Optimizing some parameters of air-filled ion pulse ionization chambers for effective radon detection

Air-filled ionization chambers in pulse ion mode can provide precise low-level detection of indoor/outdoor radon. However, it suffers from relatively high collection duration of positive and negative charges as well as environmental changes. In order to overcome these shortcomings, the Garfield++ software which is a powerful toolkit to develop a realistic simulation framework for radiation detectors was applied. Also, Garfield++ interface to Magboltz was used to calculate the gas parameters for air at various humidity conditions for a wide range of electric field strengths in order to compare the results with other available data in the literature. The Garfield++ software was also used to calculate maximum charge collection duration for a wide range of tube radii, wire radii, and electric potentials which are the main parameters affecting the charge collection duration. Furthermore, the influence of environmental conditions including air pressure, temperature, and humidity on the charge collection duration was investigated. The results show that for an air-filled radon ionization chamber with a moderate size (e.g. 1 L) with a bias voltage of hundreds of volts, the time constant of the external circuit can be chosen to be less than 100 ms, which is much lower than the values used in other studies. Therefore, with a proper design of an ionization chamber and its electronic system, it is possible to determine radon activity concentrations of up to a few tens of kBq.m-3 in pulse mode. In conclusion, the data obtained in this study assists in developing radon air-filled ionization chambers in order to determine chamber dimensions and bias voltage as well as designing the readout electronics according to the characteristics required in an intended application.