Numerical modeling of the gas gain of low-pressure Tissue-Equivalent Proportional Counters

Proportional counters are radiation detectors widely used in many applications. The design of the counter, to best fit each application, needs an accurate knowledge and physical modeling of the electron avalanche process. A particular proportional counter is the tissue-equivalent proportional counter (TEPC), the reference detector for experimental microdosimetry, which consists of a spherical or cylindrical chamber filled with low-density tissue-equivalent gas to simulate the energy deposition in tissue sites of micrometric size. The lower operation limit of standard TEPCs operated in the pulse-height analysis mode is about 0.3μm. In order to overcome this technological limit, different avalanche-confinement nano-microdosimetric TEPCs capable of measuring microdosimetric spectra in the nanometric domain were designed and constructed. In this work, a novel numerical tool developed for the Monte Carlo simulation of the electron avalanche process inside a low-pressure TEPC is described. The Monte Carlo code allows to simulate complex 3D electric field configurations exploiting COMSOL finite elements analysis. Several models for the electron interactions (i.e. scattering and ionization) are included in the code. The code has been benchmarked with the experimental results of a wall-less avalanche-confinement TEPC in terms of absolute gas gain for different operating conditions (i.e. gas pressures and electrode voltages). The results show that the code is capable of reproducing the absolute value of the gas gain for the avalanche-confinement TEPC simulating some tenths of nanometers in site size. Moreover, the code can reproduce both the extension and the shape of the proportional counter working windows. The code was also applied for simulating the probability of absorption of electrons by the central third electrode: the helix. The results show a non-negligible probability of absorption in the common range of operation. This code will be further applied for optimizing the TEPC design, capable of simulating site sizes closer to the nanometer region.