Dynamical neutron diffraction in a thick-crystal interferometer

Abstract
The Massachusetts Institute of Technology (MIT) neutron interferometer consisting of two thick perfect-crystal plates of silicon is described, its operation is analyzed within the framework of dynamical diffraction theory, and experimental results illustrating some aspects of its operation are presented. It is found theoretically that the details of the distribution of intensity in the neutron beams leaving the device and the changes in the intensity distribution induced by phase-shifting elements placed between the crystals depend sensitively on the coherence properties of the neutron radiation as it enters the interferometer. A slit placed in front of the first crystal plate will modify the incident-beam coherence properties enough to affect the interferometer operation, if the width of the slit is comparable to or smaller than the Pendello$uml—sung ength which characterizes the dynamical diffraction in the crystal plates. High-resolution scans of the intensity distribution of the neutron radiation leaving the interferometer were performed while the incident beam was defined by a wide slit (approximately 1 mm in width) or alternatively a narrow slit (approximately 0.1 mm in width, comparable to the Pendello$uml—sung length), with phase-shifting devices placed between the crystals. The data agree closely with the theoretical calculations, and demonstrate the spatially extended coherence properties of a neutron wave traversing such an interferometer.