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thesis entitled

Fast Radiochemical Separations for Neutron
Deficient Short Lived Nuclides

presented by

Christine V. Hampton

has been accepted towards fulfillment
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Fast Radiochemical Separations for
Neutron-Deficient Short-Lived Nuclides

By

Christine V. Hampton

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the Degree of

MASTER OF SCIENCE

Department of Chemistry

1990

(650*- dJ'H'

ABSTRACT

FAST RADIOCHEMICAL SEPARATIONS FOR N EUTRON-DEFICIENT
SHORT LIVED NUCLIDES

BY

CHRISTINE V. HAMPTON

We review both simple and exotic radiochemical techniques used for the separa-
tion, purification, and analysis of neutron-deficient short-lived nuclides. These range
from classical wet-chemical methods to automated and instrumental techniques. We
give special attention to chemical adjuncts of the He-jet recoil-transport technique
and to high-resolution mass spectrometry. We also review recent developments in

biological applications, including 13N production and use.

This work was commissioned for the special issue of
Journal of Radiomalytieal and Nuclear Chemistry
commemorating
the 59th anniversary of
the discovery of fissxon.

It is dedicated to all present
and future scientists, so that we may
recognise and heed the
Inner Truth.

iii

Contents

LIST OF FIGURES v
1 Introduction 1
2 WET-CHEMICAL METHODS 3
2.1 Overview .................................. 3
2.2 Separation of Actinides by Ion Exchange ................ 5
2.3 Solvent Extraction Involving Very Few Atoms ............. 7
2.4 Volatilization Techniques ......................... 8
2.5 Thermochromatography ......................... 1 0
3 The He-JET RECOIL-TRANSPORT TECHNIQUE 13
3.1 Mechanism ................................ 13
3.2 Wet Chemistry with the HeJ RT Technique ............... 16
3.3 Cluster (“Plasma”) Chemistry with the HeJRT Technique ...... 16
4 13N AS A BIOLOGICAL TRACER 21
5 HIGH-RESOLUTION MASS SPECTROSCOPY 24
6 SUMMARY 31

References 33

List of Figures

2.1

2.2

2.3

3.1

3.2

3.3

3.4

4.1

5.1

5.2

Elution curve of actinides from a Dowex-50 ion-exchange column using
a-hydroxy-isobutyrate as eluant. Taken from Ref. 8. .........

Percent extraction into the organic phase as a function of the pH of
the aqueous phase for various elements having 2+ and 3+ oxidation
states. The plot includes points obtained for minute tracer quantities
of Lr and No, demonstrating that No2+ and Lr3+ are the most stable
oxidation states. Taken from Ref. 9.

Thermochromatographic spectrum for metallic chlorides produced above

600°C. The wall coatings are listed at the top of the figure, and the

temperature gradient is given by the diagonal line. Taken from Ref. 11. 11

Modular HeJRT system for delivering short-lived nuclides from the
target area to a distant counting area. See text for details. Taken from

Ref. 16. ..................................

Set-up for performing fast (nonequilibrium) ion-exchange on activities

delivered by a He] RT system. Taken from Ref. 20. ..........

Efficiency of transport of 23Mg and 26Si as a function of concentration
of the ethylene dopant in a He] RT system. Taken from Ref. 21. . . .

a-particle spectrum demonstrating that a small amount of 12 in the He

is necessary for a He] RT system to transport Hg isotopes. Taken from
Ref. 22.

Liquid chromatogram of the ”N compounds produced in untreated soil

and soil incubated with “denitrification” bacteria. Taken from Ref. 23.

Mass spectrum taken with LOHENGRIN showing the separation of

two A = 94 isobars with a resolution of A/ AA z 1.5x 10". Taken from
Ref. 35.

Fourier-transform ion cyclotron resonance mass spectrum of 3‘5 Cl+ and

35Cl‘. Taken from Ref. 33 .........................

