Electrostatic energy scanning for the GNS 3MV Van de Graaff accelerator

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Vickridge, I.C. 1996 Electrostatic energy scanning for the GNS 3MV Van de Graaff accelerator . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 96/26 12 p.

Abstract: Narrow resonant structures in nuclear reactions such as 18O(p, gamma) and 15N(p,alpha gamma) may be exploited to determine the concentration depth profile of the target nucleus by measuring the reaction yield as a function of incident proton energy from just below to above the resonant energy. Accurate and precise depth profiling requires extremely fine control of the incident beam energy. Varying the beam energy by varying the analysing magnet current does not provide the best control since even if the analysing magnet field is reproduced exactly at a particular point within the magnet, the effects of differential hysteresis will introduce uncontrollable variations in actual beam energy. An alternative means of varying beam energy, which circumvents this problem is to deflect the beam electrostatically before the energy analysing slits, allowing the automatic stabilisation system of the accelerator to adjust the beam energy to maintain the beam centrally between the slits. We have investigated two possible configurations with a view to implementing such an electrostatic energy scanning system on the GNS KN Ban de Graaff accelerator. Previously published work has considered only circular magnets, and our analysis shows that the focussing effects of the inclined pole faces on the KN analysing magnet have a significant effect on the performance of the system. We show that it should be feasible to scan up to at least + or - 15 keV by using two pairs of electrostatic deflectors symmetrically disposed before and after the magnet, whilst not exceeding 1000 volt across the deflector plates.