Large-Area Proportional Counter Camera for the U. S. National Small-Angle Neutron Scattering Facility

An engineering model of a multiwire position-sensitive proportional-counter (PSPC) was developed, tested, and installed at the U. S. National Small-Angle Neutron Scattering Facility at ORNL. The PSPC is based on the RC-encoding and time-difference decoding method to measure the spatial coordinates of the interaction loci of individual scattered neutrons. The active area of the PSPC is 65 cm × 65 cm, and the active depth is 3.6 cm. The spatial uncertainty in both coordinates is ~1.0 cm (fwhm) for thermal neutrons; thus, a matrix of 64 × 64 picture elements is resolved. The count rate capability for randomly detected neutrons is 104 counts per second, with <3% coincidence loss. The PSPC gas composition is 63% 3He, 32% Xe, and 5% CO2 at an absolute pressure of ~3 × 105 Pa (3 atm). The detection efficiency is ~90% for the 0.475-nm (4.75-Å) neutrons used in the scattering experiments. These results confirm that the parameters of the RC encoding method can be scaled to successfully design large-area PSPCs.