The Australian National University's (ANU) Department of Nuclear Physics conducts accelerator based research using numerous different cutting-edge accelerators located at the Universities Campus.

Accelerators

LINAC

The LINAC, currently in Stage I, comprises twelve split loop resonators housed in four cryostats. These add 6 MV/q of energy to a pulsed beam. The facility has space for 32 resonators which will have a minimum capability of 15 MV/q. The lead plated split loop resonators of the Stage I LINAC will be complemented by multi-stub resonators developed here. The lead plating in the existing split loop resonators has been substantially improved. The LINAC will be upgraded by the installation of innovative multi-stub resonators at betas of 0.03 and 0.06 which are under development. Initially these will also exploit the improved Pb-Sn plating techniques. Liquid helium is provided by a TCF 50 Sulzer helium refrigerator of 328 Watts @ 4.2 K proven capacity. The LINAC beam optics equipment are controlled by a locally developed computer system which will soon be extended to the 14UD and experimental equipment.

14 UD Pelletron

The 14UD, in stand-alone operation since 1973, services seven well established beamlines plus three new ones in the LINAC Hall. It runs between 3500 and 5000 hours per year depending on competition from development projects and is operated by experimenters. Ion sources include a NEC Multi-Sample SNICS and a Gas Cathode equipped SNICS II. These feed a high resolution injector system and a comprehensive pulsing system. The pulser boasts a programmable chopper which produces intervals of beam as short as 50 ps with repetition rates from sub microseconds to multi-milliseconds. A double gridded buncher, operating at 9.375 MHz, plus two harmonics, compresses the beam into pulses less than 1 ns wide at injection into the 14UD. Two orthogonal post acceleration choppers clean up the pulses. The phase detector is in front of the image slits of the energy analysing magnet. The high voltage terminal of the 14UD includes a gas stripper differentially pumped by two turbopumps and two ion pumps. This is followed by the existing foil stripper and electrostatic triplet quadrupole lens. All terminal equipment is controlled via optical cable. The 14UD routinely operates above 15.5 MV. The voltage performance upgrades include the installation of NEC compressed geometry accelerator tube, which increases the insulated length by 12%, and the local development of a robust resistor grading system for the column and tube.

Current Research

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Exposure Dating

The earth is continually bombarded with extremely high-energy (GeV) particles originating from outside our solar system. These ‘cosmic rays’ produce a variety of nuclear reactions in the atmosphere and within the Earth’s surface, resulting in the creation of cosmogenic isotopes. Exposure dating is based on the principle that cosmogenic isotopes accumulate in surface rocks as a function of time. After a geological process freshly exposes a rock surface, these cosmogenic nuclides build up at a known rate. Measurement of their present-day abundance, in conjunction with knowledge of the rate at which they are produced, allows an ‘exposure age’ of the surface to be determined.
Concentrations of cosmogenic isotopes in typical earth materials are incredibly low, being less than one in a million million (10-12) relative to their stable counterparts. Hence, the ultrasensitive technique of accelerator mass spectrometry (AMS) is required. The powerful and versatile 14UD tandem accelerator at the ANU has proved to be one of the best tools in the world for such measurements.
The long half lives of 10Be, 26Al, and 36Cl make these cosmogenic nuclides useful for studying landscape evolution on geological timescales (103-106 years). In particular, exposure dating has revolutionised the study of the history of glaciers and ice sheets. By directly dating glacial debris and eroded bedrock, the timing of the advance and retreat of the ice (a sensitive indicator of climate) can be determined with unprecedented reliability. Current projects include using this technique to take a fresh look at the history of glaciation and climate change in Australia, New Zealand, Papua New Guinea and Spain.
Studies of relics of the last Ice Age in the Snowy Mountains and Tasmania have led to a complete revision of the glacial history of Australia. Hypothetical ideas about glacier extent and its timing that stood for nearly a century have been replaced with a robust chronology placing Australia into a global context. It transpires that there was not just one but at least four major advances of glacier ice during the last 70,000 years. The coldest part of the last ice age was about 20,000 years ago and only lasted a few thousand years. The ensuing global warming is the greatest in recent geological history. On the basis of the altitude of the ice age landforms, mean temperatures around Canberra are about 9°C warmer today. This research provides an important baseline from which to assess climate variability and future climate change.

Sediment transport and deposition

Radio-isotopes have been used for many years as tracers of soil erosion processes. In particular, erosion and sediment transport studies have made significant use of 7Be, 137Cs and 210Pb, by counting the decay radiation. These isotopes bind with soil particles, providing a marker which can trace the movement of the soil. The 3 isotopes provide complementary data because their different chemical properties make their binding properties slightly different and this can be exploited to elucidate different physical processes in the moving soil. Berylium-7 and 210Pb are produced naturally and continuously in the environment and are best suited to probe the top few centimeters of the soil profile, but the 137Cs, which probes to approximately 30 centimeters, is fallout from atmospheric weapons tests, and hence is not being replenished. The 137Cs half life is 30 years and its sensitivity as a tracer is therefore decreasing. AMS measurements of plutonium isotopes provide an alternative to using 137Cs, and may well prove to be a superior tracer for such studies. The techniques for AMS Pu measurements were pioneered at the ANU, and routine measurements of the two isotopes indicate that plutonium provides similar quantitative information to Caesium but has better sensitivity. A soil depth profile comparing the two elements is shown in the figure. A study of sediment movement through the Herbert River catchment, Queensland, Australia, comparing 137Cs and 239, 240Pu measurements is presently underway. This study, which is partially funded by an ARC discovery grant, aims to trace sediment movement from the catchment to the Great Barrier Reef. Caesium readily desorbs from the sediment particles in saline environments. The relative extent to which plutonium desorbs is currently under investigation, and initial studies indicate that the plutonium binds much more tightly to the particles than does the caesium. The image show the initial sampling sites where material has been collected to date.

Gas-filled Magnet detection System

The group has continued to develop AMS measurements using our gas-filled magnet (GFM) system, with the type of ions measured in this device now including 26Al and 53Mn, along with 32Si. The GFM allows ions of AMS interest to be separated from their much more abundant stable isobars (26Mg,53 Cr and 32S). The gas-filled magnet detection system consists of an Enge Split-Pole magnetic spectrograph coupled to a position sensitive multi-anode gas ionisation detector (see upper figure). Ions travelling in a magnetic field region, which is filled with nitrogen gas at an appropriate pressure, do so with a circular trajectory with a singular average-charge state. This average charge state is proportional to the Z of the ion, thus giving rise to a physical separation between ions of differing Z at the exit of the magnet (see lower figure). The ability to detect 26Al in a GFM by suppressing the 26Mg isobar enables AMS of 26Al to be performed with the Al- beam instead of the traditional method, injection of AlO-. The advantage of this is that the Al- beam is up to twenty times more prolific than AlO-, thus superior counting statistics can be obtained for the equivalent measuring time. This should lead to 26Al to Al ratios with statistics near those obtained for 10Be. The AMS of 53Mn is being pursued with the hope that isotope could be used as a new cosmogenic nuclide for erosion-rate applications. Its long half-life, at 3.7 Ma, means it should be suitable for examining landscape processes at the timescale of the order of 1 Ma. In situ, 53Mn is produced by cosmic rays (mainly spallation) on Fe. Thus the 53Mn may be useful in ancient landscapes were only Fe-bearing rocks remain after all other minerals have weathered away.