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Heavy Ion Accelerator Facility Department ofNuclear Physics Australian National UniversityGeorge DracoulisPublished online: 27 Apr 2009.

To cite this article: George Dracoulis (1999) Heavy Ion Accelerator Facility Department of Nuclear Physics AustralianNational University, Nuclear Physics News, 9:1, 9-19, DOI: 10.1080/10506899909411106

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laboratory portrait

Heavy Ion Accelera Department of Nuc Australian National

T h e major accelerator-based nuclear research facility in Austra- lia is operated by the Department of Nuclear Physics, one of the larg- est Departments within the Re- search School of Physical Sciences and Engineering in the Institute of Advanced Studies a t the Australian

. National University. As well as'hav- ing an inordinately long address, the Institute of Advanced Studies is unique in the Australian University scene. From its beginning as a re- search institution in 1947, it has grown to be a collection of seven major Research Schools, covering disciplines as diverse as Medicine, Archeology and the Social Sciences. The Research School of Physical Sciences and Engineering (originally known as Physical Sciences ) was one of the foundation Schools, with Nuclear Physics as its core discipline.

Figure 1 shows the layout of the present laboratories, which deliver a wide range of heavy ion beams to the experimental areas. The 14UD Pelletron is coupled to a Supercon- ducting Linear accelerator whose components were transferred to the site under an agreement with the Engineering and Physical Sciences Research Council of the United Kingdom.

The 14UD Pelletron, the center- piece of the facility, was the first large accelerator constructed by the National Electrostatics Corporation (NEC). Much of its development, which has given it an outstanding record in stability and reliability a t high voltage operation well in ex- cess of its original rating, was car- ried out on-site, with the close in- volvement of Department staff.

The participation of both tech-

:or Facility .ear Phvsics

J

University

nical and academic staff in facility projects, such as the recent installa- tion of the LINAC, and the fact that operation of the accelerators and ion sources is carried out largely by the academic staff and students themselves, gives the laboratory an unusual character in facilities of this scale. It functions as a de-facto National Facility, with local, na- tional and international users and collaborators, and supports a di- verse scientific programme of basic and applied research.

Background and Current Research The achievements of the Depart-

ment have rested in many ways on its ability to develop accelerator and detector facilities with limited resources, and to operate these con- tinuously and reliably. When the 14UD accelerator started produc- ing beams in 1974, the laboratory

had in concurrent operation the 14UD, a 2 6 iMeV cyclotron, a 6 M V E N tandem, capab le of stand-alone use o r injection by the cyclotron, and a 2 M V single- en d e d m a c h i n e, together with separate beam-lines, target areas, data collection systems, etc. for all, a long way from its begin- nings with a Cockcroft-Walton generator in 1951.

By the time the EN tandem was being superseded, it had supported over a decade of nuclear research, particularly in the study of the level structure of light and medium- weight nuclei with proton and a- beams. However, it was already being used for studies of single- nucleon transfer reactions with heavy ions near the Coulomb bar- rier, a move which was indicative of a worldwide trend. Research gradually became centred on the

Figure 1. Layout of accelerotors and beam-liizes.

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Figure 2. View of the Particle-Detector-BL

14UD as i t became operational. Publication of the “ Observation of the Yrast and Statistical Cascades in (Heavy Ion, xny) Reactions” (Physical Review Letters 3 4 (1975) 99) by J.O.Newton et al. [ 11 marked the first successful experiment on the 14UD (carried ou t in fact be- fore final acceptance tests) and it also coincided with the start of the modern era of continuum y- ray studies.

