accelerator mass spectrometry and nuclear physics

371 Nuclear Instruments and Methods in Physics Research B17 (1986) 377-384

North-Holland, Amsterdam

Section I. Use of accelerators and lasers for analysis

ACCELERATOR MASS SPECTROMETRY AND NUCLEAR PHYSICS *

Walter KUTSCHERA

Argonne National Lrrborafoty. Argonne, IL 60439, USA

The use of accelerator mass spectrometry (AMS) for problems related to nuclear physics is discussed. Examples are given for

AMS measurements of long half-lives. cross sections, exotic particles. and solar neutrinos.

1. Introduction

The potential of accelerators as a sensitive mass spectrometer was recognized in the very early days of accelerator physics. In 1939 Alvarez and Cornog [1,2] used the newly built 60-in. cyclotron [3] in Berkeley to find out which of the mass-3 isobars, ‘He or ‘H, was stable. Since everybody at this time believed that ‘He was heavier than 3H, they were actually planning to attempt a half-life measurement of 3He. The idea of the experiment was to produce these isotopes by bombarding deuterium with deuterons, feeding the irradiated gas into the ion source of the cyclotron and tuning it to 3He2+ ions. By measuring the ‘He/“He ratio as a function of time they were hoping to find the half-life of 3He. This early experiment is described with all its interesting twists in a recent reminiscent article of Al- varez [4]. It is quite amusing to read that Alvarez and

Cornog never performed the experiment they originally had planned to do. What actually happened is that they accidentally discovered natural ‘He when they were checking out the background in the expected ‘He’+ region, operating the cyclotron with helium. Since the helium had come from an approximately 100 million year old deposit in a gas well, the stability of 3He was readily established. Alvarez and Cornog measured the 3He/4He ratio in gas-well and atmospheric helium and found correctly that they differ by a factor of ten. Although the absolute ratios, IO-* and IO-‘, respectively, were too low by one order of magnitude, the power of using an accelerator as a sensitive mass spectrometer was clearly demonstrated. Absolute ratio measurements in this range are difficult to perform, since all the normalizing factors between the measurement of the abundant isotope (usually a current measurement) and the rare isotope (ion counting) must be known.

The experiment of Alvarez and Cornog [ 1,2] incorpo-

rated many features of the field we now call accelerator

* This research was supported by the US Department of

Energy under contract W-31-109-Eng-38.

mass spectrometry (AMS). Besides the high sensitivity mentioned above it utilized fully stripped ions. namely 3He2+. This allowed for an easy separation from ‘H+ ions due to the cyclotron resonance condition o/B = Q/M. The minute mass difference between 3HeC and ‘Hf ions, Am/m = 5 x 1O-6 would have been impossible to resolve. The separation of isobars by stripping off all electrons is now utilized for the detection of several long-lived radioisotopes in AMS experiments. For example, P. Kubik et al. [5] performed various experiments with 36CI (T,,Z = 3.0 X 10’ yr), where 36C1’7+ ions were separated from the stable isobar 36S’6+ in a

magnetic analysing system. This method can be used at sufficiently high energies for all cases where the isobar of interest has the higher atomic number than the

background isobar. Another important aspect of the ‘He experiment was the identification of ‘He’- not only by the cyclotron resonance condition, but also by the energy measured through the range in Al foils. Today, energy and range, and in particular energy-loss measurements are an essential part of most AMS experiments to identify the atomic number of the detected ions. After the stability of ‘He was established [l]. Alvarez and Cornog [2] set out to find the radioactivity of ‘H. They bombarded deuterium with deuterons, separated hydrogen from the rest by diffusion through hot palladium and showed that ‘H was indeed radioactive with a very short-ranged radiation and a long half-life (now known to be 12.3 yr). The ‘He experiment was not only the first one to use an accelerator for mass spectrometry, but is also was the first one to solve a nuclear physics problem with AMS.

