Scan profiles for neutron spectrometers. II. Rectangular-profile elements by acceptance-diagram methods

Abstract

The recent development of neutron collimators with rectangular transmission pro®les (intensity versus angular divergence) extends hope of improved count rates on neutron scattering instruments. It is usually assumed that a more effective use of beam angular spread in these devices should increase count rates by about a factor of two. However, real beams have both angular and wavevector spread and both these spreads are governed by the allowed collimation. In this extended view, the gains from ideal rectangular-pro®le elements (angle ®lters) are shown to be much larger (about a factor of four). The mirror re¯ections used to achieve the rectangular pro®les in real devices complicate the resolution effects. Speci®cally, the re¯ections disturb the wavevector±angular divergence correlation in the beams, leading to unusual peak shapes characterized by triple peaks on powder diffractometers. Thus, these re¯ecting collimators are likely to be universally useful only before the monochromator and immediately preceding the detector, where wavevector± angle correlations have no effect. This reduces the potential gains to a factor of two or so. Note that the gains are as previously expected but for quite different reasons than imagined. This remains a very signi®cant gain in a ®eld where most work is intensity-limited.