Other areas which developed in parallel included mass measure- ments of exotic light nuclei, which was to presage the growth of heavy ion reactions as a tool for determin- ing the properties of nuclei far from stability. An early high point was the measurement [2] of the mass of 220, deemed at the time to be very neutron-rich. Techniques that were being developed then form the ba- sis of the local expertise in hybrid gas-filled detector systems [3,4], which has turned out to be crucial in applications to elemental analy- sis. The ability to mount long-term programmes requiring access to extensive beam-time has been an- other factor in the competitiveness of the research. This underpinned

111, inside CAESAR, the y-ray array.

the successful measurements of qua- drupole moments using the re-ori- entation method (see for example ref [5]), and is a key factor in more recent contributions in nuclear structure and heavy ion reaction studies.

The current research areas in- clude the study of cluster structures in nuclei and other aspects of corre- lated states and heavy-ion reactions, which is carried out largely by the Charissa collaboration under the auspices of the ANU-EPSRC agree- ment; nuclear structure and y-ray and electron spectroscopy; hyper- fine interactions and fields; studies of the fusion and fission of heavy nuclei; accelerator mass spectrom- etry, and various other applications such as elemental analysis using heavy-ion elastic recoil detection. Some aspects of these will be dis- cussed later. Most of these projects are to some extent collaborative, linking in to experimental studies on other facilities. They involve colleagues from overseas, other Australian universities and insti- tutions, other Research Schools within this institution, and other Departments.

Nuclear Spectroscopy and Nuclear Structure

The Department’s reputation for definitive spectroscopic studies has been earned by building on the flex- ibility of the locally developed pulsed-beams from the 14UD and by developing both a range of in- strumentation (modest in scale though it is) for y-ray and electron spectroscopy, and also by main- taining expertise in a range of tech- niques. The y-ray array CAESAR with a particle-detector system in- stalled is pictured in Figure 2.

Since the early 1980s, thecombi- nation of techniques has led to the characterisation of many high-spin metastable states in heavy nuclei, including some of the highest-spin cases so far identified, for example in the radon and francium isotopes [ 6 , 7, 81. More importantly, when the measurements were coupled with a theoretical approach which sought to treat all the experimental properties on an equal footing [9], including transition rates and mag- netic moments, the accepted view of how these states were formed was challenged.

They had been thought to be examples of the stabilisation of a pancake-type distortion of the nucleus caused by a concentration of protons and neutrons orbiting the equatorial plane. This was shown to be only one factor, the more crucial one being the coupling of the motion of specific orbits to the octupole vibration of the core nucleus. This sort of coupling was well-known for one-particle cases, but the successful translation to the many-particle case provided the basis for an explanation of the static and dynamic electromagnetic prop- erties of many exotic states.

The most recent result in this area is identification of a long-lived isomer in 212At, found using long- pulsing techniques, and character- ized using time-correlated spectros-

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copy with sensitivity to high and low energy photons [lo]. A key factor in spin assignments is the ability to measure conversion coef- ficients directly using the supercon- ducting solenoidal spectrometer (SUPER+) operated in Lens mode [ 111. The spectra in Figure 3 show y-ray and electron spectra in equiva- lent delayed time regions. The weak K-line and intense L-line means that E3 character can be deduced for the 224 keV transition directly depopu- lating the isomer. Although this transition is enhanced to 26 single- particle units, it is less collective than expected. The diminution is a t t r i bu ted t o blocking of the octupole vibrational correlation by the occupation of a specific single- particle orbital [lo], a subtle but significant result.

Identification and characteriza- tion of metastable states in de- formed nuclei is another area where a large impact has been made, using instrument a t i on which exploits time-correlations. An example of the value of the high sensitivity thus obtained was the resolution of a long-standing anomaly in forbid- den decays-what had been a mys- teriously fast decay in 179\V was naturally explained by exposure of an intermediate decay path involv- ing quantum interference effects between close-lying states [12]. Serendipitously, the intermediate states revealed arose from a struc- ture interpreted as a rotation in the nuclear system, tilted (at approxi- mately 45") to the principal axes of deformation.