After this important experiment, the method of using accelerators for sensitive mass spectrometry was forgot- ten for almost 40 years. During this period the main purpose of accelerators was to deliver beams of ever increasing energy and atomic number for experiments after the accelerator. The accelerator itself almost never was an intergral part of the experiment. The AMS technique was revived in 1977 simultaneously in several laboratories using a cyclotron [6,7] and tandem acceler-

1. ACCELERATORS/LASERS FOR ANALYSIS

378 W. Kurschera / AMS /or nuclear physics

ators [g-11]. It was quickly recognized that this technique had a great potential for dating with 14C (T,,z = 5730 yr) and various other applications with naturally occurring long-lived radioisotopes such as ‘“Be (1.6 x

IO6 yr), 26AI (7.2 x lo5 yr) and 36Cl (3.0 x lo5 yr). The rapidly growing field of AMS applications is docu- mented in the proceedings of three conferences [12-141. Some of the recent applications will be discussed in other contributions to this conference. In the present

paper the discussion will be concentrated on the use of AMS in its original field, namely nuclear physics.

Nuclear physics has grown into a rather complex field and it is sometimes difficult to state what we mean by using this term. In the context of this paper AMS experiments will be described which are related to prop- erties of nuclei and particles, and to nuclear reactions. The following topics will be discussed: Long half-lives (sect. 2). cross sections (sect. 3), exotic particles (sect. 4), and solar neutrinos (sect. 5).

2. Long half-lives

It is well known that a wide range of half-lives can be measured by following the change in decay rate with time. For such a measurement, the absolute number of radioisotope atoms. N, need not be known. However, if the half-life is too long for such a measurement, N must be measured to obtain the half-life from the relation d N/dr = -X N with X = ln2/T,,,. Many long half-lives have been determined in the past by measuring N with

low-energy mass spectrometry (LEMS), if a sufficiently large number of atoms was available to overcome the interference from stable isobars. The large discimina- tory power of AMS allows one to extend these measurements to difficult cases which LEMS cannot handle.

So far. three half-lives have been measured with AMS (see table 1). The technical challenge in these

Table 1

Comparison of half-life measurements with AMS and other

techniques

Isotope T,,* (yr) Technique Ref.

32 Si 330 k40 depth distrib. (ice) [IS]

216 *32 depth distrib.

(sediment) 1161 101 f 18 AMS 1171 108 f 18 AMS PRl 172 +4 decay 1191

“Ti 46.4 k1.7 LEMS I201 48.2 +0.9 LEMS 1211 54.2 +2.1 AMS PI

60Fe =3x105 cross sect. syst. WI (1.49+0.27)x10b AMS 1241

measurements is to get rid of strong stable-isobar background and to measure an absolute radioisotope-to-stable isotope ratio with some useful precision. These problems are discussed in detail in the references given

in table 1. To measure absolute ratios is different from most other AMS applications where usually calibrated standards are available. Although this reduces the precision for this type of measurements it has the advantage

that one is forced to understand every factor which influences these ratios and cannot “sweep things under the rug” by a measurement relative to a standard. In the

following I should like to briefly discuss the half-lives of

‘*Si and 6o Fe. Hal/-life oj %i: This radioisotope is of interest as a

naturally occurring species, produced in the atmosphere by spallation of @Ar with cosmic-ray protons. Although its production rate is estimated [25] to be lower by factors of about lo4 and lo2 as compared to 14C and ‘oBe, respectively, it has been detected since 1960 [26] in sediments and other terrestrial reservoirs. Any useful chronological application of “Si rests of course on the knowledge of its half-life, but in spite of considerable effort there are large discrepancies between different measurements. In fig. 1 the history of half-life measurements is presented. A clearly pronounced trend to shorter half-lives is seen. Prior to 1970 all half-life measurements used cross section systematics of various reactions to estimate the amount of ‘*Si atoms produced. Correspondingly these values were quite uncertain. Some details of these early measurements are discussed in ref. [17]. Two measurements of the depth

900 ,,,,,,,,,,,,,,,,,,,,,,,,‘,‘“,‘,,,,,’, j

w 500 u. i 1 LOO

!?I 4 300

(I ESTIMATED 71

Cl l p X

%I t t

“SI + 2n ’ X I

Fetp X

“Si +t X

ICE

SEDIMENT

0

DECAY

AMS l

8

1950 1960 1970 1980 1990 YEAR OF MEASUREMENT

Fig. 1. Summary of half-life measurements of ‘*Si since 19.53.

(No errors are given for the half-life estimates prior lo 1970.) A detailed discussion of measurements up to 1980 is given in ref.

