A fast interferometric chopper for neutrons and X rays

Abstract

FAST neutron choppers have been considerably developed in the past few years. The commonly used choppers are either rotating massive objects through which appropriately shaped neutron channels are cut¹ or crystals whose diffracting power may be modulated by magnetic fields² or dynamic strains³. All these devices suffer from practical limitations of speed, poor duty cycle and the consequent loss of intensity. We describe here our use of a scanning Bragg reflecting crystal interferometer as a fast chopper. Although they have been used with X rays, crystal interferometers work equally well with neutrons⁴. The interferometer is used as a phase sensitive neutron switch, in which a crystal translation of 0.96 Å is sufficient to redirect the beam. At larger translational amplitudes the interferometer has a corresponding multiplex advantage over other neutron choppers. Two of these interferometric neutron choppers could be used to construct a Fourier transform neutron spectrometer with 1,000 times the resolution of competing spectrometers. With a pulsed source a single interferometric chopper could be used to construct a time of flight spectrometer which would provide high resolution even with fast neutrons.