The THick Gas Electron Multiplier (THGEM) based tissue-equivalent proportional counter (TEPC) has been proven to be useful for microdosimetry due to its flexibility in varying the gaseous sensitive volume and achieving high multiplication gain. Aiming at measuring the spatial distribution of radiation dose for mixed neutron-gamma fields, an advanced two-dimensional (2D) THGEM TEPC was designed and constructed at McMaster University which will enable us to overcome the operational limitation of the classical TEPCs, particularly for high dose rate fields. Compared to the traditional TEPCs, anode wire electrodes were replaced by THGEM layer, which not only enhances the gas multiplication gain but also offers a flexible and convenient fabrication or building 2D detectors. The 2D THGEM TEPC consists of an array of 3×3 sensitive volumes, equivalent to 9 individual TEPCs, each of which has a dimension of 5 mm diameter and length. Taking the overall cost, size and flexibility into account, to process 9 detectors signals simultaneously, a multi-input digital pulse processing system was developed by using modern microcontrollers, each of which is coupled to a 12-bit sampling ADC with a sampling rate of 42 Msps. The signal processing system was tested using a NaI(Tl) detector, which has proven that is it faster than a traditional analogue system and a commercial digital system. Using the McMaster Tandetron 7Li(p,n) accelerator neutron source, both fundamental detector performance, as well as neutron dosimetric response of the 2D THGEM TEPC, has been extensively investigated and compared to the data acquired by a spherical TEPC. It was shown that the microdosimetric response and the measured absorbed dose rate of the 2D THGEM detector developed in this study are comparable to the standard 1/2" TEPC which is commercially available. / Thesis / Doctor of Philosophy (PhD)

At low doses, relevant to nuclear incidents and accidental releases of radioactivity, the detriment of radiation extends beyond direct effects. This thesis investigates genomic instability, a subclass of non-targeted effects where damage and lethality is transmitted vertically and expressed in the progeny of cells many generations after initial radiation exposure. Through a series of experiments using clonogenic assay of human and fish cell culture, studies described in this thesis describe lethal mutations, hyper radiosensitivity and increased radioresistance – processes involving repair mechanisms that dictate survival in cells exposed to low doses. Further study investigates the difference in the relative biological effect of alpha particle radiation compared to what is expected at high doses. Results demonstrate increased radioresistance in a human cell line while also revealing increased lethality in a fish cell line confirming the need for consideration of dose-dependence as well as variance in behaviors of different cell lines and species. It is hoped the conclusions of this thesis will inspire the creation of protocols with greater attention to the indirect consequences of exposure to radiation at doses relevant to nuclear incidents and accidents. / Thesis / Master of Science (MSc)