(171. The error of the 1985 value [19] is comparable to the size

of the data point.

W. Kutschera / A MS for m&ear physrcs 379

distribution of cosmogenic ‘*Si in Greenland ice [lS] and ocean sediments [16] gave half-lives around 300 yrI in rough agreement with each other. However, these half-live values strongly depend on the chronology of ice and sediment deposition and on the constancy of 32Si supply to the respective reservoirs. Both values have been corrected later to somewhat lower values by other authors [16,27]. AMS measurements have been carried out at Argonne National Laboratory 1171 and the University of Rochester [18]. The resulting half-lives,

101 f 18 and 108 + 18 yr respectively, agreed re- markedly well, but where clearly lower than the geo- physical measurements. Very recently a new half-life

value of 172 + 4 yr was found [19] by following the ‘l!Si decay over a period of 4.0 yr. The minute changes in decay rate (only 1.6% in 4.0 years) were measured in a precision proportional counter arrangement [28] relative

to a 36Cl (3.0 X lo5 yr) standard. This new result indi- cates that the question of what the half-life of “Si really is, is clearly not settled.

Half-/ife of “‘Fe: Until recently, the half-life of 6”Fe was not well known. The only measurement in the past was performed by Roy and Kohman (231 in 1957 who reported a value of 3 x lo5 yr, uncertain by a factor of three, In this measurement the number of @‘Fe atoms was estimated from spallation cross section systematics of high-energy protons in copper. The AMS measurement at Argonne [24], which for the first time used a tandem-superconducting linac system for an absolute 60Fe/Fe ratio measurement, established a significantly longer half-life of (1.49 f 0.27) x lo6 yr. This improved value is of importance, since 60Fe belongs to the group of extinct radioisotopes, whose presence in the early solar system is detectable in certain meteoritic inclu- sions through isotopic anomalies of the stable decay product. So far, positive evidence for isotopic excess of

26Mg, “‘Ag and ‘29Xe has been found. This has been attributed [29] to in situ decay of primordial 26A1 (T,,, = 7.2 X 10’ yr), “‘Pd (6.5 X lo6 yr) and 1291 (1.6 X 10’ yr), thus establishing a time scale between the last synthesis of matter piror to the formation of the early solar system. Isotopic excess of 60Ni due to u’Fe decay has not yet been found [31-331. However, future searches with meteoritic material of very high Fe/Ni ratos many well find a positive evidence. There still exist a few uncertain half-lives accessible to AMS techniques at sufficiently high energy. One of them is ‘*%In with an estimated [30] half-life of = 1 x 10’ yr. If this half-life turns out to be at least that long it could become another piece in the interesting puzzle of under- standing the early solar system by measuring extinct radioactivities in meteorites.

Recently, live 26AI has been observed in the interstel- lar medium through satellite-based y-ray spectroscopy [34,35] measuring the 2+ -+ 0” y-line from 26Mg. Con- siderations [36] on the stellar origin of this finding make

60Fe another candidate for observing ongoing nuclear synthesis [37]. In this case one would look for the well known y-line from 6oCo (5.27 yr) decay, an interesting

project for the next generation of y-ray experiments in space.

3. Cross sections

Many production cross sections of radionuclides can be measured by radiochemical techniques. However, when the half-life becomes too long for an efficient

decay counting it is better to count the atoms directly by AMS. In an early measurement of this kind the cross

section of the reaction 26Mg(p, n)‘6A1( T,,r = 7.2 X 10’

yr) was measured [38] by bombarding 26Mg foils with protons, adding “Al for normalization and measuring 26AI/‘7A1 ratios in the range of lo-“. It is quite re-

markable that in spite of ‘6Mg/26AI ratios of around 10” no chemical separation of 26Mg from 26A1 was

27*,“* =“d’+

SAMPLE +6 NON-IRRADIATED

I,.

Fig. 2. Ion spectra from an irradiated and a blank rample in

the cross section measurement of the ‘6Mg(p. n)26A1 reaction

[38]. The 26A~/27Al ratios are 9x10-” and 4~ 10-‘2. respec-

tively. The spectra were measured in the focal plane detector of

a split-pole magnetic spectrograph.