15

17

19

23

26

30

Chapter 1

Introduction

Exotic, short-lived nuclides have intrigued scientists ever since the discovery of ra-
dioactivity, even more so since the discovery of fission, since many primary fission
products lie far from stability and have very short half-lives. A large body of radio-
chemical techniques has accrued over the years in connection with the study of fission
products and other short-lived nuclides—an excellent library/ summary of such tech-
niques can be found in the on-going series, The Radiochemistry of the Elements,1
produced under the auspices of the US. National Academy of Sciences, and in the

forthcoming compilation by Rengan and Meyer.2

In general, the half-life of a radionuclide decreases rapidly the farther that nu-
clide lies from the line of stability. In recent years interest has increased in study-
ing such short-lived species on two accounts: 1) Their large decay energies en-
able them to populate a myriad of states in their daughter nuclei, thus providing
a wealth of information about nuclear structure. 2) Many applications—including
those in medicine and biology—require the shortest-lived species commensurate with
the experiment to minimize long-term radiation damage. Although still in use—
especially for the purification of radionuclides intended for medical and biological

applications—straightforward wet-chemical methods have become increasingly com-

bined with and/ or supplanted by ingenious instrumental techniques. As improved cy-
clotrons and other accelerators enable the production of shorter- and shorter-lived ac-
tivities, speed becomes an essential factor. Also, because nuclides far from stability are
commonly produced by spallation / fragmentation and high-energy heavy-ion-induced
reactions—inherently messy reactions, producing a wide mixture of products—it be-
comes important to be able to determine the A as well as the Z of a radionuclide.
Thus, some type of mass identification is often combined with radiochemical tech-

niques.

In this paper we present an overview of selected wet-chemical methods, both
classical and exotic. This is not intended to be an exhaustive survey but is dictated by
our own experimental experience and bias. We also review the He- jet recoil-transport
(HeJ RT) technique, which has been a mainstay in the clean delivery of short-lived
nuclides to detector systems. We include several chemical and quasi-chemical adjuncts
developed in our laboratories. Next we examine some recent methods for producing
and handling 9.97-m 13N, which is becoming more and more used as a biological
tracer. We conclude with an examination of high-resolution mass spectroscopy as
a complement to radiochemical techniques for short-lived nuclides, making a few
suggestions and predictions for future developments. (As complementary reading to

this paper, we recommend an earlier chapter on “Techniques for the Study of Short-

Lived N uclei.”3)

Chapter 2

WET-CHEMICAL METHODS

2. 1 Overview

Sixty or seventy years ago, when the study of radioisotopes was called “isotopy” and
isotopes were referred to as “atomic types,” the only radioactive elements that could
be obtained in weighable quantities were Th, U, and Ra. In those early days of search-
ing for new radio-elements and developing separation methods, there occurred some
discussion about whether spontaneous separation of the isotopes could be expected
as a result of the gravitational field of the earth. Perhaps at higher levels in the
atmosphere lighter gases might be concentrated relative to heavier ones, and perhaps
various depths of the oceans might exhibit variations in isotope ratios were it not for
convection currents. Centrifugal forces and biological processes were also inspected
to determine whether or not they could effect separations. None was found. What
became adopted as standard radiochemical separation techniques were typical ana-
lytical methods for elemental analysis, together with the radioscope (electroscope)
for measuring activities. Typical examples were electrodeposition, distillation, dif-
fusion, vaporization, photochemical reactions, precipitation, solvent extraction, and
“the positive-ray method of separation” used for radio-elements that could be formed

only in “invisible” quantities.4

Otto Hahn, for example, reported5 that mesothorium-2 (228Ac) could be separated
from mesothorium (“28Ra) by two methods: by precipitating MsTh2 with NH; solu-
tion and by the electrolysis of weakly-acidic solutions of MsThl —MsTh2 separates

out at the cathode.

From the work of Szilard and Chalmers, Hevesy and Paneth reported4 that “when
an organic substance that contains I in combination with C is bombarded by neutrons,
radioactive isotopes of I are produced by the capture of neutrons. The molecular
binding is severed, and the radioactive I atoms are not united with C. (7-ray emission
breaks the chemical bond.) Because of this it is possible to obtain the active I in a very
concentrated form by precipitation of the free I with Ag or by shaking up the organic

liquid well with water.” Thus was born the well-known Szilard-Chalmers process(es).

Differences in reaction rates were also employed for separations. By using an
enzyme as catalyst and a countercurrent scrubbing apparatus, the difference in rate

of 12CO; and 13C02 reaction with water increased the concentration of 13C from

1.06% to 1.36%.

Some gaseous mixtures could be separated by photochemical reactions. If phos-
gene composed mainly of C035Cl35Cl, CO35Cl37Cl, and C037Cl37Cl was “rayed”
with light filtered to the exact wavelength corresponding to an absorption line of

C035Cl35Cl, then that molecule was preferentially decomposed, releasing 35Cl.

