ANALYSIS OF COMPOSITE NEUTRON MONOCHROMATOR SYSTEMS CONSISTING OF CURVED CRYSTALLINE LAMELLAS

Composite neutron monochromator systems consisting of curved crystalline lamellas were studied. The conditions were derived under which such systems can transform a parallel polychromatic neutron beam into a diffracted beam in which the neutron momentum is dependent upon the secondary ray direction. Such a transformation was proposed recently by Maier-Leibnitz, who considered a system of monocrystalline plane lamellas having a small mosaic spread η and stressed its advantages in neutron physics. One advantage is, for instance, the elimination of superposition of the several order (hkl) reflexes in the Laue method. The present paper shows that the original system of plane lamellas is a particular case (corresponding to the curvature radius R = ∞) of infinitely many possible systems of curved lamellas. All these systems are equivalent, i.e. realize the same transformation of neutron beams, but they differ because the curvature radius of the lamellas depends upon their orientation with respect to the incident beam. The curvature of perfect crystals substitutes for the mosaic spread of the plane lamellas in the elimination of primary extinction. These results suggest alternative approaches to the experimental realization of the neutron beam transformation. © 1969.