Fluctuation phenomena in the proportional counter.
thesisposted on 19.11.2015, 09:17 by John Peter. Sephton
The energy resolution of a proportional counter depends fundamentally on (i) fluctuations in the number of electron-ion pairs produced in the primary ionization and (ii) fluctuations in the size of the subsequent electron avalanche. The avalanche fluctuation is typically 3-4 times larger than the ionization fluctuation. Previous theoretical and experimental work has indicated that fluctuations in both processes can be reduced by improving the efficiency of the ionization mechanisms involved. The ionization efficiency can be increased by means of the Penning effect. Energy spent exciting metastable states is used to increase the ionization yield. This is achieved by introducing a second gas whose first ionization potential is slightly lower than the first metastable level of the main gas. Cross ionization of the admixture gas by metastable states in the main gas can thus occur. With appropriate conditions, the Penning effect becomes the dominant ionization process. The operation of a proportional counter filled with a range of Penning mixtures is investigated experimentally. Penning mixtures are shown to give improved resolution over the more conventional filling of argon-methane. The Penning mixture, argon-xenon, seems to be a practical alternative filling. In order to make rather more fundamental studies of avalanche fluctuations in Penning mixtures, a uniform field chamber is constructed. With this chamber it is possible to directly compare experimental measurements of f (the relative variance of the avalanche size) with values predicted by various avalanche models. (With a cylindrical counter the ionization efficiency varies from cathode to anode and such a comparison is less clear.) The models assume different variations of the local value of Townsend's first ionization coefficient ai with the distance travelled by an electron from the previous ionization. The results obtained are in reasonable agreement with two of the models but these are unsatisfactory from a theoretical point of view. The results are, however, reasonably close to predictions based on experimental measurements of ionization probability.