sn) is from nuclear processes, and then
use these values to estimate the nuclear component for the heavier targets, much
as we were going to do with the C and Al targets. Since calculating the ‘
measured cross section is dependent on knowing the decay energy, we would
have to use the yields from the decay energy spectra, Figure 19. Although there
is an observable 8Li yield in the PID spectra for the intermediate targets, their
yields are not statistically significant when broken down by decay energy. .
Therefore, we cannot use them to place any meaningful lower limit on the

radiative capture cross sections.

46

Table 3 - The radiative capture cross section calculated from the detailed
balance theorem. '

Decay Observed
Target Energy Counts Cmeas(Ey) n(B,) om.) (SIM)
(MeV) (mb) (mb) (mb) (mb) (uh) (uh)
U 0.0-0.5 29.4 116.1 13.9 17.6 225.44 0.53 10.3 19.1 110.4
U 0.5-1.0 21.2 114.9 10.2 17.1 186.05 0.53 10.4 7.9 15.5

 

 

Pb 0.0-0.5 29.9 120.0 10.8 17.2 188.56 0.50 10.3 17.8 111.91
Pb 0.5-1.0 23.6 118.6 8.7 16.9 154.16 0.54 10.4 8.1 16.4

 

Although we have only been able to determine an upper limit to the
radiative neutron capture cross section for 8Li, it is still of some use for its
astrophysical implications considering the large range of theoretical estimates. _
These estimates are shown in Figure 23 along with our measured limits from the
U target. The estimates shown are for the direct capture to the ground state
unless otherwise noted. We only compare our data to the ground state capture
since we assume the 9Li is in its ground state when we measure the inverse
reaction 9Li('y,n)3Li. The direct capture to the ground state should be stronger ‘

than to the one bound excited state at 2.691 MeV20 since the transition strength

3 .
goes as E, .

A neutron-unbound state exists at 0.247 MeV above the one neutron
separation threshold. Although there are no experimental determinations of this
state’s spin-parity”, two shell model calculations by Mao and Champagne21
predict it to be J " = %’. Given this spin-parity, p-wave neutrons will dominate
the resonance capture cross section. Since neutrons at such a low resonance
energy are primarily s-wave, the resonance is not expected to contribute much to ‘
the cross section. Also, the only transition from this excited state is through

electric quadrupole radiation , further suppressing the resonance contribution.

Of the four theoretical estimates shown, two are based on models of the

9Li structure, and two are based on the systematics of similar nuclei. Of the

47

model calculations, we are in sharp disagreement with the spd-shell model ,
calculations of Mao 8: Champange21 and consistent as an upper limit with the
cluster model calculations of Descouvemont? Similarly, we are in disagreement
with the estimates of Maleney 6: Fowler,23 and consistent With the estimates of

Rauscher.24

Considering their astrophysical interest, radiative capture cross sections

are often discussed in terms of a reaction rate per particle pair, N A (do) , where

NA is Avogadro’s number. The reaction rate is given by

(av)=j¢(v)a(v)vdv, (10) .

where o(v) is the normalized Maxwell-Boltzman distribution, o(v) is the cross

section, and v is the relative velocity. This reaction rate is approximately
constant for low-energy, s-wave neutron capture because the cross section for the

direct capture of s-wave neutrons is proportional to l/v. By fitting a UV function i

to our data points, we obtain an upper limit to the reaction rate,

N A (av) = 9560 cm3 s‘1 mole". This rate, and the four theoretical estimates are

 

 

given in Table 4.
Table 4 - Reaction rates for the four theoretical estimates and our data.
Reaction Rate
cm3 5'1 mole-1
Malaney & Fowler 4.294x104
Mao 8: Champagne 2.10x104
NSCL-MSU 9560
Descouvemont 5280

Rauscher, et a1. 4650

 

48

 

 

 

 

 

 

 

 

103 - I I T I l I I I I l I I I I I I I I I 1
I - - - - Mao & Champagne, 1991 I
1‘. Descouvemont, 1993 "
‘. ------ Rauscher, etal, 1994
' ‘9 \ ----- Malaney & Fowler, 1989
- \‘ \ \ o NSCL-MSU, 1996 -
102:
3 I
3 _ .
a I
G I
.9 ..
0
9
D10":
1
1 00 I I I I l I I I I I I I I I ' I I I I
0.00 0.25 0.50 0.75 1.00

En (MeV)

Figure 23 - Radiative capture cross sections for 8Li(n,y)9Li. The data points are
upper limits measured using the coulomb dissociation method with a U target.
All curves are for direct capture to the ground state, except Malaney 8: Fowler,
which is for direct capture to the ground state and first excited state.

49

In an inhomogeneous early universe, nucleosynthesis in neutron rich
regions could produce an observable amount of A > 12 isotopes, whereas the
standard-big bang nucleosynthesis ends with the production of 7Li. The primary
reaction chain to heavy elements begins with 7Li(n,y)3Li(a,n)llB and has a weaker
channel beginning with 7Li(a,y)118. The 8Li(n, )9Li and 8Li(d,n)98e reactions
provide so called leaks from the main sequence, reducing the heavy element
production. These leaks could reduce the heavy element production by half
according to Malaney 82: Fowler.5 Our data suggest that the reduction might not
be so large.

In conclusion, we have used the coulomb dissociation method to
determine an upper limit to the 3Li(n,'y)9Li radiative capture cross section. The
very small yield from low-Z targets prevented us from making any estimate of
the nuclear contribution to the measured dissociation cross section from high-Z
targets. Without this estimate, our measurement can only be interpreted as an
upper limit. Even so, current estimates of this cross section range over an order

of magnitude, and our measurement is only consistent with the lowest estimates.

5 Discussion for Future Experiments

Had we determined a 9Li(y,n)3Li cross section for C and Al, we would
have then used it to estimate the nuclear cross section for the higher-Z targets.
Doing so would require a model for the nuclear dissociation. In one type of
model nuclear reactions would occur via central collisions, in another via
peripheral collisions. If the observed dissociation cross section was primarily
from central collisions, then the cross section would depend on the cross
sectional area of the target-plus-projectile, or roughly scale as A2/ 3, where A is

the atomic number of the target. If the dissociation cross section was primarily

50

from peripheral collisions, then the cross section would depend on the cross

sectional circumference of the target-plus-projectile, or scale as A1 / 3.

Further consideration is necessary to determine which model is more
appropriate. The detector geometry is a factor in determining whether we
observe central or peripheral collisions. Our detectors subtend only a narrow
forward angle, and by detecting both the neutron and the fragment in the
forward direction, we favor peripheral collisions. We can also observe the energy
distribution of the neutrons; while neutron velocities from peripheral collisions
will be peaked around the beam velocity, neutrons from central collisions will
exhibit an exponential decay distribution. Since the 8Li events from the high-Z
targets contain some amount of coulomb dissociation, which result from
peripheral collisions, their neutron energy distributions might not provide the

best evidence for peripheral or central nuclear collisions.

The data from the 9Li ——) 7Li + 2n events should contain a weaker coulomb
component because of the greater 2 neutron separation energy, 82,. = 6.1 MeV as
compared to $1,, = 4.1 MeV. The low-Z targets are still preferable for measuring
the nuclear effect, and whereas there are no 8Li events for the low-Z targets, there
are plenty of 7Li events as Figure 17 Figure 18 show. Figure 24' shows the
neutron distribution for the 7Li events from the C and Al targets (the target-out
background has been subtracted). Clearly, the neutron energy spectrum is
peaked near the beam velocity, indicating that we are detecting primarily

peripheral events.

 

‘ The neutron energy distribution shown in Figure 20 is not appropriate for this discussion because by
selecting neutrons from events with a given decay energy, we have selected a narrow range of neutron
energies.

51

One of the assumptions for calculating the nuclear dissociation cross
section by scaling cross sections from low-Z targets was that the dissociation
cross section would have two contributions, one nuclear 9Li —> 8Li + n, and one
coulomb 9Li —) 3Li + n. As we clearly see from the PID spectra, 9Li -> 7Li + 2n
and 9Li —) 6He + 3n are very strong channels. These channels’ actual relative
strength compared to the 9Li —) 3Li + n channel may be misrepresented in the
PID spectra because they could be producing more neutrons. A measurement of
the 2n cross section for the 9Li —> 7Li + 2n reaction would provide more
information about the nature of the reaction. The strong presence of the these
two channels increases the difficulty in modeling the nuclear dissociation cross
section for the 9Li('y,n)3Li reaction. These considerations will have to be
addressed when a better measurement of the 3Li(n, )9Li cross section is

attempted.

52

 

80 IIrTIIIII'IITT'IIII'TIII

20-

 

YIeld (counts/2 MeV)
4:- o:
o o
I I I I I I I
o
\
.\
./
./
./
I I I J I

 

 

 

 

'IIII'IIII'IIII'IIII'IIIII

0 1 0 20 30 40 50
Neutron Energy (MeV)

Figure 24 - The neutron energy distribution for neutrons in coincidence with
7Li fragments from the C and Al targets. This shows the results from a target-
inltarget-out background subtraction. The narrow energy distribution near the
beam velocity is indicative of a peripheral collision.

