Monte Carlo Simulation and Experimental Characterization of Tissue Equivalent Proportional Counter for Dosimetry of Neutron and Photon Fields

Abstract

The Tissue Equivalent Proportional Counter (TEPC) is a radiation detector generally used for experimental microdosimetry. The TEPC offers a physical approach in measuring energy depositions in a microscopic volume by filling the gas cavity with a low-pressure gas. It allows observation of the behavior of the patterns of energy distribution in the microscopic scale, which is critical in understanding the interaction of radiation especially to the sensitive structures of the cell. Over the last two decades, TEPCs of different shapes and sizes tailored to specific applications have been developed due to the growing interest in space exploration as well as particle beam therapies where various types of radiation contribute to the dose.

The purpose of this dissertation is to observe the response of the Benjamin-type TEPC developed by the Korea Astronomy and Space Science Institute (KASI) for mixed radiation fields. Experimental measurements were performed in order to evaluate its capability in measuring the microdosimetric spectra for a 2 um-simulated site in a pure propane gas when irradiated with Cs-137 gamma and Cf-252 neutron sources. Furthermore, Monte Carlo (MC) modelling of the TEPC using the Geant4 simulation toolkit was also carried out to validate the experimental results and to evaluate its capability in reproducing the measured spectra.

The frequency-weighted lineal energy distribution (yf(y)) and dose-weighted lineal energy distribution (yd(y)) were constructed from both the simulation and experimental data. The frequency-mean lineal energy (y ̅_F) and dose-mean lineal energy (y ̅_D) were subsequently calculated. For the photon irradiation, the shape of the experimental spectrum was accurately predicted by the simulation when low lineal energy events were not considered. The difference in the measured and calculated y ̅_F and y ̅_D values for the photon case were 2.8% and 4%, respectively, when low lineal energy events were disregarded.

Similarly, the shape of the calculated spectra for the neutron irradiation was in overall agreement with the experimental one particularly in the region of the neutron-induced peak. However, underestimation in the simulated spectra along the low lineal energy region was observed. Removal of the gamma region resulted in a satisfactory agreement between the measured and calculated y ̅_F and y ̅_D values within 5%. Although discrepancies were observed for the neutron case, the results in this work indicated that the simulation model based on the Geant4 toolkit successfully reproduced and predicted the experimental microdosimetric spectra with reasonable accuracy for both irradiation conditions.