Simulation of the performance of an electron-tracking Compton camera using a two-dimensional image sensor and analytical method

This report proposes a high-accuracy Compton camera (CC) designed to improve gamma-ray visualization, which addresses the limitations of size and complexity of standard CCs. As a single-layer CC has low sensitivity, a stack of numerous single-layer CCs is used. Therefore, simplicity is as important as accuracy. There are two types of CCs. The first is the conventional CC with a simple structure, which detects the incident-ray direction using multiple gamma rays but suffers from high background and noisy signals. The second is the electron-tracking CC, which detects the incident-ray direction using a single gamma ray. However, it requires measuring the three-dimensional (3D) trajectory of recoil electrons generated by Compton scattering, which necessitates complex equipment such as gas chambers or dedicated semiconductor detectors. Instead of measuring the 3D trajectory of recoil electrons, we developed a method to analytically determine the 3D direction from a two-dimensional recoil-electron image captured by a charge-coupled device (CCD). We classified Compton scattering based on scattering conditions and formulated equations that describe incident directions for all scattering conditions. As this method uses a simple commercial CCD with a small pixel size, it enables simple and high-accuracy measurements. Our simulation using Geant4 indicated that for 511 keV gamma rays with a 10 μm pixel size, the full width at half maximum of the scatter plane deviation was less than 50°. Additionally, this method was applicable when the recoil-electron energy was 50 keV or more. Furthermore, we discuss the adaptation limits of this method induced by measurement errors.