[en] Continuing research on advanced methods of thermal neutron detection and position sensing with gas-filled counters was aimed at improving their performance and extending the limits of their applicability. High electron drift velocities obtained from measurements on gas mixtures containing CF₄ motivated us to evaluate the properties of ³He-CF₄ and Ar-CF₄ mixtures to show that these gases have the potential of improving the count rate capability, spatial resolution, and photon discrimination of neutron PSPCs (position-sensitive proportional counters) and fission counters. In support of the U.S. National Small-Angle Neutron Scattering (SANS) Facility we developed a large-area (65-cm x 65-cm) PSPC camera. RC position encoding was chosen for simplicity of construction, but since previous experience with this encoding method had been limited to smaller PSPCs (area < 25 cm x 25 cm), the main objective of this development was to show that RC encoding parameters and construction methods could be scaled up for larger area PSPCs. The use of the new counter gas mixtures enabled the development of position-sensitive transmission line fission counters (TLFCs) for neutron flux monitoring and a one-dimensional, curved PSPC for large-angle (130⁰) neutron diffraction experiments. The main objective of these developments was to extend the capabilities of the LC-encoding method by mitigating the effects of interelectrode capacitance, and thereby increase the count rate capability