Part II

A Large-Area, Position-Sensitive Neutron Detector
with Neutron/y-ray Discrimination Abilities.

6 Motivation

We have constructed a large-area, position sensitive neutron detector with
neutron / y—ray discrimination capabilities, called the Neutron Wall Array.
Although the Neutron Wall Array was originally intended for a particular
experiment, it has already been used in a number of other experiments and has
been incorporated into the design of the future experiments of other research
groups using the NSCL facility. Section 6 discusses the original motivation
behind the construction of the Neutron Wall Array and the original design
requirements. Section 7 is a brief discussion of some principles of neutron
detection methods used by the Neutron Wall Array. An outline Of the physical
construction and a demonstration of the primary functions of the detector are
shown in section 8. Section 9 presents some of the operational procedures used in
the experiments to date. Finally, section 10 outlines the three experiments that

have used the Neutron Wall Array.
6.1 Radioactive Nuclear Beam Advances

Our original motivation for building the Neutron Wall Array was to
improve and extend measurements made at the NSCL of neutron-rich nuclei.
These measurements are part of an extensive research program the NSCL has
developed around its production of radioactive nuclear beams (RNBs). One of

the biggest opportunities RNBs offer is the chance to systematically study

53

54

isotopes far from the limits of stability. Many of these unstable isotopes are very-
neutron rich, and neutron detection has played an important roll in probing their
structure. The nature of RNBs has also lead to a change in the size and scope of

the neutron detectors that are needed.

The primary difference in working with RNBs is a great reduction in
intensity compared to beams of stable nuclei. RNBs are the result of sifting
through the many fragmentation products produced when an intense primary
beam of heavy ions strikes a thick target. Consequently, radioactive beam
intensities are much lower than primary beam intensities. A typical experiment
with a stable beam would have an intensity of about 1012 particles per second; in
contrast, an experiment with a radioactive beam might have an intensity of only
hundreds or thousands of particles per second. With such low beam intensities,
it is preferable to cover as much solid angle as possible to reduce the beam time

necessary to acquire sufficient statistics for an experiment.

The reduction in beam intensity is even more acute if two or more
neutrons must be detected in coincidence. Unlike most charged particle
detection, neutron detectors do not have an efficiency of 100 percent; a common
value for one-neutron detection efficiency is about 10 percent. If two neutrons
are to be detected in coincidence, then the efficiency falls to 1 percent. Since the
two-neutron detection efficiency goes as the square of the one-neutron efficiency,

it is useful to try to increase the one-neutron efficiency.
6.2 The 11Li(};2n)5’Li Experiment

One of the early clues to the richness of RNBs was the anomalous large
cross section for 11Li interacting with various target nuclei.25 For years the

nuclear density was thought to be nearly constant throughout the table of

55

isotopes, but studying 11Li has demonstrated that far from stability, the final
nucleons added to a system may form a low-density halo around a more
common core. A recent experiment that studied 11Li is an excellent example of
the added complexities of working with neutrons from 'RNB experiments. First,
the experiment required two neutrons to be detected in coincidence, and second,
the available beam intensity was only 500 llLi per second. The Neutron Wall
Array was constructed to replace the neutron detectors used in this experiment
with a device that has better resolution and efficiency. What follows is a brief

description of this experiment and the limitations of the neutron detector used. .

In 1991, Sackett et al. performed an experiment to measure the soft-dipole-
resonance parameters and ground state n-n correlations of 11Li. To accomplish
this, they made a complete kinematics measurement of the coulomb dissociation
of 11Li as it passed through the virtual photon field of a Pb nucleus. In this
situation, if the 11Li absorbed a virtual y—ray from the Pb nucleus with sufficient
energy to excite it above the two-neutron separation energy, it could dissociate
into a 9Li and two neutrons. By measuring the velocities of the incident 11Li, the
9Li fragment and the two neutrons, it is possible to determine the 'y-ray energy

and then determine the (7,2n) decay cross section.

The experimental setup used by Sackett et al. is shown in Figure 25. A
beam of 30 MeV/ nucleon 11Li’s were produced by the A1200 fragment separator.
The beam was incident on a Pb target after passing through two position-
sensitive parallel-plate avalanche counters (PPACs) used to determine the
incident direction of the 11Li. After dissociation in the Pb target, the 9Li
fragment’s energy and position were measured in a Si-CsI telescope about 15 cm
downstream from the target. The two neutrons passed through the telescope

and were detected in two arrays consisting of 54 small scintillation detectors.

56

 

 

 

neutron
detector
array
........ ' / target and
......... 5° Si-CsI
7) ............ telescope

 

 

-
o
I
I
o
u
C
o
'0
u
g Q
o O
o. .
I
o
v
I
.Q
Q C
I

position
sensitve
PPACs

 

Figure 25 - Experimental setup to measure the complete kinematics of
11Li—->9Li+2n events (Sackett et al., 1991).

The arrays were placed 5 and 6 meters downstream and subtended a maximum
half-angle of 5 degrees. Each neutron’s energy was determined by measuring its
time of flight from the target. The neutron’s direction was determined by
knowing which detector it hit.

The neutron detectors used were small cylindrical scintillation counters,
shown in Figure 26. The liquid scintillator volume was housed in an aluminum
can; one end of the can was open and optically coupled to a large plastic light
pipe attached to a photomultiplier tube. The neutrons were incident parallel to
the axis of the cylinder. When these detectors are stacked into a tight array, the
cylindrical design creates a minimum 50 percent dead space between the
detectors, decreasing the array’s efficiency. To help boost the efficiency, a second
array was added behind the first. The experimenter choose a second array
instead of increasing the thickness of the detectors’ to increase the efficiency. The
reason for not increasing the detectors thickness is that it adds uncertainty to the

flight path’s length and thus to the measured neutron energy. Unfortunately,

57

adding the second array complicates the analysis by enhancing the effects of

cross-talk and what we call out-scattering.

Cross-talk is the familiar problem of one neutron creating signals in two
separate detectors. Out-scattering is a process whereby a neutron scatters from
the non-active part of a detector and is then detected in a different detector with
a distorted position and TOF (energy) measurement. While methods exist for
identifying and eliminating cross-talk events from the data, there are no methods
available identifying out-scattering events. Therefore, the neutrons should pass
through as little non-active material as possible. Unfortunately, the neutron

detectors used in the array had a total mass to active mass ratio of over 4 to 1.

For small decay energies, the reaction products were forward focused in

the lab and the setup had a reasonable geometric efficiency. Given a one-neutron

aluminum
housing

plastic lightpipe

  
  

photomuliplier tube

/
/

 

voltage

glass plate divider

scintillator
volume

Figure 26- A cutaway diagram of a neutron detector used by Sackett et al.
Neutrons are incident from the left. The scintillator IS 5 inches 1n diameter
and 3 inches 1n depth for a volume of 59 in3 (0. 97 liters).

58

detection efficiency 13an of 10%, a 11Li reaction probability in the target Pram
of 1%, a beam intensity I of 500 11Li/ sec, and the factor 16 for the empty space in

the array, a simple estimate of the counting rate is
eneutmngneutronpmctionl =
0.1 x I x 0.1 x 12» x 0.01 x 500 = 0.0125 events / sec '
For larger decay energies, this counting rate drops as the geometrical efficiency

drops.

The intense interest in neutron-rich nuclei and the intriguing questions
posed by this experiment lead us to attempt to improve it. The first
improvement is to measure larger decay energies, where the geometrical
efficiency of Sackett’s setup is small. The second improvement is to increase the
energy resolution of the decay energy spectrum. Unfortunately, increasing the
energy resolution requires decreasing the target thickness, thus reducing Preaction.
These factors reduce the counting rate to an unacceptably low value. Although
some improvement has been made in the intensity of 11Li beams, it is not enough
to compensate for these new requirements. Therefore, we are left with
improving the detectors in the experiment to increase the geometrical efficiency

and the one-neutron detection efficiency.

The first area of improvement was the neutron detector array. To
overcome many of the limitations imposed on Sackett’s experiment by the array,
we proposed, designed, and constructed what is now called the Neutron Wall
Array. We had four main objectives when designing the Neutron Wall Array:

1. Increase the angular acceptance of the array by increasing its area,
2. To greatly decrease the dead-space between the individual detectors,
3. To reduce the inactive (non-scintillator) mass through which the neutrons

must pass to reduce scattering,

59

4. To increase the total area of the detector array while decreasing the ratio of
the number of electronic channels to scintillator volume.

This last point is simply one of cost-savings; as an example, if we duplicated the

existing detectors so we could cover the same solid angle as we intend on

covering with the new detector, we would need over 500 individual detectors

with over 500 channels of associated electronics.

