Evaluating the capability of detecting recoil-electron tracks using an electron-tracking Compton camera with a silicon-on-insulator pixel sensor

Abstract

Sub-MeV gamma-ray line observations are crucial for the understanding of high-energy astrophysical processes. In this energy range, the presence of a significant amount of background poses a serious problem. Therefore, it is essential to reduce this background to achieve high-sensitivity observations. An electron-tracking Compton camera can efficiently mitigate the impact of background on scientific results thanks to its ability to fully resolve the Compton process. In addition if we utilize a semiconductor detector as the sensor, the electron-tracking Compton camera can achieve high-energy resolution for the detection of gamma-ray lines. To operate the electron-tracking Compton camera effectively, the sensor needs to coincidence events for each detection signal and resolve the complex structure of recoiled-electron tracks of a few hundred μm. We focused on the silicon-on-insulator pixel sensor XRPIX2b developed for X-ray observations, which satisfies requirements. We developed a prototype electron-tracking Compton camera using XRPIX2b and carried out a quantitative evaluation of its detection capability for estimating the recoil directions of electrons at various angles. As a result, we successfully detected short recoil-electron tracks with a length of 100μm corresponded to scattering angles of 30°-40° for 511 keV gamma rays entering the sensor. In term of angular resolution measure and scatter plane deviation of a reconstructed image, the evaluations showed results of 15° and 40° (full width at half maximum), respectively, in cases where 511 keV gamma rays were scattered at 90° and recoil electrons tracked the surface of the sensor detection plane. We confirmed that these results were consistent with simulation results, within approximately a 15% margin.