Correcting time-of-flight neutron diffraction data for chopped delayed neutron backgrounds: The action of a drum chopper in a pulsed neutron beam

Delayed neutron choppers in pulsed source neutron beams serve to reduce the background caused by delayed neutrons in pulsed source instruments. We analyze the effect of a drum chopper placed in an incident pulsed beam which contains delayed neutrons and compute its influence on the detector counting rate. Expressions are found for the time and wavelength dependence of the counting rates for both prompt and delayed neutrons, in both the monitor and the scattered neutron detectors. On the basis of these results, an exact method for determining the delayed neutron background is suggested for use in measuring and correcting pulsed-source diffractometer data, by operating the chopper in random phase with respect to the source. Other approximate methods are investigated. We report measurements which confirm the analysis. (orig.)