14 10 36 MASS OF NUCLEI SPECTROMETRY 14C dating, and then ... · applications of AMS measurements are also being investigated, both for natural and man-made isotopes. Vigorous collaborations

The interaction of cosmic radiation with the atmosphere and surface layer ofthe Earth results in the production of minute quantities of radioactive nucleisuch as 14C, 10Be and 36Cl. These isotopes decay sufficiently slowly to be usefulin dating man-made artefacts or natural features in the environment. Oneapproach is to measure the radiation emitted during their decay. In practice,this is only practicable for 14C dating, and then only for large samples.

Direct measurement of the concentration of the undecayed isotopes is a muchsuperior method. The Accelerator Mass Spectrometry (AMS) group uses the14UD accelerator to provide a mass-selected beam of the isotope from asample, having sufficient energy to enable the counting and unambiguousidentification of each ion, using techniques from nuclear physics. A sensitivitydown to 1 atom of isotope in 1015 normal atoms has been achieved, fromsamples of only a few milligrams. This previously unattainable sensitivity hasopened up whole new areas of research in fields as diverse as global climatechange, bio-medicine and archaeology.

Department of Nuclear Physics

Heavy-ion AcceleratorFacility

wwwrsphysse.anu.edu.au/nuclear

ACCELERATORMASS

SPECTROMETRY

The detection of scattered beamparticles, and of recoiling targetnuclei, are the basis of theextremely powerful materialsanalysis techniques calledRutherford Backscattering andElastic Recoil Detection Analysis.Collaborators from the AustralianDefence Force Academy (ADFA)and the Department of ElectronicMaterials Engineering (EME), useaccelerated heavy-ion beams and aunique large solid angle gas-ionization detector to obtain thedepth profiles of all elements in asample simultaneously.

The motions of protons and neutronswithin the nucleus give rise toelectric and magnetic fields, whichboth affect and are affected bynearby atomic electrons. Thesehyperfine interactions change theorientation of the nucleus, and canperturb the spatial distributions ofemitted radiations.

Measurement of perturbed radiationpatterns allows the investigation ofa wide range of nuclear structure andmaterials science problems. Forexample, the nuclei of ions movingswiftly within ferromagnetic hostsexperience high transient hyperfine

INTERACTIONSOF NUCLEI

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The group has a strong program of developing new measurement techniquesand apparatus, to extend the range of isotopes that can be studied. Newapplications of AMS measurements are also being investigated, both fornatural and man-made isotopes. Vigorous collaborations with both Australianand international scientists have been established for projects including thedating of glacial advance and retreat as an indicator of past global climaticchanges, tracing the effect of land clearance on the salinity of the Murray-Darling river system, and dating the time of arrival of Aboriginal People inAustralia.

Vigorous international links have been developed with physicists from many countries.These often take the form of short or long-term visits by individual scientists, or bysmall groups, who may perform experiments in collaboration with local researchers.Experiments are also performed under an agreement with the EPSRC in the UK,whereby a large group of external users carry out their own experiments.

INTERNATIONALLINKS

Local researchers also travel tooverseas facilities (e.g. in France, Italy,USA) to make use of apparatuscomplementary to that available inCanberra.

Strong collaborations exist withtheoreticians in all the research fields.

magnetic fields (several thousand Tesla).These allow the measurement ofmagnetic moments of very short-lived(10-12 seconds) nuclear states, criticallytesting nuclear theories. Transient fieldsare also studied as a unique probe ofion-solid interactions.

In collaboration with EME, perturbedangular correlation measurements areused to study atomic-scale electricfields due to dopant-defect interactionsin semiconductor materials, throughimplantation of radioactive isotopesusing the 14UD accelerator, or adedicated ion implanter developed withADFA. The control and understandingof dopant-defect interactions is crucialin the design and fabrication ofsemiconductor devices.

CONTACTS Department of Nuclear Physics, RSPhysSE, Australian National University, ACT 0200+61 (0)2 6125 2083wwwrsphysse.anu.edu.au/nuclear

A large boulder deposited 20,600 years ago by aglacier at Blue Lake in the Snowy Mountains ofAustralia. This date was obtained by determininghow much 10Be isotope had built up in the rocksurface over time due to cosmic-raybombardment.

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AUSTRALIA’S TOP NUCLEAR PHYSICSLABORATORY

• Located on the ANU campus• 15 million Volt electrostatic accelerator• Superconducting post-accelerator• Ph.D. research opportunities, working

closely with distinguished staff havinghigh international profiles

• Ph.D. graduates obtain positions in topresearch laboratories worldwide

• ARC-funded postdoctoral positions• International collaborations• World-leading research in nuclear

properties and nuclear reactions• Outstanding applications in bio-medical

and environmental science

THEFACILITY

The Department of Nuclear Physics operates two accelerators,producing charged atoms (ions) with up to 10% of the speed oflight. This is enough to overcome the electrostatic repulsionbetween atomic nuclei, and initiate nuclear reactions.