This tilted motion should be a general phenomenon but one de- pendent on the balance of deforma- tion and rotational coupling. The first example in an odd-proton nucleus, lSIRe, has also recently been found in collaborative work with the group from Surrey [13]. A related thrust of the collaborative studies has been the effort to iden-

15000

2 10000 5 8

5000

0

2000 v)

3 1000

0

I

100 200 300 400 500 600 700 800 transition energy [kevl

Figure 3. Ganirna-ray and electron spectra obtained in a period from 50 to 170 p between beam brrrsts, shoruitrg the decay of a receittl~-L~iscovered 220 p isomer in 212At

tify, and characterise, high-senior- ity multi-quasiparticle states. This has resulted in the assignment in 178W of the highest-seniority multi- particle state with an associated collective band yet found [14]. The band is seen clearly in the spectrum obtained by selecting transitions feeding the 25-isomer7 as shown in Figure 4.

The philosophy we have empha- sised is the importance of finding the collective bands associated with the intrinsic (and often isomeric states), as both a means of charac- terising the intrinsic configurations and a way of probing the collective motion and particularly its rela- tionship with the pairing. Estimates of the reduction in neutron and proton pairing due to blocking have been predicted for this sort of case in our recent studies [15]. By com- paring the properties of a suite of rotational bands associated with multi-quasiparticle states from a restricted set of proton and neutron orbitals, as a function of the num-

ber of orbitals blocked, it has been possible to evaluate the pairing re- duction [15] as shown in Figure 5.

The possibility of probing pre- dicted pairing reductions has also been pursued by measuring gyro- magnetic ratios in related multi- quasiparticle states. The collective rotational g-factor gR depends on the partition between proton and neutron contributions to the mo- ments-of-inertia. In the absence of proton-neutron pairing, this parti- tion will vary depending on how many proton or neutron orbitals are blocked in a given multi-quasi- particle state.

In 179\V, for example, we have identified [ 121 isomeric states which are closely related in configuration but which have different seniori- ties. Time-dependent perturbed angular distribution measurements in this case require use of the pulsed beams, the application of an exter- nal magnetic field, and the recoil- ing of excited 179W nuclei into a heated thallium host, to avoid spin

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0

1500

1000

Lo

G 0 0

500

A

m m 104

103

102

101

i no

N

t- m m

W N

i 100 300

*" 0 400 800 1200 I time [nsl

m OI m i

500 700 900 energy [keVl

Figure 4. Time-co frelated coincidence y-ray spectiwit 11 41.

relaxation. The 35/2- isomer in 179W, which is known to have five- quasiparticle structure with three neutrons and two protons, and the

21/2- isomer, which has the same three-neutron configuration, have been measured in this way [ 161.

The experimental studies of

80

- 60 LD

0 M

40

20 20 40 60 80

3 (Migdal) [MeT1K2] Figure 5. Comparison between observed arid predicted momertts-of-irtertia in a range of mrrlti-qrrasiparticle states of different seniority [ I 51.

multiquasiparticle states have also been used to hone model calcula- tions (see for example [17]) to reli- ably predict the existence of fa- vored configurations in nuclei near stability. Some of these will be meta- stable and while a few could be accessible, many more are likely to be too neutron-rich to be reached with conventional reactions. They are bound to receive early attention when neutron-rich beams, such as 9Li, become available from the new generation of Radioactive Ion Beam accelerators.

Incomplete Fusion Reactions for Spectroscopy

While the advent of such beams is likely to change the face of the spectroscopy of neutron-rich nu- clei, incomplete fusion reactions already provide a useful, albeit lim- ited, window into relatively neu- tron-rich nuclei a t medium spins. A recent example of our application of this technique was the identifica- tion [18] of the rotational band based on the 31 year, 16' isomer in 178Hf, using the 176Yb(9Be,a3n) re- action with the compact particle- detector system of Figure 2.

