Microscopic Calculation of the Gas Gain in Cylindrical Proportional Counters

• Published in: Radiation protection dosimetry 1989-10, Vol.29 (1-2), p.23-30
• Main Authors: Ségur, P., Pérès, I., Boeuf, J.P., Bordage (INVITED), M.C.
• Format: Article
• Language: English
• ISSN: 0144-8420; 1742-3406
• DOI: 10.1093/oxfordjournals.rpd.a080525

In this work, the first ionisation coefficient is calculated in equilibrium and non-equilibrium situations. All the calculations are made using microscopic methods (numerical solution of the Boltzmann equation and Monte Carlo calculations). Thus the only data needed are electron-molecule cross sections. We give the cross sections we used for the calculations in methane. We then calculate equilibrium values of the first ionisation coefficient in argon, methane, in some mixtures of these gases and for tissue equivalent mixture. The non-equilibrium calculations show that, generally, three phenomena may modify the equilibrium values of the gain. Among these, the most important in practical cases is the delay effect existing since electrons cannot reach the equilibrium state, due to the fast increase of the electric field. It follows that their mean energy and the ionisation coefficient are lower than the corresponding equilibrium values. Experimental determination of the gas gain in argon-methane mixtures and in pure methane are compared with our equilibrium calculations. The discrepancies observed between measured and calculated values are explained in terms of non equilibrium effects.