To meet our objectives, we are using the well known geometry of long
rectangular scintillator cells, placed perpendicular to the beam axis. Each cell is
viewed at both ends by photomultiplier tubes (PMTS). By using 25 cells of
scintillator, each 2 meters long, we will cover an area of 4 m2 in each wall. Using
the same flight path as the previous experiment, the array will subtend a half-
angle of 15 degrees where the previous array only subtended a half-angle of 5
degrees. A time signal for the time-of-flight measurement is obtained from the
mean time of the PMT signals. The position of the event along the cell is
determined from the time difference between the signals. The thickness of the
cell in the beam direction is determined by the energy resolution we wish to
achieve with a 5 meter flight-path. To increase the efficiency, we created two

identical arrays of 25 cells, so that one can be placed behind the other.

Complicating the design of the detector is a requirement that the detector
be capable of pulse-shape-discrimination (PSD) in order to distinguish between
neutron and y—ray events. When the time-of-flight method is used to determine
energy, a time-independent 'y-ray background will introduce a constant
background in the neutron spectrum; because we wish to study a continuous
neutron energy spectrum, we must have some method of eliminating this 'y-ray
background from the neutron spectrum. Also, 'y-ray identification plays an

important role in cross-talk rejection. The complication in the design arises

60

because the only scintillators capable of PSD are a few liquid hydrocarbon
scintillators. Instead of the self-supporting plastic bars of scintillator that are
usually employed in the configuration we are using, our scintillator is housed in
long, cast Pyrex tubes that are sealed at both ends. Although the Pyrex cell adds
non-active material through which neutrons must pass, the ratio of total-mass to

active-mass is much less than with the previous detector configuration.

7 Neutron Detection at Intermediate Energies

Given the general operating parameters of the NSCL’s cyclotron, neutrons
from nuclear reactions can have anywhere from a few MeV to a few hundred
MeV in energy. The next four sections discuss the basic principles of detecting
neutrons in this energy range, so called fast neutrons. A more detailed

discussion of fast neutron detection can be found in reference 26.

The lack of charge makes a neutron very difficult to detect directly.
Therefore, almost all neutron detection methods involve imparting some or all of
the neutron’s kinetic energy to a charged nucleus that is then detected. The
simplest means of doing this is to transfer some of the neutron’s energy through
elastic scattering, creating a recoil nucleus. Since the energy of a recoil nucleus
will be a random fraction of the neutron’s energy, we cannot determine the
neutron’s energy by measuring the energy of the recoil nucleus. To determine
the neutron’s energy, we measure the time the neutron takes to travel from the
target to the detector. Then, if the length of the flight path is known, we can
determine the neutron’s velocity and energy regardless of the energy deposited

in the detector.

61

The more energy the recoil nucleus has, the easier it is to detect. Simple
two body kinematics gives the maximum recoil energy E elm for elastic neutron

scattering as
4A

RIM = (1+A)2 En!

 

(11)

where A is the mass number of the recoil nucleus and En is the neutron’s incident
energy.27 The maximum recoil energy is obtained when A=1; therefore, it is
preferable to use hydrogen as the primary scattering target for neutron detection.

Detectors based on hydrogen scattering are called proton recoil detectors.
7.1 Scintillation Detection

Scintillation detectors are among the most popular devices used as proton
recoil detectors. Scintillation detectors convert the kinetic energy of ionizing
radiation into detectable light pulses. In the case of neutron detection, the
ionizing radiation is an elastically-scattered recoil proton from a hydrogen atom
in the scintillator material. Most recoil protons will deposit all of their energy in
the scintillator since the range of the recoil proton is typically much less than the
dimensions of the detector. Although many materials are available as
scintillation detectors, the most commonly used materials are organic solvents

and plastics that have incorporated an organic scintillant.

Once a light pulse is produced in the scintillator, a photomultiplier tube
(PMT) is use to detect the light. In a PMT, the photoemission of electrons from
light passing through a semitransparent photocathode creates an electrical
charge that is then amplified. The amplification takes place inside the PMT as
the electrons emitted from the photocathode are collected in an electron
multiplier. The electron multiplier uses a series of high-voltage dynodes to

accelerate the electrons and create secondary electron emissions that are

62

accelerated by the next dynode in the series. After a series of accelerations, the
charge is collected on an anode. The typical gain of such a multiplier is about 106

or 107.
7.2 Efficiency

Since neutron detection relies on nuclear scattering, it does not have 100
percent efficiency. Accurately determining the efficiency is necessary when
trying to determine absolute cross sections, such as in Part I. To demonstrate
some of the basic principles behind calculating the efficiency, we consider the
efficiency from proton scattering for low energy neutrons. The efficiency
consists of two primary contributions: the scattering efficiency and the counting
efficiency.

Scattering efficiency is simply the probability that the neutron will scatter
from the hydrogen in the scintillator. When only elastic proton scattering is
considered, the scattering efficiency escattering of a neutron detector is
9.,an = 1- exp(—Nost) , . (12)
where as is the (n,p) scattering cross section, N is number density of hydrogen,

and t is the thickness of the detector.

Counting efficiency refers to the number of events that surpass a
threshold and are counted by the electronics. To estimate this efficiency, we need
to know what the light output distribution is for a monoenergetic neutron beam
incident on a scintillation detector. The light output, as a function of energy, for
heavy charged-particles in an organic scintillator is nonlinear. In fact, for many

organic scintillators the light output L as a function of recoil proton energy is
L = us:6 , (13)

63

where k is a constant. Since the scattering cross section as a function of recoil
energy dN/dE is a constant for monoenergetic neutrons below 10 MeV, the light
output distribution is
dN = dN/dEP = constant ___ k’L‘m’, (14)
dL dL/dEP ngfi/z

 

 

where k’ is a proportionality constant (see Figure 27). Given this distribution,
when a threshold L1 is set below the maximum light output Lo, the counting

efficiency is

36
8counting = 1 — [£1] = 1 — ET- ° (15)
L0 E7:
The total efficiency is the product of the two, 5 = amusemdng. The solid curve

in Figure 28 shows the detector efficiency as a function of neutron energy as

calculated using equations 12 and 15.

 

 

 

 

 

 

 

 

 

 

 

 

EP

Figure 27 - The two basic contributions to the light output distribution for a
monoenergetic neutron beam incident on a detector (from Knoll, ref. 27).

To make the efficiency calculation more accurate we also must consider
the pulse-height resolution of the detector. This has the effect of rounding off the
end of the uniform differential energy distribution dN/dEp. If we assume the
detector’s pulse-height resolution has a guassian dependence with a width 0,
then the differential light output becomes

E, ’
dL _ N15 jo exp(-(E—E )2 / 202)dE
dN ' %kE‘/2 '

 

 

(16)

Other, more subtle, effects that influence the efficiency are multiple scattering
and edge effects. For large volumes of scintillator, the possibility of multiple
scattering must be considered. For smaller scintillator volumes, edge effects can

significantly effect the efficiency. Edge effects refer to a decrease in the efficiency

65

created by recoil protons near the edge of the detector escaping from the
scintillator before they deposit enough energy to be detected. All of these effects
are included in the code TOTEFF, and are used to calculate the efficiency of
right-cylindrical detector where neutrons are incident to the end of the cylinder.
Figure 28 also shows the results of the TOTEFF code. The triangle data points
represent the efficiency from (n,p) scattering, and the square data points
represent to the total efficiency for the detector. At about 20 MeV, reactions in

the carbon begin add to the efficiency and at about 40 MeV they dominate the

 

 

 

 

(n,p) scattering efficiency.
20% T j I I I I I I I I j I I I l r I I I I I I T fi
: —I:I— TOTAL 1
—A— H(n,n)H
157 j —— Simple Efficiency Calculation 1
o _
> ' H d
U . VA A b A I I -D- - ‘
C "‘ 3‘ I _ ’D’D’D D D-D‘D‘U‘ I
.9 10% - / . 9 011-0 _
é) . 1 .
LU ‘ ‘ A‘At .
I AAA 1
5% 1 A‘A‘AA ..
I 1"“ q
- r
u .l
0% I I I I I I I I I I I I I I l I j 17 I l f f I I
0 1 0 20 30 40 50

Neutron Energy (MeV)

Figure 28 - Neutron detection efficiencies. The solid curve represents a simple
estimate of the efficiency based on equations 12 and 15. The points represent
the results of the simulation code TOTEFF. The triangle data points represent
the efficiency from (n,p) scattering, and the square data points represent the
total efficiency for the same detector.

66

7.3 Time-Of-Flight Resolution

Determining the neutron’s energy by measuring the time the neutron
takes to travel a known distance is called the time-of-flight (TOF) method. Three
factors that contribute to the time resolution are: the rise time of the light pulse
in the scintillator, the intrinsic time resolution of the electronics, and the intensity
of the scintillation light. These three factors are characteristics of the scintillator
and electronic equipment available. Other factors effecting the time resolution
are the collection efficiency of the available light and the thickness of the

scintillator. These last two factors are properties of the detector’s design.