A substantial part of the current research program is aimed a t using such reactions with relatively light projectiles to study medium to high spin states in deformed nuclei - other hafnium isotopes [19] for ex- ample, lutetium and tantalum iso- topes, including a nucleus of some astrophysical interest,18'Ta [20], but also trans-lead nuclei which are no- tionally spherical, such as 211Po [21].

Shape Co-existence Another regime of nuclear struc-

ture where the Department has had a long-term effort is in heavy nuclei which are neither spherical nor well- deformed, but instead exhibit mul- tiple minima in the potential well. States characteristic of each mini- mum may then be present, such as

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rotational bands in the prolate-de- formed minimum, or vibrational states in a weakly-deformed oblate minimum. Our early measurements for example, established a competi- tion beween such shape co-existing states in the very neutron deficient platinum isotopes [22] beginning with the primary identification of ''9t. A collaboration with the Berkeley group later led to identifi- cation of lsoHg and showed [23] that the energy of the prolate states in the mercury isotopes minimised at the middle of the neutron shell. These studies have extended down in.pro- ton number to the light osmium iso- topes where in-beam measurements have been coupled with in-sitii decay studies [24] to make complementary measurements of high and low-spin states. As well, detailed studies of the spectroscopy of the odd-proton iri- dium isotopes [25], the odd-proton thallium isotopes [26] and the odd- neutron mercury isotopes [27], have been pursued to characterise con- figurations involved.

The focus of current research is on the very neutron-deficient lead isotopes where the nuclear poten- tial well is predicted to have an additional minimum at sphericity. This work has also involved col- laborative studies with the Argonne group using their fragment mass analyser and the Jyvaskyla group using their gas-filled separator to identify residual nuclei in the face of fission competition [29, SO]. The presence of both spherical and de- formed minima leads in some cases to the need for substantially differ- ent techniques because of the pres- ence of long-lived isomers, mea- surements carried out using the flex- ible pulsed beam system. The level scheme recently established for 190Pb [28] typifies this.

Such spectroscopic measure- ments on heavy nuclei benefit from the ability to identify short and long-lived states but also from an

Figure 6. Prefissiori iteirtroii iitirltiplicities as a frriictiori of excitation energy in the coiiipoiirid mclerrs (froin Hitide et al. 1331). Full curves are statistical triode1 calctila- tiom, iubile dashed ctirves allow iteritroir-evaporatioolr dtrrirzg a saddle-to-scissiorz time of 3 X s.

understanding of the reaction pro- cesses, particularly in fission-lim- ited cases, where a prudent choice of reaction is mandatory.

Fission and Fusion Studies The Department's innovative

work on the dynamics of heavy ion reactions showed that global stud- ies involving absolute cross-section measurements were imperative to reach a proper view of the fusion- fission process [31,32].That work, in the early 1980s, was followed by studies on the dynamics of nuclear fission which focussed on the time- scale of the process [33, 341. The first comprehensive results to clearly show the effect of a delay time (of the order of 3 x seconds) in controlling the fission process are shown in Figure 6.

Fission studies have continued

to be a major element of the re- search programme, and a particu- larly influential part. As well as establishing terms such as "neu- tron clock" in the jargon of the field, new insights into the dynam- ics have resulted, particularly on the role of nuclear viscosity [35, 361. In the more recent experiments, fission fragments are detected and characterised in an arrangement of large area multiwire proportional counters pictured in Figure 7, de- veloped in the laboratory.

As already intimated, fission and fusion studies are closely related and can complement the under- standing. For example, information on fusion angular momentum dis- tributions which can be extracted from the fusion data allows more precise model calculations of fission fragment angular anisotropies,

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Figrire 7. Fission Spcctroineter.

which carry important information on the dynamics of the fission pro- cess [37].

