Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
INTERACTIONS OF NUCLEI WITH MATERIALS The motions of protons and neutrons within the nucleus give rise to electric and magnetic fields, which both affect and are affected by nearby atomic electrons. These hyperfine interactions change the orientation of the nucleus, and can perturb the spatial distributions of emitted radiations. Blue Lake in the Snowy Mountains, formed by glaciers Direct measurement of the concentration of the undecayed isotopes is a much superior method. The Accelerator Mass Spectrometry (AMS) group uses the 14UD accelerator to provide a mass-selected beam of the isotope from a sample, having sufficient energy to enable the counting and unambiguous identification of each ion, using techniques from nuclear physics. A sensitivity down to 1 atom of isotope in 1015 normal atoms has been achieved, from samples of only a few milligrams. This previously unattainable sensitivity has opened up whole new areas of research in fields as diverse as global climate change, bio-medicine and archaeology. A large boulder deposited 20,600 years ago by a glacier at Blue Lake in the Snowy Mountains of Australia. This date was obtained by determining how much 10Be isotope had built up in the rock surface over time due to cosmic-ray bombardment. The group has a strong program of developing new measurement techniques and apparatus, to extend the range of isotopes that can be studied. New applications of AMS measurements are also being investigated, both for natural and man-made isotopes. Vigorous collaborations with both Australian and international scientists have been established for projects including the dating of glacial advance and retreat as an indicator of past global climatic changes, tracing the effect of land clearance on the salinity of the MurrayDarling river system, and dating the time of arrival of Aboriginal People in Australia. The detection of scattered beam particles, and of recoiling target nuclei, are the basis of the extremely powerful materials analysis techniques called Rutherford Backscattering and Elastic Recoil Detection Analysis. Collaborators from the Australian Defence Force Academy (ADFA) and the Department of Electronic Materials Engineering (EME), use accelerated heavy-ion beams and a unique large solid angle gasionization detector to obtain the depth profiles of all elements in a sample simultaneously. Measurement of perturbed radiation patterns allows the investigation of a wide range of nuclear structure and materials science problems. For example, the nuclei of ions moving swiftly within ferromagnetic hosts experience high transient hyperfine INTERNATIONAL LINKS magnetic fields (several thousand Tesla). These allow the measurement of magnetic moments of very short-lived (10-12 seconds) nuclear states, critically testing nuclear theories. Transient fields are also studied as a unique probe of ion-solid interactions. In collaboration with EME, perturbed angular correlation measurements are used to study atomic-scale electric fields due to dopant-defect interactions in semiconductor materials, through implantation of radioactive isotopes using the 14UD accelerator, or a dedicated ion implanter developed with ADFA. The control and understanding of dopant-defect interactions is crucial in the design and fabrication of semiconductor devices. Department of Nuclear Physics Heavy-ion Accelerator Facility 25 Elastic Recoil Detection Analysis Doped Gallium Nitride Film 20 Ga 15 (ch1 ) 200 Si 150 10 'E(1) The interaction of cosmic radiation with the atmosphere and surface layer of the Earth results in the production of minute quantities of radioactive nuclei such as 14C, 10Be and 36Cl. These isotopes decay sufficiently slowly to be useful in dating man-made artefacts or natural features in the environment. One approach is to measure the radiation emitted during their decay. In practice, this is only practicable for 14C dating, and then only for large samples. 'E (MeV) ACCELERATOR MASS SPECTROMETRY H 100 Mg 50 H 5 O C 0 N 0 100'E(2)200 300 0 0 20 40 60 80 Energy (MeV) 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 by small 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. Local researchers also travel to overseas facilities (e.g. in France, Italy, USA) to make use of apparatus complementary to that available in Canberra. Strong collaborations exist with theoreticians in all the research fields. CONTACTS Department of Nuclear Physics, RSPhysSE, Australian National University, ACT 0200 +61 (0)2 6125 2083 wwwrsphysse.anu.edu.au/nuclear wwwrsphysse.anu.edu.au/nuclear 385.7 Our nuclear structure research investigates the properties of, and interactions between individual excited states of metastable nuclei, whilst the complex interactions between colliding nuclei are the subject of nuclear reaction dynamics studies. In our applied research, the accelerated beams are used in AMS measurements and in advanced materials characterization, to investigate subjects as diverse as landform evolution and internal electric fields in semiconductors. delayed gammas 202Po 912.1 The different strands of research within the Group have the ultimate goal of developing a unified picture of the dynamics of nuclear break-up, fusion and fission. 