In this paper, we study how pixel size influences energy resolution for a proposed pixelated detector-a high sensitivity, low cost, and real-time radon monitor based on a Topmetal-II − time projection chamber (TPC). This monitor was designed to improve spatial resolution for detecting radon alpha particles using Topmetal-II − sensors assembled by a 0.35 µm CMOS integrated circuit process. Owing to concerns that small pixel size might have the side effect of worsening energy resolution due to lower signal-to-noise ratio, a Geant4-based simulation was used to investigate the dependence of energy resolution on pixel sizes ranging from 60 µm to 600 µm. A non-monotonic trend in this region shows the combined effect of pixel size and threshold on pixels, analyzed by introducing an empirical expression. Pixel noise contributes 50 keV full-width at half-maximum energy resolution for 400 µm pixel size at 1 ∼ 4 σ threshold that is comparable to the energy resolution caused by energy fluctuations in the TPC ionization process (∼ 20 keV). The total energy resolution after combining both factors is estimated to be 54 keV for a pixel size of 400 µm at 1 ∼ 4 σ threshold. The analysis presented in this paper would help choosing suitable pixel size for future pixelated detectors.

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I would like to take this opportunity to express my thanks to those who helped me with various aspects of conducting research and the writing of this thesis. First and foremost, my advisor Dr. James P. Vary for his guidance, patience and support throughout this research and the writing of this thesis. His insights and words of encouragement have often inspired me and renewed my hopes for completing my graduate education. I would like to thank my group members Shreeram Jawadekar and Mamoon Sharaf for wonderful discussions and help in producing some of the results. I would also like to thank my committee members for their efforts in this process. In the parallel programming part, I especially thank Dr. Glenn R. Luecke from math department for valuable learning experience in his high-performance computing courses. Results that obtained by parallel programming using MPI routines reported in this thesis are partially supported by the HPC@ISU equipment at Iowa State University, some of which has been purchased through funding provided by NSF under MRI grant number 1726447. vii