TRANSMISSION PROPERTIES OF A COUNTER-ROTATING PAIR OF DISK CHOPPERS

Counter-rotating pairs of disk choppers are used to provide pulsed neutron beams for thermal neutron scattering experiments. An important advantage of this type of device, compared with the single disk chopper, is that each disk can be equipped with several slots. A choice of slot widths is then made possible, simply by changing the relative phasing of the two disks. The resolution of an instrument with counter-rotating chopper pairs can therefore be altered without having to modify the incident wavelength or the speed of the choppers. A potential problem, when there are several slots in each disk, is that there will be "cross talk", i.e. neutrons will be transmitted by a pair of slots other than the intended combination. We examine the transmission properties of a counter-rotating pair of choppers, and describe methods to determine whether or not cross talk can occur. An important conclusion is that the axial separation between the two choppers must be included in the analysis; this separation, which is generally small (of order several cm), significantly increases the potential for unwanted cross talk. We briefly discuss the implications of cross talk in the design of a high resolution time-of-flight spectrometer.