Neutron optical imaging study of neutron moderator and beam extraction system

Abstract

The study of the performance of a cold-hydrogen moderator and a supermirror-based neutron beam extraction system of the flight path 12 at LANSCE has been performed based on energy-resolved neutron optical imaging. We have developed a pinhole camera system with a 2D position-sensitive 3He multiwire proportional chamber neutron detector with delay line position encoding (0.75 mm pixel size), together with a standalone time-of-flight electronic system with 1.2 μs dead time. We have determined the efficiency, resolution, and counting rate saturation of the detector. In particular, we have considered an impact of these parameters on the quality of the images. The neutron images of the moderator were taken as a function of the neutron wavelength given by the time-of-flight information. The images were recorded as arrays of 256×256×2000 pixels; x and y coordinates, and time of flight. Information obtained from the images includes a distribution of the brightness on the neutron moderator, the efficiency and geometrical accuracy of the beam extraction system, and the reflectivity of the supermirror-coated elements of its optics. Our results demonstrate that the pinhole optical camera-based neutron imaging method combined with time-of-flight information is an extremely efficient tool to characterize neutron sources and neutron beam extraction systems.