One aspect of the fission studies is aimed at understanding the de- viations from transition state model predictions for reactions with ac- tinide targets a t beam energies be- low the fusion barrier. These devia- tions have sometimes been attrib- uted to quasi-fission, from the for- . mation of a very elongated system at the fusion barrier, but there has been conflicting evidence. Recent high precision measurements of fis- sion fragment angular distributions and the kinematical differentiation

between components from trans- fer-fission and fusion-fission, pos- sible with the fission spectrometer, support the interpretation that large sub-barrier anisotropies can indeed be explained by understanding the dependence of the competition be- tween quasi-fission and fusion-fis- sion on the orientation of the de- formed target nucleus [38,39]. De- viations from the TSM predictions also are found at high beam ener- gies, where compound nuclei are formed with temperatures higher than the fission barrier. Interest- ingly, these deviations also ap- pear to be correlated with the

nuclear structure of the target nu- clei [40].

Fusion Barrier Distributions The major part of the current

fusion studies is related to the topic of barrier distributions. To fuse, nuclei have to runnel through the fusion barrier, the residue of the sum of the long-range repulsive elec- trostatic and short-range attractive nuclear forces. However, excitation of other nuclear degrees of freedom (e.g. rotation, vibration) of the tar- get or projectile during the colli- sion could result in a distribution of fusion barrier heights.

These propositions bore fruit in 1991 in this laboratory when tech- niques were developed which al- lowed very accurate measurements to be carried out. By drawing on the idea of Rowley, Satchler and Stelson (411 that a barrier distribution func- tion could be deduced from the second differential of the cross- sectiodenergy product, with re- spect t o the beam energy, the ef- fect of the structure and excita- tion modes of the projectile and target was revealed for a number of systems.

This success and further im- provements in instrumentation stimulated precise measurements of fusion excitation functions, princi- pally here [43, 44, 451 and more recently a t Legnaro [46]. Rowley [42] has encapsulated the progress in Figure 8, in which he has com- piled recent data and calculations which use quanta1 tunnelling com- bined with a classical orientation treatment.

Fingerprints of the underlying structure can be seen: A single peak for the spherical-spherical case, 40Ca on 40Ca; a complex broad distribu- tion for I6O on IS4Sm, shown to be characteristic of a prolate deformed rotor; in comparison a distribution for I6O on 186\V also due to prolate deformation, but markedly differ-

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ent because of the large and nega- tive hexadecapole moment of the target; a two-barrier structure for l6O on ‘44Sm which can be repro- duced by including the 2’ and 3- phonon couplings; a single but broad peak for l6O on 92Zr, again because of (in this case unresolved) phonon couplings, whereas the fi- nal case of 18Ni on 60Ni shows a complex structure, reproduced by theory with phonon couplings in- cluding mutual double excitation of the modes in target and projec- tile nucleus.

Tha t such structure can be ob- served is remarkable. It is also c l ea r t h a t depend ing on t h e izriclear structure, the effective barriers can be moved down in energy substantially. This has clear implications for optimising fusion leading to a particular com- pound nucleus. Choice of a target and projectile with specific struc- ture could be exploited given that even small shifts in the barriers can have order of magnitude ef- fects o n the cross-section in “sub- barrier” fusion, a proposit ion which may be significant for the production of exotic nuclei such as superheavies.

This programme has brought insights into a consistent interpre- tation of sub-barrier tunnelling ef- fects and stimulated theoretical ideas on heavy ion fusion [48]. Cur- rent studies include work with rela- tively light projectiles such as ’Li and 9Be in which break-up, pre- sumably in the Coulomb field, is a complicating factor. The work is in collaboration with Brazilian and Birmingham groups and relates as well to the possible uses of incom- plete fusion reactions for spectros- copy, as discussed earlier. Such stud- ies are also likely to have implica- tions for the study of halo nuclei, and the possibilities for the fusion of unstable beams to form new, neutron-rich nuclei.

1500

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500

0

I I t .

50 60 50

~~

55 65 95 105 Energy (MeV)

Figure 8. Barrier distribritiorrs for a irrriirber of systeiiis coiiipared to calcrrlatioizs 1421.