The 14UD Van der Graaff accelerator, housed in a 40m tall tower,can operate at over 15 million Volts, and delivers ion beamswith pulse widths from a nanosecond to seconds. Beam energiescan be doubled by a superconducting linear post-accelerator(Linac). The varied interactions of beam nuclei with target nucleiare studied using state-of-the-art detector arrays, developedin-house, and located at the ends of the 10 beam lines.

Our nuclear structure research investigates the properties of,and interactions between individual excited states of metastablenuclei, whilst the complex interactions between colliding nucleiare the subject of nuclear reaction dynamics studies. In ourapplied research, the accelerated beams are used in AMSmeasurements and in advanced materials characterization, toinvestigate subjects as diverse as landform evolution and internalelectric fields in semiconductors.

These main areas of research are complementary, overlappingin terms of shared techniques, and the understanding achievedof interrelated aspects of nuclear behaviour.

Atomic nuclei are completely invisible, beingless than 10-14 m across, and a collision oftwo nuclei takes only 10-20 seconds. In suchseemingly infinitesimal and transient events,a wide variety of phenomena occur.Understanding them represents afascinating intellectual challenge, and alsoimpacts on other fields of science.

The Nuclear Reaction Dynamics groupcarries out highly regarded research into thefundamental processes of nuclear fusion,where two nuclei merge into one, andnuclear fission, where one nucleus splits intotwo. This work is built on our broad expertisein the design and development of uniqueand efficient particle detection systems,matched to the high-quality particle beamsfrom the accelerator.

To fuse, nuclei can tunnel through thepotential barrier created by the sum of thelong-range repulsive electrostatic andshort-range attractive nuclear forces. Theexcitation of other nuclear degrees offreedom (e.g. rotation, vibration) during thecollision results in a distribution of barrierheights.

NUCLEARREACTION

DYNAMICS

These distributions can be extracted fromextremely precise fusion probabilitymeasurements, through a simple and elegantmathematical transformation. They give aunique picture of the complex interactionsof the nuclear surfaces as they come intocontact.

After contact, the two nuclei can merge intoone (fusion) or separate after exchangingmass (quasi-fission). The latter inhibits theformation of new heavy elements. We haveshown that quasi-fission is more prevalentthan expected, and have developed anexperimental framework to investigate thedelicate balance of forces that determineswhether fusion or quasi-fission occurs.

Fusion can also be inhibited by break-up ofthe colliding nuclei before contact, a processimportant in the interactions of the fragilenuclei found at the boundary of stability(becoming available from new radioactivebeam facilities). Our precision measurementsfor stable, yet fragile, light nuclei havethrown light on the interplay betweenbreak-up and fusion.

The different strands of research within theGroup have the ultimate goal of developinga unified picture of the dynamics of nuclearbreak-up, fusion and fission.

NUCLEARSTRUCTURE

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Spectroscopic studies are aimed atidentifying and characterising individualquantum states in nuclei, usinginstrumentation for high-resolutiongamma-ray and electron detection. Thenuclei that are studied are not just thespecific combinations of protons andneutrons which form the stable species,but the very wide range which areaccessible by combining stable nuclei withany one of the beams available from theaccelerator. The result is efficientproduction of a nucleus, under conditionswhich force it to emit the gamma-rays orelectrons connecting quantum levels, thusrevealing the states that characterise itsproperties, motion and degrees offreedom.

The nucleus is a highly symmetricmesoscopic system whose structure andmotions are coupled: mesoscopic, ratherthan microscopic like a group of individualnucleons, or macroscopic, like a chunk of

fluid or solid matter. Further, the energiesof single-particle excitation andcollective motion such as rotation andvibration are very similar and essentiallyin competition. The quantum states thatarise occur in a specific pattern, but theyare sparse and therefore observable asindividual states. If they do overlap theycan interfere, leading to modifications ofthe decay probabilities which areindicative of the coupling.

Identification and characterisation ofexotic metastable states is one focus ofcurrent spectroscopic studies. Thisexploits their relatively long-lived nature(nanoseconds to milliseconds), and pulsedbeams from the accelerator, to achievevery high sensitivity. Recent results havegiven new insights into the couplingbetween collective vibrations andoctupole-shaped proton and neutronorbits near the surface of spherical nuclei,and into the constituents of superfluidmotion in deformed nuclei.

Another focus of the research program isthe identification of nuclei that are veryfar from stability. Some exhibit differentshapes at low energies, which is amanifestation of the competitionbetween dual minima in the nuclearpotential. Their study, which ofteninvolves the primary identification ofnuclei never before formed, is particularlyimportant for testing models of nuclearstability.

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