442.7 delayed electrons 571.2 High resolution delayed electron and γ-ray spectra, from which the nuclear energy levels of 202Po were deduced fluid or solid matter. Further, the energies of single-particle excitation and collective motion such as rotation and vibration are very similar and essentially in competition. The quantum states that arise occur in a specific pattern, but they are sparse and therefore observable as individual states. If they do overlap they can interfere, leading to modifications of the decay probabilities which are indicative of the coupling. Identification and characterisation of exotic metastable states is one focus of current spectroscopic studies. This exploits their relatively long-lived nature (nanoseconds to milliseconds), and pulsed beams from the accelerator, to achieve very high sensitivity. Recent results have given new insights into the coupling between collective vibrations and octupole-shaped proton and neutron orbits near the surface of spherical nuclei, and into the constituents of superfluid motion in deformed nuclei. Another focus of the research program is the identification of nuclei that are very far from stability. Some exhibit different shapes at low energies, which is a manifestation of the competition between dual minima in the nuclear potential. Their study, which often involves the primary identification of nuclei never before formed, is particularly important for testing models of nuclear stability. 676.8 Fusion can also be inhibited by break-up of the colliding nuclei before contact, a process important in the interactions of the fragile nuclei found at the boundary of stability (becoming available from new radioactive beam facilities). Our precision measurements for stable, yet fragile, light nuclei have thrown light on the interplay between break-up and fusion. The 14UD Van der Graaff accelerator, housed in a 40m tall tower, can operate at over 15 million Volts, and delivers ion beams with pulse widths from a nanosecond to seconds. Beam energies can be doubled by a superconducting linear post-accelerator (Linac). The varied interactions of beam nuclei with target nuclei are studied using state-of-the-art detector arrays, developed in-house, and located at the ends of the 10 beam lines. These main areas of research are complementary, overlapping in terms of shared techniques, and the understanding achieved of interrelated aspects of nuclear behaviour. After contact, the two nuclei can merge into one (fusion) or separate after exchanging mass (quasi-fission). The latter inhibits the formation of new heavy elements. We have shown that quasi-fission is more prevalent than expected, and have developed an experimental framework to investigate the delicate balance of forces that determines whether fusion or quasi-fission occurs. Using Gammasphere in the USA The Department of Nuclear Physics operates two accelerators, producing charged atoms (ions) with up to 10% of the speed of light. This is enough to overcome the electrostatic repulsion between atomic nuclei, and initiate nuclear reactions. The nucleus is a highly symmetric mesoscopic system whose structure and motions are coupled: mesoscopic, rather than microscopic like a group of individual nucleons, or macroscopic, like a chunk of 571.2 To fuse, nuclei can tunnel through the potential barrier created by the sum of the long-range repulsive electrostatic and short-range attractive nuclear forces. The excitation of other nuclear degrees of freedom (e.g. rotation, vibration) during the collision results in a distribution of barrier heights. 526.2 • These distributions can be extracted from extremely precise fusion probability measurements, through a simple and elegant mathematical transformation. They give a unique picture of the complex interactions of the nuclear surfaces as they come into contact. 442.7 • • • The Nuclear Reaction Dynamics group carries out highly regarded research into the fundamental processes of nuclear fusion, where two nuclei merge into one, and nuclear fission, where one nucleus splits into two. This work is built on our broad expertise in the design and development of unique and efficient particle detection systems, matched to the high-quality particle beams from the accelerator. Spectroscopic studies are aimed at identifying and characterising individual quantum states in nuclei, using instrumentation for high-resolution gamma-ray and electron detection. The nuclei that are studied are not just the specific combinations of protons and neutrons which form the stable species, but the very wide range which are accessible by combining stable nuclei with any one of the beams available from the accelerator. The result is efficient production of a nucleus, under conditions which force it to emit the gamma-rays or electrons connecting quantum levels, thus revealing the states that characterise its properties, motion and degrees of freedom. NUCLEAR STRUCTURE Counts • Located on the ANU campus 15 million Volt electrostatic accelerator Superconducting post-accelerator Ph.D. research opportunities, working closely with distinguished staff having high international profiles Ph.D. graduates obtain positions in top research 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 Atomic nuclei are completely invisible, being less than 10-14 m across, and a collision of two nuclei takes only 10-20 seconds. In such seemingly infinitesimal and transient events, a wide variety of phenomena occur. Understanding them represents a fascinating intellectual challenge, and also impacts on other fields of science. Velocity components of fission events • • • • NUCLEAR REACTION DYNAMICS Some members of the SOLITAIRE team AUSTRALIA’S TOP NUCLEAR PHYSICS LABORATORY The group’s CUBE fission detector array THE FACILITY 676.8 912.1 385.7 17+ 17+ 16517.7 151513152.8 1211- 407.4 10656.1 9 555.9 14+ 138.2 542.0 455.3 575.2 + 830.9 12 142.7 10+ τ =141 ns 436.1 11679.1 385.7 τ =23 ns 912.1 915.7 526.2 τ =168 ns ∆ 8+ 6+ 442.7 4+ 202Po 84 118 537.4 571.2 2+ 676.8 Energy 667.3 16+ 0+