Transient Fields and Hyperfine Interactions

In a completely different aspect of Department research, systematic measurements have also been at the heart of the elucidation of the ori- gins of the very large and anoma- lous magnetic fields experienced by ions moving swiftly through matter [49], fields which can be used to measure magnetic moments of very short-lived nuclear states.

It was a line of research begun a t the ANU in the early 1980s by H.H. Bolotin from Melbourne University. An important early discovery of the study of the ve- locity and atomic number depen- dence was that the transient field for platinum in iron is reduced c o m p a r e d w i t h n e i g h b o r i n g lighter probe ions such as tung- sten and osmium [50]. The re- duced field for Pt was eventually attr ibuted to quenching of 4s va- cancies due t o a level matching

effect involving the N shell of Pt and the L shell of Fe.

T h e in-beam hyperfine pro- gramme, which has produced a large number of results in this field, is now largely ANU-based with an emphasis on measuring the mag- netic moments of short-lived states in neutron-deficient nuclei (comple- menting the spectroscopic studies) and on studying aspects of static hyperfine fields. As well as utilising transient fields, recent studies have involved use of static fields follow- ing implantation, exploiting the geometry of the y-ray array and allowing the measurement of g-fac- tors of a range of low-spin states in nuclei where shape co-existence was proposed [51].

The need to be confident about the fields acting under these differ- ent experimental conditions and discrepancies between different types of measurements has led to other experiments whose results

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1.0

1

m z O.* 0.6

I I I T

-

: -

InP implanted with Ge dose

1012 ions/an2

$ Pt after pdecay 1 I I I I 1 0.00 0.02 0.04

117 (PS-1)

Figure 9. Dependence of the effective IhIPAC fields on the inverse lifetinze of probe states in different niiclei.

suggest that the static hyperfine fields experienced by ions implanted into iron hosts differ a t short times. Specifically, it can take about 10 picoseconds after implantation to reach equilibrium. (See Figure 9 which shows reduced effective hy- perfine fields following implanta- tion, compared with off-line mea- surements, as a function of the life- time of the probe state.) The effect is associated with the implantation process and probably involves the

quenching of the hyperfine field during the thermal spike produced by the stopping process, followed by a finite equilibration time for the impurity-host spin system [ 5 2 ] .

Hyperfine Interactions in Materials A closely related area uses simi-

lar tehniques to probe the local environment in materials. To date much of this work has centerd around the measurement of electric field gradients in compound semi- conductors, using an off-line per- turbed angular correlation (PAC) method [53,54]. The work has been carried out in close collaboration with the Department of Electronic Materials Engineering (EME), and also with the ISKP, Universitat Bonn. The program uses the 14UD accelerator to produce radioiso- topes and implant them directly into samples. For most measurements, sample modification is performed using ion implantation with the EME accelerator, after the radio- isotope has been introduced into the sample, and after the initial

0.00 - 50

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3-0 .05 - - c1:

-0.10 - 0

o.on I- -1 50

20

10 3-0 .05

----I r

- c1:

-0.15 I I I I 1 0 100 200 300

tine (ns)

Figure 10. Perturbed Angular Correlation (PAC) ratio spectra for five InPsanzples, pre- implanted with “lln, which have been irradiated with Ge to irzdirce amorphization. The lowest crrrve corresponds to an irndistrrrbed saniple and the highest to a completely arnorphized sample.

disruption due to the first implan- tation has been removed by anneal- ing. PAC spectra are measured us- ing an array of four barium-fluo- ride detectors and a dedicated ac- quisition system.