1. ACCELERATORS/LASERS FOR ANALYSIS

380 W. Kutschera / A MS for nuclear physics

necessary, because 26M g does not form stable negative ions. In fig. 2 two spectra from this experiment measured with the Argonne FN tandem and a magnetic spectrograph as heavy ion detection system are shown.

In a very recent experiment with the Argonne

tandem-superconducting linac system we have measured the spallation cross section of 165 MeV protons on nickel producing “OFe. A preliminary value of about 0.2 mb was found. Spailation reactions are of interest for the cosmic-ray production of 60Fe in iron meteorites which contain typically about 10% nickel. A first hint of WFe in a meteorite was found in these experiments. The main technical problem in these experiments is the

separation of ““Fe from a very s trong, ever-present ‘(‘Ni background. Since the two ion species are not dis- criminated in the acceleration process they arrive with the same energy of about 360 MeV at the final detection system. In the half-life measurement of hOFe [24] the separation was achieved by passing the ions through an Al foil stack (2.8 mg/cm*). The ions emerge with a different energy from this foil and therefore arrive at different positions in the focal plane of a magnetic

(a) 6oNi _-__---_----__-,

IO5 d

104Z 5

IO35 \

IO2 g

z ‘0 v

I

Fig. 3. Mass-60 ion spectra measured in the focal plane detec-

tor of the split-pole spectrograph. The comparison of a 60Fe/Fe

ratio measurement of 1 x lo-’ with foil stack separation (a)

and of 5 X 10m9 with gas separation (b) is shown.

spectrograph. The intense WNi component can now partially be shielded from entering the focal-plane detector. Since the splitting into several charge states

and the energy straggling tails limited the sensitivity of this method to 60Fe/Fe ratio measurements of about lo-” a different separation technique was developed for the cross section and meteorite measurements. The vacuum chamber of the spectrograph was filled with nitrogen gas of a few Torr pressure and the foil stack removed. In this arrangement the ions experience many charge changes and differential energy losses during their flight path of about 3 m through the spectrograph. As a result of these process they end up in a mean charge state and with improved focal plane separation. In addition, since there is no splitting into different charge states the ion detection efficiency is considerably increased. In fig. 3 a comparison of 60Fe detection from enriched samples for the foil and gas technique is shown.

It is obvious that WNi is suppressed much better by the gas method. In fig. 4 two spectra with an enriched sample of low 60Fe/Fe ratio and a meteoritic sample are shown. Although only one 60Fe count in three hours of running was observed, the cleaness of the spectrum allows to identify this count unambiguously as WFe.

(b)

Fig. 4. Mass-60 spectra measured with the gas-filled spectro-

graph for an enriched sample with wFe/Fe = 7X IO-l2 (a)

and the iron meteorite Treysa with 60Fe/Fe = 3 X 10.. I4 (b).

W. Kutschera / AMS for nuclear physics 381

The measured “Fe/Fe ratio of = 3 X lo-r4 is con- sistent with the expectation from preliminary measure-

ments [39] of #Co activity produced by 60Fe decay in some other iron meteorites.

4. Exotic particles

It seems that somehow the term “exotic” char- acterizes particles which either do not exist or cannot be found in measurable quantities. This means that every search is bound to end in a negative or a false positive

result. There are quite a few examples for this situation in the recent past including searches for free quarks, superheavy elements, anomalously heavy isotopes, gravitons, monopoles, tachyons and so on. In this brief

discussion of exotic particle searches with AMS only searches for free quarks will be considered. A more thorough review on the dectection of rare particles with AMS is given in ref. [40].

It is one of the most intriguing facts of modern particle physics that quarks, which nobody doubts to form the sub-structure of hadrons, cannot be freed from them. Well over a hundred experiments have looked for free quarks, all with negative results except for one [41]. Although the Stanford levitation experiment never claimed to have detected free quarks, they present [41] very strong evidence for the observation of fractional charges on small superconducting niobium spheres (weighing about 10e4 g each). It is quite natural that AMS got involved in these searches, since a fractionally charged particle has a quite unique signature in accel- erating and beam analysing processes. The main problem is that the mass of a hypothetically free quark is not known, Fortunately, AMS can nicely adjust to this problem by avoiding any magnetic focussing or analys-