Even simple separations such as those of the radioactive inert gases were multi-
step processes. Most techniques were developed primarily for stable elements, then
successfully applied to long-lived isotopes. However, as shorter-lived species were
recognized and produced (primarily as intermediates in the natural decay series),
faster, more sophisticated techniques were developed, often involving physical as well

as chemical procedures.

We present examples of a few of the most important current techniques. Some of
the methods are offshoots of the original techniques, but many are much more recent
developments. At times innovations in wet-chemical procedures lagged behind those
in nuclear-chemistry procedures. For example, the first production of element 61 was
undoubtedly made by Pool and Quill,6 who bombarded Nd with neutrons, allowing
the radioactive Nd isotope(s) to decay by 6‘ emission up to element 61. However,
fractional crystallization was the only rare-earth separation technique available, so
the activities had all decayed away before they could prove the existence of element
61 by chemical means. The first chemical proof of element 61 had to await the
development of sophisticated ion-exchange separations.7 In the following sections we
concentrate on ion-exchange, solvent extraction (precipitation), volatilization, and

thermochromatography.

2.2 Separation of Actinides by Ion Exchange

One of the bulwark routine procedures for separating lanthanides and actinides is
cation exchange,8 which played an important role in identifying most of the heavier
actinide elements. A sulfonic-acid cation-exchange resin, such as Dowex-50, is used
with an ammonium a-hydroxy-isobutyrate solution as the eluting agent. The smaller
the charge radius, the better the complex an actinide ion forms with this reagent,
so the higher-Z elements elute first from the ion-exchange column. An example of
an elution curve using 0.4-M a-hydroxy-isobutyrate is shown in Figure 2.1. The
procedure requires a heated column and a relatively slow rate of elution for the
separation, so it can easily take several hours if the cleanest separations are desired.
If very short half-lives are involved, however, the column can be eluted more rapidly

in a non-equilibrium mode, still achieving decent separations (cf. §3.2). Because of

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a—hydroxy-isobutyrate as eluant. Taken from Ref. 8.

radiation damage to the organic solution and/ or the ion-exchange resin, if “massless”
sources (as for a spectroscopy) are required, then the initial separation must be
followed by a “clean-up” column, e.g., an anion-exchange separation using high-purity

HCl as the eluting solution.

2.3 Solvent Extraction Involving Very Few Atoms

Shortly after the discovery of element 103 (Lr), the question arose as to whether its
most stable oxidation state was 2"" like that of N 0 or 3+ like most of the other heavy
actinides. Solvent extraction was used to separate 31-s 256Lr from other actinides
and to determine its oxidation state.9 The organic phase was methyl isobutyl ketone
containing thenoyl trifiuoroacetonate, the aqueous phase various buffered acetate so-

lutions with pH values from 1.5 to 5.9.

256Lr was produced by irradiating a 250-p-g/cm2-thick 2‘49Cf target with an #70-
MeV 11B beam. The HeJ RT technique (cf. §3) was used to transfer recoils from the
reaction onto a Pt disc attached to a rabbit, which then transported it to the chemistry
lab in 3 s. The Pt disc had been coated with a few-pg/cmz-thick NH4C1 film for easy
removal of the recoils. This film, containing the 256Lt atoms, was dissolved with 0.1
mL of 0.1-M buffer solution and transferred to a 3-mL test tube containing 0.1 mL of
0.2-M thenoyl trifluoroacetonate solution. After le s of agitation, the mixture was
poured onto a heated Pt disc, where the aqueous phase could be pipetted off. The
organic phase was dried, flamed, and counted for 256Lr 0: particles. The overall process
took ~50 3. On average, 10 atoms were produced per experiment, but only about
1 / 10 this number were detected after performing the chemical separations because of
decay, counting geometry, and some chemical loss. In over 200 experiments z1500

256Lr atoms were produced for study. The results are shown in Figure 2.2, where

various 2+ and 3*” species are shown for comparison. Clearly, No2+ and Lr3+ are the

most stable oxidation states.

