The HVEE 3 MV AMS system at Oxford Radiocarbon Accelerator Unit is equipped with two recombinators, one used for graphite measurements on an 846 ion source, the other is used for gas or graphite measurements on a SO110 ion source (Ramsey, Higham et al. 2004). The purpose of the present project is to characterize and further develop these injectors, in the two ways indicated bellow.
The first part of the project is aimed at increasing the ion current injected into the AMS system. This current is limited by the vertical acceptance of the recombinators, because the beam size increases with increasing current due to space-charge effects. As a first step towards modifying the systems for increased through-put investigations of the ion optics from ion source to recombinator have been carried out using first-order matrix calculations and numerical simulation with SIMION 3D. Particular emphasis was put on describing the effect of the installed apertures and on the changes in the properties of the ion beam due to space-charge effects.
The second part of the project was related to the performance of the SO-110 gas source (Ramsey, Ditchfield et al. 2004). In normal operation this source can either be used with a continuous-flow CO2 injection system which is feed by an elemental analyser or an ampoule cracker or with a gas chromatograph. A test injector was constructed in order to characterize the source. Apart from determining the source efficiency at constant flow (about 1%), several experiments were performed to examine the response of the source to gas pulses of 0.1 – 10 seconds duration, the purpose being to investigate the source's suitability for GC-AMS and HPLC-AMS. It was observed that the source efficiency was fairly constant over a wide range of gas doses above a certain threshold value, whereas it decreased to almost zero for smaller doses. The source reacted rapidly to the gas pulses with a near-vertical rise and a decay time typically of about 2 seconds, however a slow component of the decay existed too, and significant pulse broadening was observed for large pulses. Finally, it was observed that the source performance depended critically on the settings applied and on source history.
References: Ramsey, C. B., P. Ditchfield, et al. (2004). "Using a gas ion source for radiocarbon AMS and GC-AMS." Radiocarbon 46(1): 25-32. Ramsey, C. B., T. Higham, et al. (2004). "Towards high-precision AMS: Progress and limitations." Radiocarbon 46(1): 17-24.
See more of Poster Session I
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)