Operation of helium-filled proportional counter at low temperatures (4.2–295 K)

Abstract

Following the previous work by our group, the operation of the helium-filled counter has been studied more in detail at liquid-helium (∼ 5 K), liquid-nitrogen (77 K) and room (295 K) temperatures. The collimated σ-ray beam from a 210Po source, which is installed inside the counter, has made it possible to measure the gas gain M from the ionization-chamber region with M = ∼1. The operation of the helium-filled counter is rather unstable at 77 and 295 K; when the applied voltage is increased, the gas gain steeply rises up to 10 3 and the pulse rise time becomes longer than 200 μm. On the other hand, the counter works stably at 5 K, although available gas gains are comparatively small, i.e., at most 170. The slope of gas gain at 5 K is more gentle than that at 77 or 295 K and the pulse rise time does not change (∼ 2 μs) so much in the wide range of applied voltage. Measured results for gas gain and pulse rise time are reported in this paper. The favourable operation at 5 K is not only due to the absence of impurity in helium gas but also to the suppression of electron emission from the cathode. This new feature of the proportional-counter operation at a very low temperature is also discussed from the standpoint of atomic processes in helium gas.