An engineering model of a multiwire positionsensitive 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 end time-difference decoding method to measure the spatial coordinates of the Interaction loci of individual scattered neutrons. The active area of Che PSPC is 65 cm x 65 cm, and the active depth is 3.6 cm. The spatial uncertainty in both coordinates is VI. 0 cm (fwhm) for thermal neutrons; thus, a matrix of 64 x 64 picture elements is resolved. The count rate capability for randomly detected neutrons is 10* counts per second, with <3Z coincidence loss. The PSPC gas composition is 632 3 He, 32Z Xe, and 5Z C0 2 at an absolute pressure of ^3 x 10 -Pa (3 atm). The detection efficiency is 'VJOZ for the 0.475-nm (4.75-A) 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.

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