The light pulse arrives at the PMT as a series of photons spread out over
some time interval. The electronics used to time the neutron requires a logic
pulse that is generated when a sufficient number of photons have arrived at the
PMT to surpass some threshold. (The threshold could be one photon, but the
quantum efficiency of the photocathode surface is not unity, so the time pulse
might not correspond to the first photon that arrives at the photo-cathode
surface.) Therefore, the time of the logic pulse relative to the actual scintillation
event can vary depending on the statistics associated with the number of
photons required to trigger the logic pulse. A more luminous pulse will create a

higher density of photons-per-unit-time, thus increasing the time resolution.

In organic liquid and plastic scintillators light is emitted isotopically. For
very large volumes of scintillator, this can lead to very different path lengths for
the light reaching a PMT. Since the number of direct photons reaching the PMT
drops off as 17/r2 , most of the light collected by the PMT will have scattered from
multiple sides of the detector before reaching the PMT. This uncertainty in the
photon’s flight path can also add to the uncertainty in the true start time of the
scintillation pulse. This effect must be considered when studying the We

67

resolution of the detector; it can also be exploited to help correct for the fifth and

final effect on the time resolution in special cases.28

The final contribution to a detector’s time resolution comes from the finite
time it takes a neutron to pass through the detector. Figure 29 shows a schematic
diagram of our rectangular scintillator detector some distance from a neutron
source. The scintillator is viewed at both ends by PMTS. As a neutron penetrates
the scintillator, it has a uniform scattering probability as a function of depth.
Since the PMTs are unable to locate the position of the scintillation in the z
direction, there is an uncertainty As in the distance 3 from the source to the
scattering event. It is useful to express this uncertainty in terms of a time
uncertainty, At = tAs / s , related by the neutron’s velocity, v = s / t. Then, for a
given thickness of detector, the absolute time resolution depends on the flight
time of the neutron through the scintillator, and therefore on the neutron’s

energy.

Increasing the detector’s thickness reduces the detector’s We resolution,
but it increases its efficiency. The thickness chosen for a detector is therefore a
compromise between the desired efficiency and the desired energy resolution. It
is often best to chose a thickness that creates a At is comparable to the other time

resolution characteristics.

68

neutron

 

 

Figure 29 - A schematic drawing of a neutron being detected in a long
scintillator cell, perpendicular to the neutron’s direction. ‘

If the neutron’s energy is to be determined by its TOF over a given flight

path,

1 s 2
En —Em(;) , (17)

and there is an uncertainty in both the flight path as and the time o, ; then the

uncertainty in the neutron’s energy is

as 2 as 2 -

Or, expressed as the relative energy resolution,

2 I
%=4[§+%]. (19)

It is often difficult to determine the overall time-uncertainty o, from each

of the contributions discussed. Luckily, there is a process by which the overall
a, can be determined. When a TOF spectrum is observed for neutrons from a
heavy ion reaction, there is also present a distinct, narrow peak associated with
the prompt y-rays from the reaction. Since the 'y-rays move at the speed of light,
their transit time through a thin scintillator detector is negligible compared to the
neutron’s transit time, thus it represents the time uncertainty for the whole

system without the added complication of detector’s thickness. (The 'y-rays

69

emission time does not contribute to the uncertainty since the y-rays are emitted
on the time-scale of electromagnetic nuclear transitions (~10'13 ns), very much
less than typical laboratory resolution of < 1 ns.) Using the width of this y—ray

peak and the given detector thickness, we can determine the relative energy

resolution 0 E / E.

7.4 Pulse Shape Discrimination

Proton recoil scintillation detectors are sensitive to more than just
neutrons. Typically, a large background of y—rays and cosmic-rays is present
during an experiment. A few possible methods exist for managing these
background radiations. One such method is to raise the threshold level of the
detector. Most prays come from nuclear transitions and their energies are
therefore limited to the binding energy, about 8 MeV. If the threshold is set
above this level, most of the y—rays can be eliminated. Although cosmic-rays are
usually high energy muons, the energy they deposit in a scintillator depends on
the length of their flight path through the scintillator. For scintillation detectors
with small enough dimensions, the threshold can be set above the maximum
possible energy deposited to eliminate the cosmic-ray background.
Unfortunately, as can be seen from equation 15, the higher the threshold, the

lower the neutron detection efficiency.

Another method of reducing the background is to require the neutron
signal to be in coincidence with another signal that is known to be associated
with the neutron. This method reduces the so called time-uncorrelated
background signals. When the TOF method is used to determine the neutron’s
energy, a coincidence between two events is always present. While all cosmic-
rays are time-uncorrelated, and many y—rays are time-uncorrelated, there are 'y-

rays from the reaction that produced the neutron, or secondary reactions. These

70

y—rays will be correlated in time with neutrons and are harder to eliminate. If the
coincident signal is also used as the basis of the TOF measurement, then the
width of the coincidence period will determine the possible neutron energies
allowed. If it is necessary to measure a large dynamic range of neutron energies,
the coincidence period might be so wide as to eliminate the background

reduction benefits of a coincidence requirement.

The most useful method of reducing background events comes from
analyzing the electric pulse from the detector. For some scintillators, the shape
of the pulse varies according to the specific ionization of the ionizing particle.
The light produced by an ionizing particle has two components, a prompt
fluorescence and a delayed fluorescence. The prompt fluorescence has a decay
constant of a few nanoseconds, whereas the delayed fluorescence has a decay
constant on the order of a few hundred nanoseconds. The proportion of light
produced by each of these two components varies according to the specific
ionization of the ionizing particle. A lightly-ionizing particle, such as a cosmic-
ray or an electron from the compton scattering of a y-ray, will produce most of its
light in prompt fluorescence. A highly-ionizing particle, such as a recoil proton,
will produce a larger fraction of light in the delayed fluorescence for the same

amount of total fluorescence as a lightly-ionizing particle.

71

 

  
 

 

 

 

I I I I. I I 1 I r I T I I I
. :3009, . ..
A head ;<>; taI|—>
52 U: 1
a s = -
‘c’:’ b C :
a) co .
E = V : '
_ 5 L : neutrons -
h I
3 . y-rays\
— I I I. I I l I I Ir—r I I I .-

 

-1 00 0 1 00 200 300
time (nanoseconds)

Figure 30 - A representation of the scintillation light output as a function bf
time for different radiations in organic scintillator NE-213.

Over the years, many procedures have been developed to exploit this
property, commonly called pulse-shape-discrimination (PSD). Mest methods are
able to discriminate between y-ray and neutron signals for neutron energies
above 3 MeV. A method developed at the NSCL uses two ADCs: one mtegrates
the total charge of the pulse, and the other integrates the charge for some fixed
time-fraction of the pulse. The fixed time-fraction is usually either the head of
the pulse or the tail of the pulse. Comparing the charge in this fraction of the
pulse to the total charge in the pulse, we can determine what particle created the
pulse.

To quantitatively discuss the PSD property of a scintillator, we calculate
the so called Figure-Of—Merit (FOM). The FOM is defined as a function of the
total integrated charge. If we illuminate a detector with both y-rays and neutrons

and record the charge in the tail of the pulse, we obtain a histogram as shown in

72

 

 

 

I I j I I I I r I
' neutrons ':
‘9 _
C ..
3
o .-
0
q
d
I I T l I l I I I

 

 

Charge

Figure 31 - The integrated charge in the tail of the pulse for neutron and 'y-ray
signals, when the total integrated charge for both signals is the same. The
FOM for this spectrum is 1.35.

Figure 31. The POM is then defined as the separation between the two centroids
of the peaks divided by the sum of the FWHMs of the two peaks. We consider a
FOM above 1 to be acceptable.

8 Neutron Wall Array Characteristics

The Neutron Wall Array consists of two individual detector arrays, each
consisting of 25 individual detector cells. The area of each array is 2x2 m2; the
inactive area is less than 12 percent of the total area. The scintillator used in the
array is NE America’s NE-213.’ The total scintillator volume is 500 liters.
Currently, the Neutron Wall Array resides in the NSCL’s N4 vault. The overall

time resolution of the array has been shown to be about 1 ns FWHM with a

 

' NE America was recently purchased by Bicron Corporation of Newbury, Ohio.

73

corresponding position resolution along the length of the cell of 7 cm FWHM for
neutrons depositing greater than about 10 MeV of energy in the detector. This
position resolution is comparable to the 7.62 cm height of the individual detector
cells. Using our PSD circuit, described in section 9.1, ’we have obtain excellent

PSD for neutrons above energies of 3 MeV.