2.4 Volatilization Techniques

Volatilization techniques have been used in many forms. Radioactive noble gases have
been separated from aqueous or molten solutions by a volatilization technique some-
times referred to as “sparging,” in which an inert gas is swept over or through the
solution, carrying with it the volatilized radioactive gas.10 RuCl has been separated
from Mo, Zr, Nb, Te, and other chlorides based on differences in their volatilities.11
Oxides of Tc and Re in a mixture with Ru, Os, Ir, Se, Te, and Po react with K2Cr04
at 900°C, forming stable, non-volatile compounds (possibly pertechnetates and per-

rhenates). The other vaporized oxides do not form precipitates.11

An ion-source technique inspired by the observation of a fluoride impurity was
developed at the Orsay synchrotron, using the ISOCELE-2 on-line isotope separator.12
CE, is introduced as a reactive gas into the target ion source. Without CF4, a mixture
of Cs, Ba, and La isotopes is produced at M/Z = 129. With CF4, volatile metallic
fluoride ions are formed that undergo consecutive unimolecular decomposition of the
molecular ion, MP: , to form MFLI, MF:_2, etc. Cs forms CsF+, which decomposes
to Cs+; Ba forms BaF'z", which decomposes to BaF+ (major component) and Ba+;
and La forms LaFg', LaF; (major component), LaF+, and La+. By selecting a
combination of ion-source temperature and mass, the first selective on-line separation

of neutron-deficient nuclides was observed.

Researchers at OSIRIS13 have also used the CF 4 technique to separate Sr and Ba
as monofluoro ions and Y and the lanthanides as mono- and difluoro ions. They have

also developed a technique for 89"90Br and 139"““1, using Al vapor introduced into the

 

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2.5 Thermochromatography

On-line gas-phase separations can also be achieved using thermochromatography,
which combines gas-phase reactions with thermal separations after the ion source.11
This was first developed for the separation of fission products, but it shows consid-
erable promise for fast separations of very-short-lived accelerator-produced nuclides.
A carrier gas flushes the fission products from the source into a tube similar to a
gas chromatographic column. The carrier gas is doped with a reactive gas that can
produce volatile inorganic compounds—for example, N; with 13% CCl4 produces in-
organic chlorides. The column contains a combination of wall coatings for selective
retention of the various compounds. By creating a falling temperature gradient across
the column, groups of elements can be separated as a function of the volatilities of
their compounds. 11 Figure 2.3 shows results for chlorides produced above 600°C with

the wall coatings listed at the top and the temperature gradient represented by the

slanted line. The less-volatile compounds fall on the right; more-volatile on the left.

Selective “multidimensional” separations can be made by combining two or more
of the following parameters: 1) reaction with an added gas or column wall coat-
ing, 2) preferential chemisorption on the column wall, 3) low-temperature deposition
of less-volatile compounds, and 4) decomposition of thermally-labile compounds at
higher temperatures. An example is the separation of Ru and Tc from 252Cf fission
products.ll Tc, Ru, 1, and Xe oxides were separated at 900°C with N2(10% 02) gas
on an uncoated quartz column. Part of the I was retained by using an additional
column coated with Ag. The Tc, Ru, and I (partial) were retained in a 150°C trap,

while the more-volatile Xe compound moved through it. The trapped mixture was

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flushed with HCl to form volatile Tc oxychlorides and ICl; Ru remained in the trap.
Finally, the Tc and small amount of I were sent to a room-temperature trap, into
which the Tc was preferentially transported by adding 2% H20 to the carrier gas.

Such a procedure is an ideal one for automation.

Even greater selectivity can be obtained by combining thermochromatography
with the mass-selected beam of an on-line isotope separator. At OSIRIS“ a mass-
selected beam enters the thermochromatography apparatus and is collected on a
hot W filament. The separator column consists of heated concentric tubes, various
segments being cooled to produce the thermal gradient. The most-volatile species
leave the system through the inner tube, while other species progress through the
column at differing rates. Relatively unambiguous determinations of Z can be made

with this combination of techniques.

Chapter 3

The He-J ET
RECOIL-TRANSPORT
TECHNIQUE

3.1 Mechanism

The He- jet recoil-transport (HeJ RT) technique is one of the most important methods
for transporting short-lived nuclides from the target to a near or distant counting area.
First developed15 in the early 1960’s in a quest for nuclides near the elusive doubly-
closed-shell 100Sn, it has undergone many modifications and improvements, but its
operational mechanism is still only empirically understood.3 Basically it consists of
a chamber behind the target through which “doped” He flows. Reaction products
recoil out of the target into the He, which thermalizes and transports them to the

counting area.

