Investigations on 3He filled proportional counters aimed at various neutron applications

Solid State Physics Division is involved in development of gas filled neutron detectors for more than two decades, for various applications such as neutron beam research and area-beam monitoring. These detectors show excellent characteristics and long life and are continuously under modification to suit the specific applications. Developments are aimed at improving the operational stability of detectors in hostile environments like high gamma field and extreme operating temperatures and enhancing life of the detector. Position sensitive detectors (PSD) mounted at neutron scattering instruments around Dhruva and CIRUS have demands of high efficiency, larger dimensions with finer position resolutions and the geometry that intercepts maximum scattered beam. We present the basic characterization of 3He filled detectors supporting the development of wide spectrum of detectors. Performance of He3 filled neutron detectors depends on the additive gases and their concentration. Gas gain and Diethorn gas constants are measured and results are used to design the operating parameters of large area and high pressure detectors. Detector fill gas is optimized for proportional region and low gamma sensitivity. Investigations are carried out on 3He filled neutron detectors with additive gases with low photon absorption cross section CF4 and CO2. Detector with CO2 as additive gas shows good gamma discrimination up to flux of 8 R/hr. These investigations are also used to design high efficiency and high resolution PSDs. Newly assembled 15 PSDs in a stack of 3 layers show the gain in efficiency of 600 % and improvement in resolution by 2 mm. (author)