ing components. A purely electrostatic system will bring all particles independent of their mass from the ion source to the final detector. The most simple experiment of this kind was performed [42] with the 700 kV Cockcroft-Walton injector at Fermi Lab. This particular experiment was designed to search for + fe particles extracted from a niobium filament which was first cooled to 4.2 K and then heated to several hundred K. During this process energy spectra for any positively charged particle falling within the mass range from 10 MeV/c’ to 100 GeV/c’ were measured in a Si surface barrier detector. No indication of particles with one third of the energy of protons were seen. Whereas this experiment was directed towards testing a hypothesis [43] which could have explained the Stanford findings, AMS experiments at Toronto [44], Caltech [45] and Rochester [46] use tandem accelerators for more general searches of fractionally charged particles in matter. The sample material is atomized in an ion source, accelerated through the tandem, and by using various charge-chang-

ing processes combined with energy and electric rigidity measurements the desired particles are filtered from the background. These measurements cover a wide range of fractional charges and masses. In some cases [45] sensi- tivities of around lo-l8 fractional charged particles per “normal” atom were reached. So far no positive evidence has been found. With a sensitivity of lo-‘*, AMS searches have reached a practical limit in performing free quark searches. Since it is impossible to search through all materials in the world, what is needed most is a very good “educated guess” of what material could

have trapped a few quarks posibly left over from the big bang.

5. SoIar neutrinos

The importance of detecting solar neutrinos on earth can probably best be emphasized by quoting W.M. Fowler from his 1983 Nobel Prize lecture [47]: “Until the solar neutrino problem is resolved, the basic princi- ples underlying nuclear processes in stars are in question”. The well-known solar neutrino problem is based on the discrepancy between the neutrino flux predicted by the standard solar model [48] and the result of the 20-year effort of R. Davis and collaborators [49] to measure this flux on earth. The prediction is high by about a factor of three and canot be explained within our current knowledge of nature.

Fig. 5 shows a comparison of the solar neutrino flux spectrum on earth with the energy range covered by different inverse beta decay reactions. Important are the different thresholds of the reactions indicated schematically by the left edge of the reaction box. Only three reactions are sensitive to the main solar neutrino flux originating from the pp reaction. Altogether there are three different types of inverse beta decay reactions shown in the figure, requiring different detection techniques, The short-lived isotopes 37Ar [49] and “Ge [SO,511 can most sensitively be detected by decay counting in well-shielded small proportional counters. The long-lived isotopes ‘“Kr, 98Tc and M5Pb require direct atom counting techniques. For the detection of a small number of “Kr atoms laser resonance ionization spectroscopy has been investigated [52] and for the detection of ‘*Tc [53] and “‘Pb [54] exploratory AMS experiments have been performed. The ultilization of the “‘In (v,, e-yy)“%n reaction is proposed [55,56,57] to be done with a real time detector, which in principle would allow to perform neutrino spectroscopy. So far, only the 37C1 experiment has been realized and all evidence for a lack of solar neutrinos is based on this single result. Since it is not sensitive to the essentially model-independent flux of pp neutrinos, an experiment with a lower threshold is most desirable. Most likely this will be the “Ga experiment and efforts for its

I. ACCELERATORS/LASERS FOR ANALYSIS

382 W. K&schwa / AMS for nuclear physics

b p windowfoil

I I ionizolion AE

chamber AE

AE

I I

I I "Galv,.e-1 "Gel11 Ldl

I I /

“Br (v..e-1 “Kr I2 1 I lo5 yr)

1 I I I I I I

98Molve.e-)98TcfL 2x106yr

‘0 ‘E s 0

7 E c

0 g

;i 8 0 Pep

: E

VI ,: ._

: E

i s 105-1

0.1 0.3 1 3 10

Neutr~no Energy IMeL’)

Fig. 5. Solar neutrino flux spectrum at the earth as predicted

by the standard solar model [48,50]. The boxes above the spectra schematically indicate the energy range covered by the respective reactions. The left boundary of the boxes corre-

sponds to the threshold energy.

implementation are under way in Germany, USA and USSR.