There are many variants, from simple thimble—like chambers to elaborate multi-
ple-target arrangements, from nozzles that deliver a emitters directly onto detectors
to long (up to 200 m) capillaries that deliver 7 emitters to low-background areas.
There are also many variants on dopants for the He and contradictory opinions as

to why each works: Ultrapure He does not work, nor does the HeJRT work at room

13

l4

temperature in the absence of a plasma, such as produced by a cyclotron beam—for
use at a reactor, an x-ray or ultraviolet source must be provided. The plasma, act-
ing on the dopants—typically small amounts of benzene, ethylene, water vapor, even
“pump oil”—produces high-molecular-weight “clusters,” perhaps quasi-polymers or
nucleated droplets, and these clusters pick up and transport the radioactive species.
(A cryogenic variant uses pure He at very-low temperatures, perhaps creating “clus-

ters” or droplets of a different type.)

Perhaps the HeJRT Operation can be understood most easily by examining a
specific set-up. Figure 3.1 shows an exploded photograph and a schematic diagram of
a modular HeJ RT system used at Michigan State University.16 The system is attached
to the end of a beam pipe at A, and the beam is collimated at B and degraded if
necessary at C (“absorberdorber”). D is the target holder, and E and F are the
actual HeJRT chamber—the He enters through F and exits through the 1.4-mm
(inside diameter) polyethylene capillary attached to E. (G is an insulator, and H is
the faraday cup.) The modular design allows for tailoring the system to a particular
experiment, e.g., multiple targets and capillaries with large chambers for efficiency
vs a single tiny chamber with a rear-exiting capillary for fastest transport. With this
system we have been able to study le-ms activities at reasonable efficiency—at a

station some 50-m distant from the target.

There have been many developments and variations on the HeJRT system, too
numerous to cite here. Three of particular interest are: l) The coupling of a gas jet
with a hollow-cathode ion source for on-line isotope separation." 2) An automated
system for fast chemistry using a He + KCl jet.18 3) A “classical” HeJRT system
making use of many recent improvementsw—this paper also gives references to much

of the recent literature.

 
      

    

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Figure 3.1: Modular HeJRT system for delivering short-lived nuclides from the target
area to a distant counting area. See text for details. Taken from Ref. 16.

16

3.2 Wet Chemistry with the He] RT Technique

Whatever the composition of the “clusters” in HeJ RT systems, it is evident that the
radioactive recoils become attached to the surfaces of the clusters, for the delivered
activities behave chemically as simple, inorganic species and not as ions or atoms
embedded inside a polymer. We used such behavior to enable us to perform very

fast, non-equilibrium chemical separations.“0

One example was the separation of 32-3 63Ga, 38.1-m 63Zn, and 23.0-m 60Cu,
all produced by a 30-MeV p beam on a natural-Zn target. The set-up is shown in
Figure 3.2. The flow from the HeJRT capillary was mixed (turbulently) with 8N
HCl and directed onto the top of a Dowex-l anion-exchange column. A large mesh-
size (50-100) resin was used to minimize the column hold-up time. Ge(Li) 7-ray
detectors monitored both the column and the eluant, and we were able to improve
the 63Ga/f’s’Zn and 63Ga/‘SOZn ratios by factors of 4-10 over what was delivered initially
by the HeJ RT system. And this was done with the highly-non-equilibrium conditions

of forcing the activities through the column in less than 1 s.

3.3 Cluster (“Plasma”) Chemistry with the HeJ RT
Technique

Even faster “plasma chemistry” can be performed with a HeJRT system. This de-
pends on the nature of particular types of clusters and is presently understood only
at the most rudimentary level. Different dopants—hence, different types of clusters——
vary greatly in their transport efficiencies for different elements, and these differences
can be used to identify and separate different radioactive nuclides. Unfortunately, the

best conditions for a particular separation must be found pretty much by trial and

FROM ACID FROM HeJ RT

 

 

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Figure 3.2: Set-up for performing fast (nonequilibrium) ion-exchange on activities
delivered by a HeJ RT system. Taken from Ref. 20.

18

error, for even the empirical body of data concerning these effects is meager. N ever-

theless, such methods are well worth pursuing, for they are essentially instantaneous.

