A design for improved neutron collimators

Abstract

Soller slit collimators are widely used to define beams of thermal neutrons for scattering experiments and hence to tailor the instrumental resolution. The possibility of using neutron guide principles to increase the overall transmission of a collimator for a given overall instrumental resolution has long been recognised. Advances over the last decades in the semiconductor industry now provide the technology to make such collimators. A feasible design is to use blades of flat silicon wafers on which non-polarising neutron supermirrors are vacuum deposited. In its simplest form the coating would consist of two thin layers of a material with a large scattering length, such as nickel, sandwiching a neutron absorbing layer such as gadolinium. Calculations show that such devices could increase the counting rate at the detector by up to 47% per collimator for a given instrumental resolution. A simpler design where the silicon wafers support a thin absorbing layer alone is also described and can be used to build very short, high transmission conventional neutron collimators.