Development of an AMS facility with the 6MV EN tandem accelerator at iThemba Labs - Johannesburg

In the twenty five years since the first demonstration that 14C could be detected at natural levels using tandem nuclear accelerator as a mass spectrometer, the AMS (Accelerator Mass Spectrometry) technique has developed into a major analytical tool. Presently there are about 50 AMS facilities world wide, with only 5 in the Southern hemisphere (two in Australia, one in New Zealand, one in Brazil and one in Argentina ). Here we report on the status of work at iThemba Labs and Wits University to develop a capability for accelerator mass spectrometry (AMS) making use of the 6 MV EN Tandem Accelerator. AMS makes use of nuclear accelerators of the so-called 'Tandem' type. The only tandem accelerator under operation in the African continent resides at iThemba Labs in Johannesburg, within the campus of the University of the Witwatersrand. The completion of the facility calls for a number of equipment to be modified upgraded or purchased. In this respect, the iThemba EN Tandem lab is presently undergoing a multi-million rands upgrade. New spirally inclined field tubes and shielded column grading resistors are being installed. The Van de Graaff belt is being replaced with a pelletron chain charging system in order to improve the stability of the accelerator. The stripper system will be kept as is, however, a recirculating stripping gas scheme is being implemented. The insulating gas, a mixture of nitrogen and carbon dioxide, will be enriched with 6% SF6. The low energy injection spectrometer is based on sequential injection of isotopes. The mass switching is achieved by bouncing a 90 deg double focusing magnet with a radius of 650 mm. A dedicated high current multi cathode AMS source from HV Engineering Europa will be used to produce the isotopes of interest. The design of the low energy system with all necessary optical elements (einzel lenses, steerers, electrostatic analyzer) and components is being carried out and finalized through beam optics calculations. The post accelerator spectrometer has the quadruple doublet magnet after the exit from the tandem replaced with a triplet in order to improve the symmetry of the phase space ellipse. A new double focusing magnetic analyzer, donated by the Neils Bohr Institute, with a 1000 mm radius and large acceptance will bend the ions by 90 deg. A new design calls for the replacement of the existing switcher magnet with an appropriate 90 deg mass analyzer. A 600 mm Wien filter donated by Simon Frazer University from their old AMS facility at McMaster University, will be used as velocity selector. The isotopes are finally directed into a DE/E detection telescope donated also by Simon Frazer University. This consists of an ionization chamber with four anodes and a solid state stop detector at the back. The expected research outputs with particular relevance to Southern Africa cover a wide range of scientific disciplines: Climatic Studies; Characterization of anthropogenic and fossils sources of atmospheric pollution; Characterization of Regional Groundwater Systems; Anthropology, Archaeology, History and Preservation Technology; AMS in Biomedical Dosimetry; Qualifying Therapeutic Drugs; Research in Earth Sciences and Geosciences; Ultra-sensitive Trace Element AMS (TEAMS) Applications in Materials Science; Dating Studies of Elephant Tusks and Rhino Horns with AMS; Potential Isotopic Tools for Paleodating with AMS