In Figure 3.3 we show the varying transport efficiencies for 2.20-s 26Si and 11.3-
5 23Mg as a function of ethylene concentration.21 Both activities were produced by
a 30-MeV p beam on a 27A1 target. It can be seen that the transport efficiencies
for both increase with increasing concentration of ethylene, leveling off around 150
ppm by volume. However, the transport efficiency for 23Mg increases more rapidly,

resulting in an effective separation factor of about 12.

Cabot et al.22 achieved a similar sort of effect with Hg isotopes produced by a
215-MeV “Ca beam on a 1“Nd target. Their results are shown in Figure 3.4. A
straightforward HeJRT system transports Pt and Au isotopes quite well, but the
resulting a spectrum shows no peaks from Hg isotopes. However, passing some of
the He over solid 1; (picking up a few ppm 12) before introducing it into the HeJ RT

system causes peaks from short-lived (1-4 s) 179”18ng to appear.

19

 

 

 

 

 

 

 

 

 

 

 

 

 

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the ethylene dopant in a He] RT system. Taken from Ref. 21.

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Chapter 4

13N AS A BIOLOGICAL
TRACER

Many elements important in organic and biological chemistry do not have convenient,
useful radioisotopes, and until fairly recently N was listed among these. However,
new fast methods of production, handling, and chemical separation have made 9.97-
m 13N an important tracer for biological applications. From the first use of 13N in
autoradiography to determine the site of nitrogen fixation in blue-green algae,3 this
. isotope has found uses in everything from studies of air pollution to “denitrification”

from fertilizers by bacteria in the soil.

13N can be produced by many reactions, e.g., by bombardment of suitable targets
(most often H20) with p, 3He, and 12C beams.23 Also, it has a far greater sensitivity
than most other radioisotopes of biological importance24—it is possible to detect as

little as 3x10‘20 mole.

A prime example of the use of 13N as a biological tracer is denitrification, now
being studied in many laboratories around the world.24 Denitrification is the process
by which nitrogen oxides, primarily NO; , are reduced to gaseous nitrogen products,
primarily N 2 and N 20, by bacteria that have the ability to use these oxides as alter-

native electron acceptors for their respiratory metabolism when 02 is limited. Not

21

22

only is denitrification wasteful of nitrate fertilizers, but also it is a considerable source
of air pollution, and there has even been speculation that its products are dangerous
to the earth’s ozone layer. Thus, the wide interest in studying its mechanism(s) and

determining ways to minimize the process.

Studies of denitrification using 13N are necessarily on—line because of its short
half-life. Gases such as He are used to sweep out the various 13N gases produced
in the bombardments, and various treatments, such as standard (Coleman) nitrogen
analyzers are used to select the desired compounds. The 13N compounds are then sent
to soil slurries, which have been incubated with the desired bacteria, and, after the
desired (short) reaction time, the products are monitored and/ or analyzed. Figure 4.1
shows the results from a high-performance liquid-chromatograph analysis, contrasting
the compounds removed from untreated soil vs those from soil after incubation with

a culture of Pseudomonas bacteria.

1_' .-' r‘
..s‘ ‘
£J>b..:‘.

L

23

 

 

 

 

 

 

 

 

 

 

“N03-“
6 .
Z
O
H
t-<- _ "NO." _
u:
8 ..
E NI‘I4 PS
’5; - TYPE 18 -
H
e.
”Z
UNTREATED
% : \e.
‘t 6
TIME [MINI

Figure 4.1: Liquid chromatogram of the 13N compounds produced in untreated soil
and soil incubated with “denitrification” bacteria. Taken from Ref. 23.

Chapter 5

HIGH-RESOLUTION MASS
SPECTROSCOPY

Of the physical techniques supplementing and at times supplanting wet-chemical
methods, high-resolution mass spectroscopy is perhaps the most important. Instru-
ments, of varying degrees of resolving power, range from classical magnetic spec-
trometers to electric quadrupole mass filters to recoil-product mass filters of various
kinds.33'3°'25 In the following paragraphs we give a quick overview of the types of
systems presently being used and/or under development. More details can be found

in the accompanying references.

Since the time Thompson and Aston pioneered the positive-ray method of isotope
separation, two differing philosophies for instrumentation design in mass spectroscopy
developed—one for the analysis of stable atoms and molecules, the other for the sep-
aration of radioactive species. Both relied on determining mass through observation

of the time and/or trajectory of a particle through either a magnetic (B) or electric

(E) field?”