Section 8.1 describes the general physical characteristics of the Neutron
Wall Array. Section 8.2 briefly describes the measured time resolution of the
array. Section 8.3 discusses measurements of the position resolution along the

length of a cell. Finally, section 8.4 demonstrates the PSD properties of a cell.
8.1 Size, Configuration and Construction

The elements that form the array are 2-meter long Pyrex cells filled with
the liquid, organic scintillator NE-213. Two PMTS view the cell from both ends.
Figure 32 shows a drawing of one cell. The outside of the Pyrex cell is not
treated with any reflective or specular coating; this allows total internal reflection
to be the means by which the light is channeled through the cell to the PMTS.
Monte carlo simulations of the detector show that approximately 20 percent of
the light from an event reaches each PMT; the other 60 percent of the light
escapes from the cell because the angle it forms with the surface normal of the
glass is less than the critical angle. Figure 33 is a photograph taken looking
through one end of a cell; the NSCL logo was placed at the other end. Multiple
reflections of the logo in the sides of the detector are visible. (The cell used fOr
the photograph had a square cross section and was not one of the cells used in

the Neutron Wall Array, which have a rectangular cross section.)

The Pyrex cells were custom made for this application and have a wall

thickness of y, inch. To make the cell, we purchased custom Pyrex tubing made

74

to our cross sectional dimensions and at least 85 inches long. The tubing arrived
from the manufacturer with the ends of the tube open. The Michigan State
University Scientific Glass Blowing Shop then closed the ends of the tubes so that
they could be coupled to the PMTS. The PMT used was a new product of Philips
Photonics, model XP4312B/O4. This model is a fast, 12—stage PMT with a 3 inch
photocathode surface. In order to make use of the whole photocathode surface,
the end of the tubes were closed off with a 3 inch circular Pyrex disk. Closing
the tubes was a two step process. First, a cylindrical Pyrex tube, 3-inches in
diameter, was fused onto the end of the rectangular tube. The cylindrical tube
was then trimmed at the nearest point to the junction where the cross section
became circular, about 1 inch. The second step was to fuse a circular plate, 3-
inches in diameter, onto the cylindrical tube. Except for cleaning, the surfaces of
the cells required no further polishing or finishing and were used as they came

from the manufacturer.

75

scintillator expansion
reservoir with Kynar
fittings and Teflon tubing
photomultiplier tube
with ,u-metal shield

2m

14.“..qu

 

 

 

 

voltage divider
Pyrex cell with

NE-213 scintillator-
cross-sectional area
is 6.35x7.62 cm2

Figure 32 - A drawing of an individual detector cell from the Neutron Wall
Arra .

  

Figure 33 - A photograph looking lengthwise through a cell. The NSCL’s logo
was placed at the opposite end of the cell. Only the center image is real, the
others are all reflections from the sides of the cell.

76

The PMTS are attached to the end of the cell with the optically clear epoxy,
BC-600. A special jig was designed to hold the PMT and cell in place for the 24
hours the epoxy requires to cure. Surrounding each PMT is a u-metal shield; this
prevents any extraneous magnetic fields from interfering with the operation of
the PMT. We designed a simple passive voltage divider for our PMTS. Both the
anode and last dynode signals are used by the acquisition electronics. Figure 34
shows the voltage divider’s circuit. The total resistance of the resistor chain was
selected to use the maximum current available from the power supply. Having a
large current flow through the resistor chain helps maintain the linearity of the

signal amplification.

Once the cell is sealed, a small (M1 inch diameter) tube with a glass-Kovar
seal is fused onto the cell. The cell is cleaned and filled through this small tube.

The tube is connected to an aluminum can by a corrugated Teflon tube. This can

K G D1 D2 D3 D4 D5 D6 D7 D8 D9 D10D11 D12 A

 

 

 

 

 

 

 

 

 

 

 

 

V V V V V V V V V V V V V _ V
R15 J3
R6 R7 R8 R9 R10 R11 R12 R13 R14 R16 R18 _€>
MMMMMMMMWMM 3 R21

 

 

 

 

 

 

 

 

 

 

F (:1 c2 c3
R1 R2
cs C4 $== C6
I R2
ll
1|
R17
R4 R5 R22
R1, R13 68K 1w R4,R5 241(1W R2, R18 150K 1w 11 - 12
R3 200K 1w R6, R14 91K1W R7-R12,Rzz 471(1w
R16, R20 120K1W R15 10K uw R17, R21 51 MW
c1,c2 0.01m cs 0.0047w C3, C4,C6 0.02111:

 

 

II = high voltage, 12 = dynode, I3 = anode

Figure 34 - Schematic drawing of the passive voltage divider used on the
XP4312/04 PMT.

77

is a thermal expansion chamber for the scintillator, since the scintillator’s
coefficient of expansion is greater than the glass’. The scintillator, NE-213, will
corrode most plastics; therefore, all of the tubing and fittings used are made of
Teflon or the resin Kynar, both being chemically reSistant to the scintillator.
Once the can is in place, a small (},/3 inch OD) Teflon tube is inserted through the
can and corrugated Teflon tube into the cell (see Figure 36). The scintillator is
then pumped through this small tube, filling the cell.

The PSD properties of NE-213 can be degraded by the presence of
dissolved oxygen in the scintillator. To remove any oxygen that might have been
introduced when the cell was filled, we passed dry nitrogen gas throughout the
volume of the scintillator for about 1 hour. We call this process bubbling. The
nitrogen gas will dissolve into the scintillator and force out any dissolved oxygen
gas. The nitrogen gas was delivered into the cell through the same Teflon tube
that was used to fill the cell. When the bubbling was completed, the Teflon tube

was removed and the aluminum can was sealed with a stainless-steel pipe-plug.

After the cells where completed, they were inserted into the array. The
array was supported by a large aluminum frame. In case of a catastrophic
accident, the bottom of the frame has a large catch-basin capable of containing
the scintillator volume from all 25 cells in the frame. The sides and top of the
frame are covered with aluminum plates and made light-tight with opaque
caulking. The front and back of the frame are covered with a removable
aluminum sheet attached at the edges to another aluminum frame. The
aluminum sheet is %2 of an inch thick, providing very little material for
unwanted neutron scattering. A cutaway drawing of the complete assembly is

shown in Figure 35.

78

 
 
 
  
 
   
 
 
  
 
   

individual

detector
cells, see
aluminum Figure 32
frame
aluminum
covenng air blower

nitrogen gas
manifold

‘. _. _. _. .-..- A... _. ,. .-,.0,.- _. .. .-..- -'. . _. . ..'_. _. ,- _. . . ..‘._ . .- ”r. 7..” J. .;_. .';.'..'..'..'..‘ .s..- .......................
.-3.g--.;--.-,-.- - .-.v,-~.-_-t.-4-.- 00-.- °--' 'v.‘-.-,--.--.. ‘ . , .- -------------------------------------------------------------------------------------------

Figure 35 - A mechanical cutaway drawing of one of the Neutron Wall Array.
The cells are attached with special brackets to two, hollow, 2-inch square,
aluminum posts in the center of the frame. The cells are strapped to the bracket
with a 20 mil thick stainless-steel band. To allow for small variations in the
surface of the cell, the cells are spaced y. inch apart. Once a cell is in place, the
aluminum can is unsealed and attached to a nitrogen gas manifold to provide a

greater gas volume for expansion.

The inside of each of the aluminum frames is painted flat black, and when
the front and back cover sheets are in place, the aluminum box becomes light-
tight. By making the frame light-tight, we have avoided the difficulty of making
the 50 cells light-tight individually. To prevent cross talk between the cells—that

79

is light from one cell entering a neighboring cell—a piece of black paper is

inserted between the cells.

Other miscellaneous hardware associated with arrays include a gas
monitoring system, an air blower, and a fiber optic timing system. The gas
monitor detects high levels of xylene gas; xylene is the solvent for the scintillator.
This gas is explosive and the liquid has a flash point of 97° F. The gas monitor is
connected to the laboratory’s electronic control system. If the gas monitor
detects a dangerous level of xylene, it alerts the control system which shuts down
the high-voltage system powering the PMTS. Each of the voltage dividers
produces 5 W of heat; to help keep the inside of the frame cool, each frame has an
air blower. To keep the frames light-tight, the blowers are connected to the
frames through light-tight baffles. The fiber optic system is used for various

timing purposes and is discussed in Section 9.2.2.

Each of the two arrays is supported on a steel frame that centers the array
about the beam height in the N4 vault, where the Neutron Wall Array is used.
The steel frames are on wheels and can be moved about the vault. The frames
are also designed to fit into each other so that the arrays can be placed front to

back, with a minimum distance between the cells of about 1 foot.

