AMS uses the combination of a high efficiency (small sample) ion source, tandem acceleration and heavy-ion detection and identification techniques (only possible with relatively high-energy ions), to make highly sensitive measurements of ultra-low abundance isotopes. The AMS group at ANU is small but dynamic, and utilises the relatively high-energies (in AMS terms) capable with the ANU 14UD pelletron accelerator to analyse a large number of isotopes at very high sensitivity.
A Modified 32 sample NEC MCSNICS ion source is used to extract the isotope of interest as negative ions. The ions are accelerated to between 100 and 150 keV (depending on the isotope), mass selected with a 90° dipole bending magnet and injected into the 14UD tandem accelerator. Gas and/or foil stripping is employed to dissociate injected molecules and convert the ions to positive charge states while in the accelerator terminal. A 220 MeV.amu/e2 90° double focusing magnet is then used to select the desired charge state and mass. The selected ions then pass through a switching magnet and down one of two beam lines available for AMS analysis. Wien filters are installed on both beam lines to provide further discrimination against background events. The ions are then detected with one of the purpose built detection systems specific to isotope being measured. Detection systems currently in use or being developed include ionization chambers, time of flight detectors and a gas-filled magnet system.
Isotopes currently measured by the AMS group at ANU include: 10Be, 14C, 26Al, 32Si, 36Cl, 41Ca, 53Mn, 59Ni, 99Tc, 129I, 236U, 237Np, 239Pu, 240Pu, 242Pu and 244Pu. The group has active programs that focus on both specific project applications of these isotopes and on the technical development of the AMS detection systems used to measure them.
The earth is continually bombarded with extremely high-energy (GeV) particles originating from outside our solar system. These ‘cosmic rays’ result 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.
Radio-isotopes have been used for many years as tracers of soil erosion processes. These isotopes bind with soil particles, providing a marker which can trace the movement of the soil.
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.