For radioactive atoms, visual separations were important; emphasis was placed on
obtaining “weighable” quantities of an isotope. Two main types of separators evolved:

In the first category, high-power ion sources were coupled with large, constant-field

24

25

dispersive magnets. Initially, these isotope separators were used to analyze activities
collected in an “off-line” mode. Eventually, they were placed on-line (ISOL’s) with
an accelerator or reactor to separate and collect radioactive nuclides produced at
the target.30 Many design variations in the ion source have been attempted, from
fl-decay ionization of recoils“6 to gas-jet-coupled systems,“7 with varying degrees of
efficiency. The design objective was to reduce the kinetic energy and the number of
charge states of the radioactive nuclides, while maintaining high currents and at least
a nominal mass resolution. This increased the types of nuclides that were able to
be separated by reducing the time between creation at the target and analysis. In
order to separate nuclides of even shorter life times, the second main type of separator
was designed that could directly analyze recoil nuclides. In the Recoil Spectrometer,
counters such as E-AE detector telescopes were placed after the magnetic separator
stage to analyze the recoils at their original kinetic energy. This mass separation and
energy analysis, combined with precision timing electronics provided two-dimensional
analysis for the Z and A of an atom or ion, separated by charge states.“28 However,
for exotic, large—mass radionuclides, E—AE detector telescopes are not very effective.
Since Q values are used to determine the mass of the fragments in nuclear reactions,
any errors involved can be additive. In order to obtain higher resolution, another

magnetic or electric field would have to be placed in tandem with the first.

Thompson’s original mass spectrometer consisted of overlapping, perpendicularly
crossed magnetic and electric fields. Grenoble’s LOHENGRIN parabolic spectrometer
is similar, except the E field follows the B field in a perpendicular path with the fields
being parallel to each other.3°'35 The resulting mass resolution is 1 part in 1.5x104,
with an uncertainty in the kinetic energy of the particle around 1 MeV. An example

of a spectrum from LOHEN GRIN is shown in Figure 5.1.

Instrumentation developed for stable isotopes in the last 10-15 years allow more

(I particles
1mm
I———————-l

60 '
94Rb

l ,, i

20*

 

Figure 5.1: Mass spectrum taken with LOHENGRIN showing the separation of two
A = 94 isobars with a resolution of A/AA z 1.5x 10‘. Taken from Ref. 35.

27

or less routine mass determinations between 1.5x103 and 1x106 M/AM. Some of

these techniques have been adapted for radioactive nuclides.

The following instruments have been developed to analyze stable isotopes: the if
quadrupole (Q), combined double-sectonfield analyzers (B8513), time-of-flight (TOF),
and ion cyclotron resonance with fourier-transform analysis (FT-ICR). In addition,
over the past fifteen years a host of hybrid combinations of these techniques (QQQ,
BEQQ, BBE, etc.) have been created. Some result in ultra-high mass-resolution
analysis; others provide specialized methods for ion isolation. By coupling two or
more B, E, or Q analyzers, techniques (referred to as MS / MS) have been developed for
metastable ions and ions formed through collisionally-activated dissociations in order
to “uncomplicate” the mass spectra, identify precursor-daughter relationships, and

establish the presence of species that undergo certain decay (neutral-loss) modes.”33

Similar techniques, such as operating the sectors at different fields, have been used
in radioactive mass separators, but only as a means of deflecting contamination ions
formed in unwanted charge-exchange processes occurring in the ion souce.30 With the
incorporation of electromagnets into ISOL design, one can convert mass separators
into scanning mass spectrometers. Lampert and Chavet recently accomplished this
with EMIS MEIRA.3° Short-lived radioactive nuclides cannot be separated in bulk
quantities; however, the MS/MS scanning techniques used for stable ions could be
improved with coincidence electronics and adapted so as to enhance the presence of

such radioactive nuclides relative to background.

A few speculations on the applications of these techniques to radionuclides are
offered: — Ordinarily, the fragment ions from short-lived nuclides that decay within
the source or first analyzer would not be focused through the second sector. When
using MS/ MS techniques, the second field is scanned relative to conditions in the

first and, depending on the lifetimes of the nuclides, the fragment ions could be

28

selectively focused. — If multiple detectors were used, it would be useful to focus each
individual nuclide for further analysis by using a step/ scan technique called “single
ion monitoring”. For example, this might be useful as a clean-up procedure for 7-ray
spectroscopy. If coincidence electronics were combined with this technique, a multiple
number of nuclides could be individually analyzed in one experiment. — Volatilization
techniques (Chemical Ionization) could be combined with these scanning techniques

in order to differentiate nuclides with the same nominal mass.