80

 

 

 

 

 

 

 

 

”‘6:

r/

 

/ I
bubbling tube ’ ‘

/'

reservior /

 

 

 

 

 

 

 

 

 

 

 

 

 

lllllllllmllllllll

LI

 

 

 

4!“ 1111111111111111111111

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 36 - A schematic drawing of the bubbling procedure for a completed
cell. For about 1 hour, dry nitrogen gas is blown through the small Teflon
tube that is fed through the reservoir into the cell. '

81

 

 

 

 

 

 

 

 

 

 

TOP VIEW /\/ SIDE VIEW
bracket and
banding strip
y \ : -------- (r— """"""""" "‘3
f1 4 L i

 

 

    

 

 

fir

aluminum .
os t 11 alummum

Figure 37 - Top and side view of the cell-mounting bracket.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8.2 Time-Of-Flight Energy Resolution

Section 7.3 describes the various contributions to the .detector’s time
resolution and how they effect the neutron’s energy resolution. A good estimate
of the overall time resolution can be obtained from the prompt y—ray peak in a
TOF spectrum. From the y—ray peak shown in the upper pane of Figure 11 and
the y—ray peak shown in Figure 38 (from reference 29), the time resolution of the
detector array is about 1 ns. Using this value, the width of the cells and equation
19, we show in Figure 39 the relative energy resolution for various flight paths

and neutron energies.
8.3 Position Sensitivity

The position of an event in the cell is determined from the time difference
between the two PMT signals. To relate the time difference to a distance the cell,

it is necessary to measure a collimated source at different locations and measure

82

the time difference. This value cannot be calculated from the speed of light in the
scintillator because the time signal from the PMT is not based on direct light
from the source. Rather, the time signal is the result of light that scattering from
the sides of the cell before reaching the PMT. Figure 40 shows three different
collimated source measurements as a function of time difference (with an
arbitrary offset). Each peak is 30.48 cm from its neighbor, giving a distance-to-
time calibration of 7.65 cm / ns. The time resolution shown in the previous
section is about 1 ns, therefore, the position resolution approaches 7.65 cm for

higher energy events.

105 1 I I I I I I I I I l I I I I l I I:
: FWHM = 1 ns -> <-
104 1 prompt N 1
: y-ray peak :
s : 3
0! . .
g 1 l
E . '
3
O
O
103?
l
102 I I I I I I I I I l I I I I
-300 -200 -100

83

 

 

 

 

 

 

 

Time-Of-Flight (ns)

Figure 38 - Neutron and y-ray TOF spectrum from Ar on H0 at 25
MeV/nucleon. The prompt 'y-ray peak from the reaction is an indication of the
overall time resolution of the Neutron Wall Array.29

 

1O

 

 

 

 

 

 

 

 

 

 

q
A . ’
fl .— ' ___ av
c - _______________
a) if f. 7.? . ..............
e 1 .1"'—""—"""':,':.'.T.::::-.-" ffffff
(D d::::.'.'.::::::.‘.'.'.---- .....'..'.:S-111------_-::::: -------- =
Q. '..-.-.-.......—-—.--------"-'33 ---------- .
V q -
LL] : ——3meter, -—-4meters :
E q 5meters,----6meters ‘
‘D —--—- 7 meters, ------ 8 meters

' ----------- 9 meters; ----- 10 meters -

0.1 I I I 1 1 I I II I I —T I I I I I

1 1 O 100
Neutron Energy (MeV)

100 . .wr...fi

 

 

 

 

 

   

 

- 1

: — 5 MeV ]

J - - - 15 MeV .

’4: .1 - - - - 25 MeV .
c " """ 35 MeV
a: ~ ...... — -- 45 MeV

2 1o -\ ....... -----ssMev :-

a) : ‘ ‘32-}?4‘ “‘32:: ----------- 85 MeV :

Q. ' ~ r-e. . “51":- ...... d

V . " ‘ ’ . . :;i‘;~;. ........ '

'o ' ' ‘ "first?

 

 

_L
_L
0

Flight Path (meters)

Figure 39 - The relative energy resolution for the Neutron Wall Array. The
upper pane shows the resolution as a function of neutron energy, for various
flight paths; the lower pane shows the resolution as a function of flight path
for various neutron energies.

85

 

6 k I I I r I I T 1 —I
1 30.48 ' 30.48 :
5k - “Hm-

H

4k
3k
2k
1k

-m‘- -, v" _ _ _ ’ -_<—o..-...- _,
I l I I

l
1 O 1 5 20 25 30 35
time difference (tL - t”) (ns)

COUl‘ltS

 

 

Figure 40 - Time difference measurement of a collimated 60Co source at three
different positions. The positions are separated by 1 ft (30.48 cm).

8.4 Pulse Shape Discrimination

When we began researching the possibility of using long cells of liquid
scintillator, our primary concern was over the quality of the pulse-shape-
discrirnination we could achieve. PSD methods improve with increased light
output, therefore we were concerned with what the low energy cut-off is for
acceptable n / y-ray discrimination. We found that combining the information
from the two PMTs greatly increases our PSD resolution, and that we have
complete n/y—ray separation for recoil protons with as little energy as 3 MeV.
Given a 30 MeV neutron energy, as used in Sackett’s experiment, and using 3

MeV as a threshold, then the counting efficiency (from equation 15) is
em“: = 1— 360 = 90% . We consider this efficiency acceptable.

86

As described in section 7.4, we can distinguish between neutron signals
and y—ray signals by comparing the total integrated charge of the pulse to the
integrated charge in either the head or the tail of the pulse. Currently, the
Neutron Wall Array uses a circuit we have developed to provide a signal
proportional to the head of the pulse, called QFAST. Both the QFAST signal and
the total integrated charge, QTOTAL, are dependent on the distance of the event
from the PMT because of light attenuation by the scintillator. To compensate for
this, we create a position-independent QFAST/QTOTAL signal by taking the
squareroot of the product of the two QFAST/QTOTAL signals from the two
PMTs. For the rest of this section, QFAST/QTOTAL refers to the position-
independent values. In Figure 41 we show the QFAST signal plotted against
QTOTAL. Since QTOTAL is proportional to the light output, it can be related to
either the compton scattered electron energy or the recoil proton energy. It is
easier to represent QTOTAL in terms of the equivalent electron energy than in
terms of the recoil proton energy since the electron’s light output is linearly
dependent on its energy and the proton’s light output varies asits energy to the

three-halves power (equation 13).

Figure 42 shows the QFAST signal for various values of QTOTAL, which
is again represented in terms of electron equivalent energy. It is clear from the
figure that we have achieved excellent neutron / y—ray discrimination to below 1
MeV electron equivalent energy, or a recoil proton energy of about 3 MeV. By
fitting a double gaussian curve to these data, we can determine the figure-of-
merit. In Figure 43 we show the FOM as a function of electron equivalent

energy.

87

 

QFAST (arbitrary units)

 

 

 

 

I I I . I I I I I I I I I I I
O 2 4 6
Electron Equivalent Energy (MeV)

Figure 41 - A PSD spectrum from a cell in the Neutron Wall Army. The
neutrons and y-rays are from a PuBe source placed a few feet perpendicularly
from the center of the cell.

88

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 

E :05 MeV E =0.75 MeV E =1.0M6V
3 .fl.... #fi'11T...15 ...’.,....
‘ q10- -

2‘ - ‘ 10- d

1- - 5‘ ‘ 5- -

o-i—lv v 'fi fi 1 0' v I v

0 50 100 0 50
Er=125MeV E =15MBV
. r . . I . . 1 v . v I

TOJ " 6-
a 4-
o 5- - .
1- 2.
5‘ ‘ ‘
.0 0- 0- . . .
a) 0 50 100 0 50
'0 Er: 2.0 MeV Er: 2.25 MeV Er: 2.5 MeV
.S I I I Tfii I I I I I I
.0
v
(D
4.;
C
3
O
0

 

 

 

 

 

 

150

 

 

 

 

 

 

50 100150 50 150
channel

Figure 42 - Individual QFAST spectra for various QTOTAL values. The
QTOTAL values expressed in terms of equivalent electron energy.

89

I“
o

 

IIII'IIII'IIII'IIII'IIII'IIII'TIII

O

W0

Figure of Merit
.0 -* .-‘
01 O 01
I l I l | l l l l I l l I l l l 14 l

llllllllllllllllll

.9
o

 

 

 

IIII IIII IIII IIII IIII I—IIT IIII
I I l l I l

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Electron Equivalent Energy (MeV)

Figure 43 - The figure-of-merit from Figure 42 as a function of the equivalent
electron energy.

9 Operation

The following sections deal with various aspects of operating the Neutron
Wall Array. Of primary interest is the electronics system used to obtain the
necessary information from the cells in the array. Also of interest are the two
primary calibrations that must be performed when the Neutron Wall Array is

used.
9.1 Electronics

For every event in the Neutron Wall Array we must know the time of the
event with respect to some reference signal (e. g., the time of a fragment event or
the cyclotron’s RF signal), the position of the event along the cell, and the energy
of the recoil proton. We must also have some means of identifying whether the

event was from a neutron or from a y—ray or cosmic-ray. By measuring the time

 

90
between some reference signal and each of the PMTS on a cell, we can find both
the time of the event and the position of the event along the cell. If tL is the time

between the reference signal and the left PMT tube, and t R is the time between

the reference signal and the right PMT tube, then to first order, the time-of-flight
of the event is

TOF = ELL-ii I (20)
2

and the position from the center of the cell is

position = k(tL - tR ), (21)
where k is the scale factor between time and distance (see section 8.3). Higher
order terms correct for nonlinear effects at the very ends of the cell. We can
determine the energy of the recoil proton by integrating the charge of the signal
from the PMT, which is proportional to the recoil proton’s energy (see section
9.2.1). If we have integrated the total charge of the signal, we can then integrate
some portion of the signal to determine whether it was produced by a neutron or
a y-ray (see section 8.4). If there is a large dynamic range in the recoil proton
energies, we must also integrate an attenuated charge signal and some portion of

the attenuated signal.

