Theoretical model for response calculation of electret ionisation chambers for radon dosimetry

The application of electret chambers for radon measurements requires the exact determination of their response. Therefore, a theoretical model has been established in order to calculate the radon response for any electret ionisation chamber and to quantify the physical processes responsible for the decrease of response at lower electret voltages. Moreover, a deeper understanding of the processes inside the chamber is helpful for the discussion of possible uncertainties of measurements. The model is based on a numerical calculation of the electric field inside the chamber and in the electret reader arrangement. It is shown that the main reasons for the decrease of the radon response with decreasing electret voltage are the growing recombination loss due to a lower field strength and the influence of changes in the electret surface charge distribution on the reader characteristics. The results of the calculation are compared with experimental data.