After the separation step, determination of exact mass is often necessary. The
highest mass resolution can be obtained using an Ion Cyclotron Resonance Mass

Spectrometer, often referred to as an Ion Trap. Mass resolution values over 1x106

M/AM have been obtained.33

In 1981 researchers at Texas A&M University used Ion Traps with laser-fluoresence
spectroscopy on line with accelerators.“6 As the different ions emerged from a mass-
separating magnet, they were trapped in flight by a quadrupole ion trap. The “Physics
in Traps Panel” recently speculated” about using ion traps for high-precision mass
spectroscopy of radionuclides. Radial confinement of ions in the traps is effected by
a multi-kG magnetic field. The ions undergo cyclotron motion, inversely related to
their masses. Fourier-transform analysis can provide a mass spectrum, reconstructed
from the frequency domain. An example of such a spectrum is shown in Figure 5.2.
This mass spectrum was obtained using a 4.7 Tesla superconducting magnet with a
pulsed positive ion/ negative ion detection mode.33 The difference in the mass of the

ions is the mass of two electrons plus their binding energies.

Besides the high resolving power, there are other advantages to the Ion Trap
Spectrometer: There is a signal to noise enhancement equal to the square root of
the number of sampling intervals if Fourier transformation is used. High charge

states do not have to be reduced. It is more appropriate to use the highest charge

29

state possible because the signal increases linearly with charge for a given oscillation
amplitude. Also,the signal coupling to the external tuned electronics circuit increases

with the square of the charge.37

An Ion Trap spectrometer can be used in combination with other separators.
A high-resolution mass spectrometer composed of two Penning-type traps has been
in use at ISOLDE at CERN/Geneva.38 The first trap acts as an ion buncher and
collects the reaction products; mass analysis is then performed in the second, precision

trap. Test experiments with stable 13‘3Cs indicate a mass resolving power of 1 part in

1.1x106, corresponding to a AE of 0.1 MeV at M/q = 133.

An ion-guide quadrupole mass spectrometer has recently been designed that in-
corporates a He.) RT system to transfer recoils to a commercial residual gas analyzer,

1.39 These ions will have a kinetic en-

an rf quadrupole operating at ground potentia
ergy two to three orders of magnitude lower than those emerging from a magnetic
separator. The system should separate ions having half-lives as short as 1 ms. The

developers suggest that it can be used as an injection stage for a high-precision mass

spectrometer, such as a Penning- type ion trap.

Analyzing exotic nuclides far from stability with high resolution mass spectroscopy

is just beginning. The role of Ion Traps and Fourier transformation is very promising.

30

SSCI+ 35C|_

 

 

JK—va.v~\l.v.vy”s

muff rm

 

Figure 5.2: Fourier-transform ion cyclotron resonance mass spectrum of 35Cl+ and

35Cl'. Taken from Ref. 33.

Chapter 6

SUMMARY

In this paper we have presented a quick historical overview of the development of
fast chemical-separation techniques for short-lived nuclides. As improved accelera-
tors allow the production of exotic nuclei lying very far from stability, chemical- and
physical-separation techniques will grow more and more mutually dependent. Wet-
chemical procedures will be joined onto gas-jet transport methods, and chemistry will
play a more significant role in mass separation and high-resolution mass spectroscopy.
Also, radioactive tracers such as 13N will play significant roles in organic and biologi-
cal chemistry, necessitating development of sophisticated reaction techniques in these
disciplines. Finally, the imminent widespread development of radioactive beams40
should lead to even more sophisticated chemical and/ or physical-instrumental proce-

dures for dealing with ever more exotic species.

31

32

ACKNOWLEDGMENT

I would like to thank Professor William C. McHarris for his guidance and Professor

Krishnaswamy Rengan from Eastern Michigan University for suggesting this work.

Also, I would like to acknowledge the aid of the guidance committee: Professors
Carl Brubacker, Katherine Hunt, and John McCracken from the Chemistry Depart-

ment and Professor Jerry Nolen from NSCL and the Physics / Astronomy Department.

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