Figure 10 shows an example of the spectra obtained for lllIn in InP for a number of different Ge-im- plant doses. The curves are inter- preted as indicating a smooth tran- sition from crystaline material ( no Ge implant) to a sample which has been amorphised by the high dose Ge implant. The data can be repro- duced reasonably well by a model which assumes the lllIn probes sit either a t strongly disturbed site, associated with a n amorphised re- gion, or a weakly disturbed site [53]. A linear dependence on amor- phous fraction with dose is ob- served, indicating heterogeneous nucleation is the main mechanism of amorphization, a t least up to doses resulting in amorphization of 70 % of the irradiated volume. The perturbed angular correlation re- sults are in agreement with RBS measurements, however the PAC method shows a greater sensitivity for high implant doses.

The application of hyperfine techniques to materials is being fur- thered by a collaboration with the Advanced Materials group from the Department of Physics, Australian Defence Force Academy (ADFA), Canberra in the construction of a 150 keV ion implanter for use with radioisotopes. The construction of this machine, which is sited a t ADFA, was completed in 1998 and beam tests are underway. In the first instance this machine will be used to prepare samples for PAC and NMRON measurements.

Accelerator Mass Spectrometry A major application of the detec-

tor techniques developed in the laboratory and the accelerator it- self is accelerator mass spectrom-

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etry (or AMS). The present research programme devotes over 20 % of the 1 4 U D time to such studies [55]. Initially aimed at hydrological stud- ies using the isotope 36CI, (not sur- prising given Australia's depen- dence on groundwater resources) the programme has diversified into using other isotopes and into other fields, as disparate as global cli- mate change and bio-medicine. The high terminal voltage of the 1 4 U D accelerator makes it particularly well-suited to the measurement of 36Cl and the ability of the accelera- tor to operate stably for long peri- ods a t high voltage, has made the ANU system the most sensitive in the world for 36CI. At a typical energy of 154 MeV, a discrimina- tion of better than lo6 between 36Cl and its interfering isobar, 3 6 S , can be achieved using a special multi- element ionisation chamber.

is produced both by cosmic- rays, and also by human activity. Nuclear weapon's testing in the 1950's injected a large pulse of 36CI into the stratosphere from where it subsequently fell out over the entire Earth's surface. This "bomb pulse" can be used as a short-term hydro- logical tracer, or as a tracer of at- mospheric circulation. Ice-cores are an excellent archive of the fall-out signal, and the group has recently measured this in a n Antarctic core from a high-accumulation site a t Law Dome. The result is shown in Figure 11. Comparison with earlier measurements from the Arctic will provide information about mixing across the equator and the effect of the southern hemisphere's circula- tion pattern which is much less af- fected by land masses than the northern hemisphere's circulation.

There are many other applica- tions serving a number of inter- disciplinary areas currently under development. For example, surfaces exposed by glacial retreat, lava flows, fault movement or landslides

108

107

106

105

c

0

1930 1940 1950 1960 1970 1980 1990 Y W

Figrire 11 . hfeasrrremerits ofthearrrrrral fallorrt 0 f ~ ~ C 1 a t Law Donrein Antarctica, ruhich span the period of irrrclenr zueapoiis testing, compared tuitb ariniral ritensiirenzeiits frorit the Dye-3 core in Greenlaird. The bomb pulse is alrrrost 3 orders of inagrritrrde higher tharz the cosrirogetric prodrrctiort.

are being dated by exploiting the build-up of long-lived radio-iso- topes in surface rocks which are exposed to cosmic-ray bombard- ment. The initial applications, car- ried out mainly in collaboration with the Research School of Earth Sciences, which used 36Cl produced in calcium of potassium-rich rocks [56], have been extended to 26A1 and 'OBe produced in quartz [57]. In addition to calibrating the pro- duction rates using surfaces of known age, the method is being applied to the elucidation of glacia- tion histories in Antarctica, Scot- land, California, the Australian Alps and Tasmania; to map past sea- levels in Antarctica; to date fault- scarp movements and landslides, and to measure erosion rates across the Australian continent.

