Design and operation of a gas scattering energy spectrometer for the ISIS RFQ accelerator test stand

The design and operation of an apparatus to measure the beam energy of a radio frequency quadrupole (RFQ) particle accelerator, based on multiple scattering of H− 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 of H− ions 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− ions and measure their energies.