Figure 44 shows a simple schematic of the electronics used to obtain this
information from a PMT. All of the information is obtained from the two
electrical signals produced by the PMT’s voltage divider. One signal is. a
positive-voltage pulse from the last dynode in the multiplier chain and the other
signal is a negative—voltage pulse from the anode that collects the charge from
the multiplier chain. The dynode pulse is used for the timing signal, and the
anode is used for the integrated charge signals. Most timing electronics uSe
negative pulses, so we invert the dynode pulse with a small inductor. This

inverted signal is fed into a constant fraction discriminator (CPU). The CFD

91

produces a fast logic pulse if the voltage of the input signal surpasses a
programmable threshold. The logic output of the CFD is then used to start a fast
clock. The clock is stopped by the common reference signal. A logical OR of all
of the CFD outputs is used to create a gate signal, which controls the charge-to-
digital converts, and a trigger signal for the primary trigger logic, which controls

the data acquisition computer.

veto ,

J r stop

d inverting i constant Time-to-
transformer fraction digital
discriminator converter
an d ’ ’ ' ‘
other PMTS

@ _ clear

trigger

gate
generator at
s s 1

cable delay NSCL active S21 charge-to-
110 ns splitter/ PSD digital
circuit 4L» converters

.91..
ESE...

 

 

PMT

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 44 - Basic outline of the processing of the signals from the Neutron Wall
Array. Q1 through Q4 represent the charge signals: anode, attenuated anode,
PSD, and attenuated PSD. .

Our traditional method of doing pulse-shape-discrimination30 is shown in

the upper part Figure 45. The anode signal is split and fed into two charge-to-

92

digital converters (QDC). One of the two signals is delayed 30 ns before being
fed into the QDC. A gate signal is then timed so that it begins with the start of
the delayed signal; the QDC integrates the pulse as long as there is a gate signal.
While this method works very well, it is not suited for applications where two or
more neutrons must be detected in coincidence. The problem is, if two neutrons
arrive at slightly different times, two different gates are required. Unfortunately,
most QDC modules have multiple channels controlled by one common gate. A
few solutions exist. One solution is to use one QDC module per PMT and only
use two channels per module. The high cost of QDC modules makes this
solution impractical if a large number of PMTS are involved. Another solution is
to use a linear gate module as the input to the QDC, in effect giving each QDC
channel it own gate. We prefer not to use this method because of some bad
experience we have had with the stability of available linear gates. (This was the

method used by Sackett, et a1 .)

While investigating other PSD methods“, a suggestion arose for a very
simple circuit that would create a pulse proportional to the fast component cf
anode pulse.32 This pulse could be integrated anytime during the gate period.
With this method, we again split the anode pulse into two separate pulses. One
of the two pulses is integrated as normal. As long as the gate starts before the
pulse, and lasts for approximately 300 ns after the beginning of the pulse, the
position of the anode pulse with respect to the gate pulse is arbitrary. The other
pulse is used to create the PSD signal. This process is shown in the lower part of
Figure 45. The PSD pulse is formed by combining the anode pulse with its
reflection from a terminated delay line. The input stages of the Lecroy 2249W
QDC have a circuit that prevents the positive part of the pulse from being
integrated. Therefore, the integrated charge represents the charge in the first

93

portion of the pulse. The proportion of the anode pulse that is integrated is
controlled by the length of the terminated delay line. A schematic of the process
is shown in Figure 45.

As shown in Figure 46, the reflected pulse could make its way back to the
QTOTAL channel of the QDC. To prevent this, and to gain match with the
QFAST input, an emitter follower (EF) circuit was inserted before the delay line.
Not only does the EF prevent the reflection from entering the whole circuit, it
also acts as an active splitter, letting the full charge of the anode be integrated by
the QTOTAL QDC channel. Figure 47 shows the exact circuit that we used. A
duplicate circuit was used to produce a PSD pulse from an attenuated anode
pulse. The 8 ns delay length was found empirically to provide the best

neutron/y-ray separation.

 

QTOTAL

QTAIL

gate

QTOTAL

PSD pulse

general gate

 

 

 

94

 

Traditional Method

 

 

 

voltage

 

 

time _.,

 

 

—-)§ §<—30 ns

 

Reflection Method

 

 

A b
voltage

 

time —p
charge—to-digital
converter positive
voltage clipping circuit

 

time _.

 

7'.—
<—500 ns—> “me ->

 

 

 

—> 33(— arbitrary

Figure 45 - Pulse shape discrimination methods.

95

 

 

 

 

 

 

 

anode
I .. . terminated delay line
QTOTAL QFAST :—
Figure 46 - A simple schematic of the method of producing the PSD signal.
+12V +12V
QTOTAL
['1 R1 R2 i it
|,__ K; Typical Values:
R5 Ru 1.0 K :2
R5 8115 R2 510.0 12
R4 Rs 10.0 o
1 Rs 16.5 n
Amde _ _ ; ,. _ C1 10.0 nf
" ‘ ' C2 4.7 nf

 

 

 

Figure 47 - The circuit used to produce the PSD pulse. The circuit also acts like
an active splitter for the anode pulse. The QFAST output is gain matched to
the 50 0 input of the Lecroy 2249W QDC.

The same signal that produces the gate for the QDCs is also used as the
Neutron Wall Array trigger for the master trigger logic. The master trigger logic’s
job is to coordinate the Neutron Wall Array electronics with the rest of the
electronics required by the experiment. Such a system is described in section 2.3

and is shown in

 

96

Figure 12. Typically, the logic is designed to look for a coincidence between the
Neutron Wall Array trigger and a signal in an auxiliary detector. If such a
coincidence is detected, then the data acquisition computer is instructed to read
the value of the QDCs and the time-to-digital converts. If the Neutron Wall Array
triggers, and there is no coincidence with the other detectors in the experiment,
then the Neutron Wall Array electronics processes a so-called fast clear. This
resets all of the QDCs and time-to-digital converts. This is particularly useful
because of the very high background counting rate of the wall. With thresholds
set to 1 MeV electron equivalent energy, the background counting rate is about
16,000 events per second. By using the fast clear, these events are not processed
by the data acquisition computer; that reduces the dead time of the system. Once
the computer is activated, a veto signal is issued to all CFDs to prevent events
from entering the electronics before the previous event is completely read out .
A detailed drawing of the master trigger logic used for the 8Li(n, )9Li experiment
described in PART I is shown in Figure 48. The circled numbers represent timing
points; these points are represented on a relative timing diagram shown in

Figure 49.

 

97

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9.2 Calibrations

Two particular calibrations must be performed each time the neutron wall
is used: a pulse-height to energy calibration, and . a time per ADC channel
calibration. These calibrations are demonstrated below. Another issue alluded
to earlier is the cross-talk analysis when two ore more neutrons are to be
detected in coincidence. This analysis relies on the pulse-height calibration and

is briefly discussed in the last section.
9.2.1 Pulse-Height Calibration

Although TOF is used to determine the neutron’s energy, it is often useful
to know the energy of the recoil proton. The easiest method to measure the
proton’s energy is to measure the PMT’s pulse height which is proportional to
the proton’s energy. Unfortunately, the 1/ e attenuation length of NE-213 is
about one and a half meters—comparable to the length of a cell—making the
pulse-height value dependent on the distance between the PMT and the
scintillation event. In section 2.2 we explain how a position independent pulse-
height measurement can be made by taking the square root of the product of the

two PMT pulse-heights.

To calibrate the pulse height spectrum we use various y-ray sources. In
the same manner that neutrons are detected through (n,p) scattering, y—rays are
detected through compton scattering. We use three different ‘y-ray sources fer
the pulse height calibration: 60Co, 228Th, and 12C(l’Sx = 4.44 MeV) in a PuBe source.
Figure 50 shows a sample pulse height spectrum for each of the three sources.
The energies shown are not the y—ray energies, but the compton edge energy of
the recoil electron. The compton edge energy is associated with the channel of

the half-height value of the compton edge. Once the y—ray calibration is known,

99

 

100

we use an empirical expression relating equivalent light output from recoil

protons and recoil electrons.33 The expression is

E, = alEp — a2[1.0 — exp(—a3E;‘ )1, (20)

where Be is the electron’s kinetic energy, Ep is the proton’s kinetic energy, a1 =
0.83, a2 = 2.82, as = 0.25, and a4 = 0.93.