Yet another AMS application is in biomedicine, where a collabora- tion with a group from the Chemis- try Department of the University of Manchester, which has developed through the auspices of the ANU-

EPSRC agreement, recently dem- onstrated the capability for mea- suring precisely, and reproducibly, very low concentrations of 26AI, introduced as a tracer for the up- take of aluminium by the body, providing a new tool for the study of the biochemistry of aluminium [58]. Human volunteer studies have been performed to measure the up- take and excretion of aluminium, the dependence on the chemical form in which it is ingested, and the kinetics of the process [59].

Biomedical contacts provided the impetus for the development of other isotopes, such as plutonium and neptunium, implemented re- cently [60] with a sensitivity a t least 2 orders of magnitude better than alpha-particle counting. This capa- bility is being used to measure plu- tonium levels in teeth and urine from groups near nuclear repro- cessing plants, and to perform ba- sic uptake studies similar to those for aluminium. The additional sen- sitivity is also useful for environ-

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mental studies of migration of plu- tonium and neptunium away from processing plants and accident sites, allowing the isotopes to be followed to greater distances than is possible with conventional techniques.

Similar applications in human and environmental studies will fol- low with the most recent develop- ment in AMS of the long-lived fis- sion product 99Tc. Other isotopes which have been measured include 59Ni in meteorites, 14C in fossil cor- als, "Ca and IZ9I.

Heavy Ion Elastic Recoil Detection for Elemental Analysis (ERDA)

The newest of the applied areas in the laboratory is aimed at non- destructive compositional depth- profiling of materials [61, 621. It takes advantage of the large scat- tering cross-sections and stopping powers for very heavy ions in solids and is being used with gold beams of energies of about 200 MeV. Re- coils are detected using a large solid angle position-sensitive gas-ioniza- tiondetector.This has again exploited the laboratory expertise in hybrid detector systems and has benefited from collaborations with the Univer- sity of Newcastle, Lund and Munich groups, as well as with the Depart- ment of Electronic Materials Engi- neering in our Research School.

The technique is complementary to Rutherford backscattering spec- trometry (RBS) but allows the de- tection of light elements in heavy element matrices, an analysis which is difficult with RBS. ERDA yields separate depth-profiles of absolute concentration for all light chemical elements with atomic number less than -50.

The analytical programme ini- tially focussed on C N H layers of magnetic films and hydrogen pro- files in silica, in collaborations with groups at the University of Western Ontario and McMaster University in Canada. The most recent mea-

surements have been on analysis of thin films being developed for inte- grated photonics by other Depart- ments in the Research School, par- ticularly Plasma Physics, and the Optical Sciences Centre, illustrat- ing again the symbiotic nature of much of the research and the con- tinuing opportunities for interac- tion and application.

The Present and Future The newest element in the accel-

erator system, the Superconducting Linear Accelerator, was officially inaugurated in 1997 and is now coming into regular operation with a program of beam development, and with the completion of the beam transport systems which will de- liver beams to both target areas. We expect a general broadening of the programme using Heavy Ions and incremental extensions of the accelerator facility over the next few years, including resonator im- provemen ts.

' In parallel with accelerator de- velopments , t he experimental groups continue to develop instru- mentation for research. In the pipe- line at present are: a superconduct- ing solenoid to allow detection of evaporation residues with a large solid angle and permit coincidence measurements with high efficiency between evaporation residues and emitted particles and possibly y- rays; a multipIe electron detection system for electron-electron coinci- dences in spectroscopy studies; a new Wien filter for AMS measure- ments of heavy nuclei and various detector improvements for ERDA measurements. A new data acquisi- tion system is being developed with improved capabilities for handling multiple-element arrays and the concomitant high data rates, with the aim of servicing instrumenta- tion that might be brought to the facility by outside users.

The outline given in these pages

contains in it the genesis of a Re- search Programme which is now spread over a number of research areas. Our hope for the future is to maintain a balance of basic and applied studies and a competitive facility which provides an arena for postgraduate and postdoctoral training, and for quality research.

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