A common problem in all solar neutrino detection experiments is the extremely low reaction rate. Only a few atoms are produced in this way per year in one ton of target material. Therefore, if only short accumulation times are possible as is the case for 37C1 and ‘tGe, multi-ton detectors are mandatory. On the other hand, the long-lived radioisotopes 98T~ and *“Pb can accu- mulate over million of years in stable minerals. *05Pb is most attractive because of the very low threshold (fig. 5) and very long half-life. One kg of a suitable thallium mineral may well contain lo5 *05Pb atoms after a 10’ year accumulation time [58]. Although there are many open problems in implementing a full *05T1 experiment, we have performed [54] test experiments with the heavy- ion accelerator UNILAC at the GSI Darmstadt to investigate AMS for such a heavy isotope. Briefly, a technique similar to the 60Fe experiments was used, namely the combination of an absorber with a magnetic spectrograph to separate the isobars 205T1 and *“Pb. Fig. 6 shows in the upper part a schematic of this experimental arrangement. *05 Tl and *05 Pb ions of iden- tical energy (2.3 GeV) experience different energy losses in a gas absorber due to their different nuclear charges.

stop kcintillotor foil1

speclroqroph start detector (channel plate)

2 5 8

0 1000 2000 3000 LOO0 5000

B+Q

Fig. 6. Measurements with the UNILAC accelerator at GSI

Darmstadt for the separation of “‘Pb from 205Tl [54]. On top

a schematic of the spectrograph and the absorber arrangement

is shown. Spectrum (a) is the raw position spectrum, whereas

in (b) gating conditions on various other parameters measured

in the spectrograph are applied. The 205Pb/205Tl ratio in the

ion source sample was 10-t.

This energy-loss difference is determined by the magnetic spectrograph in a high resolution measurement of the remaining energy. In the lower part of fig. 6 two spect.ra are shown which demonstrate the separation of “‘Pb from *05T1. With the given condition a nuclear charge resolution of about lo-* was achieved. This allows for 205Pb/205T1 ratios of 10m3. A very efficient suppression of neighbouring lead isotopes of about 1 : 1016 was found due to the combined selectivity of injection resolution, rf selectivity of the linac, and beamline and spectrograph resolution.

Although the isotopic selectivity is sufficient for real samples, much has to be improved for the isobar separation, since the 205Pb/ *“Tl ratio in thallium mineral

W. Kutschera / AMS for nuclear physics

1 RADIOISOTOPES WITH T,,2 >I year

“3a

I

(4b) DECAY hIODE I

40K E

*‘Rb x

. ‘44Nd

152 Ed

a P‘ x P’ * EC

* SF

q a

e IT

g2Nb * 98T, “YPd

232Th

II 238 n ”

235” m

244n I

383

I c 10 18 10 18 18 1 I I II 8 LU 10 ’ “_ 3 ’ 8 13 I I 0 20 40 60 80 100 120 140 160 180 200 220 240 260

MASS

Fig. 7. Overview of long-lived radioisotopes with half-lives greater than one year [59]. Isotopes which have been utilized by AMS are marked with a circle.

is estimated [58] to be around 10-19. Extensive chem- istry will have to be done to bring this ratio down anywhere near to the measurable level of AMS. On the other hand, future heavy ion machines of much higher energy may allow AMS measurements with much higher isobar selectivity. For example, the planned heavy ion synchrotron at GSI will deliver energies high enough to strip off all 82 electrons from 205Pb making possible a very efficient separation from 205T18’+ ions in a charge-selecting device. Besides the isobar separation, a substantial improvement in ion source efficiency is necessary [54] for an actual solar neutrino detection experiment. However, these accelerator-based problems do not seem to be insurmountable and may well be solved in the near future with improved accelerator technology.

In conclusion one may say that AMS has grown into an extremely powerful tool to count long-lived radioisotopes of almost any element, and particles of hitherto unknown species at very low concentrations in matter. There exist a large number of long-lived radioisotopes

which are available for AMS experiments. This is demonstrated in fig. 7 which shows all radioisotopes with half-lives greater than one year. Only those marked by circles were actually utilized for AMS. It is obvious that one can imagine a great potential for future experiments with the many remaining isotopes. Among the various fields accessable to AMS nuclear physics still offers many interesting problems, and certainly the solar neutrino detection is one of the finest examples.

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