A MICROBEAM NEGATIVE-ION SOURCE BASED ON A MODIFIED HICONEX(TM) SOURCE

The first stage of the development of AUSTRALIS, a microbeam AMS syste m at the HIAF laboratory at North Ryde, Sydney has been completed. The system is designed to enable in-situ microanalysis of geological samp les for ultra-traces and for isotropic data for minerals exploration r esearch. The negative ions for analysis are produced by sputtering wit h a microbeam of Cs+. This primary microbeam source was developed by m odifying a General Ionex model 834 HICONEX ion source and combining it with auxiliary cylindrical lenses. The Cs beam bombards the sample at 45 degrees incidence angle and the secondary ions are extracted at no rmal angle. The source features a novel intermediate or ''screen'' ele ctrode to correct for the effect of the secondary ion extraction field on the trajectory of the primary beam, in order to bring the primary beam to the geometric centre. The high energy analysis system of AUSTR ALIS features a pair of deflector systems to permit fast switching of isotopes without altering the magnet setting. The paper describes the initial tests' results showing good agreement with the design paramete rs.