9.2.2 Time Calibrations

It is necessary to calibrate the TFCs used to measure the neutron’s TOF.
To facilitate the calibration, a fiber optic system has been installed in each array.
This system uses an ultraviolet laser to illuminate a bundle of fiber optic cables
simultaneously. The bundle is then separated into individual cables that are
directed to the center of each cell. The fiber optic cables have been cut to exactly
the same length. One extra cable is sent to a small plastic scintillator attached to
a fast PMT; this detector provides a constant time reference that is used as the
TFC stop signal. (In essence, it replaces the fragment detector used for the TOF
measurements). By changing the delay between the reference detector and the
input to the TFC’s stop, we obtained different peaks in the TFC spectra. One
such spectrum is shown in Figure 51, where 5 different laser peaks are present.
Knowing the incremental steps added to the delay, we calculate our time-to-

channel calibration.

 

101

 

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1
at
d
d
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01
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0 50 1 00 1 50 200
Channel

Figure 50 - Sample pulse-height spectra for three different y-ray sources. The
compton edge is used to obtain a calibration of pulse-height to electron
equivalent energy. The energies shown are the compton electron energies.

 

102

 

 

 

 

 

 

 

 

 

140 P I I I I I I I I I I 1 jfifi I l I I I -
120 - -
100 - -
w )— -
g 80 .. 20 ns 20 ns 20 ns 20 nsi .
8 60 - 4+ 4+ 0 0 .—
40 — -
20 - -
0 ’1‘“;— u‘”l‘ ‘IJL‘ .“ I“ a”? u‘ f“ L7"? ravafl“:

0 200 400 600 800 1 000

Channel

Figure 51 - A laser time calibration for a cell in the Neutron Wall Array. The
laser pulses are separated by 20 ns.

9.2.3 Cross-Talk

As mentioned earlier, placing a second array behind the first increases the
problems of cross-talk and out-scattering. To explain how we attempt to manage
these effects, it is useful to look at the main interactions that we can expect
between the neutrons and the scintillator material. The scintillator is ahnost
entirely hydrogen and carbon, and Table [reactions] lists the most likely
interactions for neutrons in the scintillator. The first two have the largest cross-

sections.

 

103

Table 5 - Listing of the predominant interactions for a neutron in the
scintillator NE-213.

 

n+p->n+p
n+C—9n+C
n+C—>n’+C-4.44MeV
n+C—)He+Be-5.71MeV
n+C->n’+3He-7.26MeV
n+C—>p+B-12.59MeV

 

The (n,p) elastic scattering is the primary source of cross-talk. Figure 52
shows a typical example of a cross talk event. In this scenario a single neutron
scatters from a proton in the first wall, making a signal. The neutron does not
lose all of its energy and proceeds to the second wall where it scatters from
another proton and makes a second signal. To discriminate these events from
the real two-neutron events, we subject each two-neutron event to three tests.

1. The light pulse from the first scattered proton implies the proton’s energy Em.
From simple kinematics, Em implies the energy En' of the scattered neutron
and thus the scattering angle 0. If the scattering angle does not correspond to
the location of the second pulse, then the event is not cross talk.

2. If the scattered neutron’s energy En', as implied by Em, does not correspond
with the energy as implied by the time-of-flight between the two walls, then
the event is not cross-talk.

3. If the second scattered proton’s energy EP2 is greater than the scattered
neutron’s energy By, then the event is not cross-talk.

If the event passes all three test, it may be a cross-talk event. If the first recoil

proton P1 does not have sufficient energy to be detected, then the event is no

longer considered cross-talk and is considered out—scattering.

 

104

 

 

 

(Xllyl)

 

 

 

 

 

— (X2,Y2) n

 

 

 

 

Figure 52 - A typical example of a cross-talk event.

Although most of the recoil protons will be detected, the recoil carbons
will be too highly ionizing to ever produce enough light for detection. (as an
example, a 40-MeV carbon produces less light than a 1-MeV electron.) Therefore,
the carbon in the scintillator effectively becomes non-active material and
contributes to out-scattering. There is no way to identify these events, so we
must rely on computer modeling to understand the effects. The majority of the
out-scattering problem results in a loss of energy and position resolution in the
second wall; the results of a monte carlo simulation of two-neutron detection are
shown in Figure 53. This simulation assumed one neutron was detected in the
first detector wall and the second neutron in the second detector wall; each
neutron had an energy of 25 MeV, and the two neutrons had a relative
momentum difference of 10 MeV/ c. Figure 53 illustrates the effects on the
relative momentum measurements caused by the position resolution, the cells’
height, and the out-scattering. Although the effects of out-scattering are
significant, they are comparable to other intrinsic resolution effects in the

detector and thus we feel they are manageable.

 

105

 

 

      

 

300 I I I I I I I I I ' r l I l I l I l I
' Fixed Height Resolution ‘
250 - - - — Position Resolution ‘
A - ----- Out-Scattering -
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O 2 4 6 8101214161820
Relative Momentum (MeV/c)

Figure 53 - The results of a monte carlo simulation that models the effects on
relative momentum resolution caused by position resolution and out-
scattering.

10 The Neutron Wall Array in Recent Experiments

The Neutron Wall Array was completed in April of 1995. To test the
simulation models developed by Wang et al., we took data from a measurement
of the TLi(p,n)7Be reaction. This reaction was used because it produces only one
neutron, whose angular distribution and energy distribution are well known.
The simulations model the effects of cross talk and out-scattering, and they can
be easily compared to the results from the 7Li(p,n)7Be experiment. A paper
describing the model and the results from this experiment are to be published in
the near future. In the time since the Neutron Wall Array was completed, it has
been used in three different experiments; a very brief description of each

experiment is given below.

106

10.1 Measuring the 3Li(n,})9Li Neutron Capture Cross Section (Zecher et al.)

This first experiment involving the Neutron Wall Array was performed at
the end of the summer of 1995. This experiment attempted to determine the
8Li(n,y)9Li neutron-capture cross section at astrophysical energies. A direct
measurement of this reaction would involve both a radioactive beam and target;
therefore, we measured the inverse reaction, 9Li('y,n)3Li, where the y—ray was
provided by the coulomb field of a large nucleus. If the inverse reaction is
known, it can be related to the reaction of interest by the detailed balance

theorem.

This reaction was of astrophysical interest because it plays a crucial role in
the formation of A>12 isotopes in proposed nucleosynthesis models resulting
from an inhomogeneous big bang. The theoretical estimates of the 8Li(n, )9l.i
reaction rate varied by greater than an order of magnitude. Unfortunately, we
did not have sufficient beam time to determine the neutron-capture cross section,
but we were able to set an upper limit to the cross section. This upper limit was
consistent with two of the four current theoretical estimates, and seems to rule
out two higher estimates. We are submitting a proposal to carry out the

experiment again in the near future.
10.2 Two-Neutron Correlations from Heavy Ion Collisions (Gaflr et al.)

Following the 3Li(n,y)9Li measurement, the Neutron Wall Array was used to
measure two-neutron correlation functions from heavy ion collisions.
Correlation functions are functions of the relative momentum of two particles
emitted from a compound nucleus formed by a central, heavy-ion collision.
Two-neutron correlation functions give insight into the space-time extent of the
compound nuclei. Similar correlation measurements have been performed using

protons,34r35l36r37 but some have suggested that the coulomb interaction between

 

107

the protons and the residual nucleus will distort the correlation function. By
using neutrons, we hope to eliminate this concern. Before this experiment, only a
few other attempts had been made to measure two-neutron correlations.3’-’~39 Gaff
et al. expect to improve both the relative momentum resolution and low-
momentum threshold of these previous measurements. To date, the analysis of
the data from this experiment indicates that the correlation function is very

sensitive to cross-talk effects, but it seems possible to correct for the effect.40

10.3 Further Halo-Nuclei Structure Experiments (I. Kruse et al., I. Wang et al.,
Y. Iwata et al.)

Recently, we repeated the 11Li('y,2n)9Li experiment that provided the
original motivation for constructing the Neutron Wall Array. In addition to
measuring the coulomb dissociation of 11Li, we also measured the coulomb
dissociation of 5He and 8He at 25 MeV / nucleon. We are also attempting to apply
the same technique that we used in 8Li(n, )9Li experiment of estimating the
nuclear component of the dissociation by measuring the dissociation by targets
with a wide range of Z. Apart from using the Neutron Wall Array in place of the
previous neutron detector array, we have made other substantial improvements
to the equipment. In place of the Si-CsI telescope, we now use a C shaped dipole
magnet located after the target to sweep the fragments and the unreacted beam
away from the Neutron Wall Array. This will reduce much of the background
created by the unreacted beam in the telescope. The analysis of this experiment

has only just begun.

 

108

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110

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33 B. Jakobsson et al., Physical Review C, 44, R1238 (1991).
39 N. Colonna et al., Physical Review Letters, 75, 4190 (1995).

40 S. J. Gaff (private communication).

 

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