Neutron detection systems for small-angle scattering

Abstract

Reactions of neutrons with nuclei of 3He, 6Li and 10B, all of which produce energetic charged particles, are the basis of high-efficiency smooth-response detectors for thermal and cold neutrons. For small-angle scattering the position of the neutron event must be determined. Several encoding schemes are used, including arrays of individual elements, arrays of linear position-sensitive detectors, and large-area gas detectors with orthogonal grids. A large dynamic range (high count-rate capability and low background noise) is also essential. At pulsed sources, two additional requirements are time resolution and extension of the wavelength range to epithermal neutrons. The Anger camera, long used for medical γ-ray imaging, has been adapted for neutron detection and has potential advantages compared with gas detectors for pulsed-source small-angle scattering in terms of extending efficiency and count rates, and has a higher intrinsic resolution. However, ability to discriminate against γ-ray background has not yet been demonstrated. For pulsed sources the flux must be monitored as a function of neutron wavelength. A high-efficiency small-area scintillator monitor may be installed in the beam stop to give direct measurement of the transmitted beam during the experiment. Such a measurement also shows the relative efficiencies of glass and gas detectors.