Analysis of a drum chopper for use on a new small angle diffractometer at IPNS

Light-weight drum choppers rotating at 15 Hz have been in use on the IPNS powder diffractometers for several years, where they serve to eliminate the delayed-neutron background from much of the spectral region of interest. Monte Carlo simulations indicate that a similar chopper operated at 15 Hz should do an excellent job of delayed-neutron removal in the new small-angle diffractometer being designed at IPNS. The simulations also show that when the same chopper is operated at 7.5 Hz it performs quite successfully as a frame-elimination chopper, effectively eliminating neutrons from alternate pulses and allowing extension of the useful wavelength range of the instrument to {approximately}28 {Angstrom}. Thus the incorporation of such a chopper should add considerably to the range and flexibility of the new instrument. 7 refs., 9 figs.