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Design of a gas scattering energy analyser for the ISIS RFQ accelerator test stand

The design of an apparatus to measure the beam energy of an RFQ, based on multiple scattering of H/sup -/ ions in xenon gas, is described. The purpose of the apparatus is to confirm the mean energy and energy spread of the nominal 665 keV beam from the ISIS RFQ. This RFQ, after comprehensive testing, is intended to replace the existing Cockcroft-Walton pre-injector on the ISIS spallation neutron source at the Rutherford Appleton Laboratory (RAL). The basis of the apparatus is a set of two cascaded assemblies each consisting of a gas scattering cell, a drift length and three small apertures which together reduce the peak intensity of the beam current sufficiently to allow a semiconductor charged particle detector to be used to detect individual H/sup i/ons and measure their energies.