Cancellation of gravity for slow neutrons in small angle scattering experiments

Abstract

In a small angle scattering experiment with long flight paths, a beam of slow neutrons falls under gravity–by a significant amount. For long–wavelength neutrons, this can have several effects: (i) the main neutron beam can fall off the bottom of the detector; (ii) the parabolic path of the neutrons in the collimation tube may prevent the neutrons from reaching the sample; and (iii) the wavelength spread of the neutrons causes vertical smearing of both main beam and scattered beam. All these effects can limit the resolution and minimum measurable wavevector. We demonstrate experimentally that the refraction of the beam by suitably designed prisms can remove these problems. We show how this cancellation may be understood i n terms of the wave properties of neutrons.