Background Contributions in the Electron-Tracking Compton Camera Aboard SMILE- 2+

The Mega electron volt (MeV) gamma-ray observation is a promising diagnostictool for observing the universe. However, the sensitivity of MeV gamma-raytelescopes is limited due to peculiar backgrounds, restricting the applicationof MeV gamma rays for observation. Identification of backgrounds is crucial fordesigning next-generation telescopes. Therefore, herein, we assessed thebackground contribution in the electron-tracking Compton camera (ETCC) on boardthe SMILE- 2+ balloon experiment. This assessment was performed using the MonteCarlo simulation. The results revealed that the background below 400 keVexisted due to the atmospheric gamma-ray background, thecosmic-ray/secondary-particle background, and the accidental background. On theother hand, the unresolved background component, which was not likely to berelevant to direct Compton-scattering events in the ETCC, was confirmed above400 keV. Overall, this study demonstrated that the Compton-kinematics testprovides a powerful tool to remove the background and principally improves thesignal-to-noise ratio at 400 keV by an order of magnitude.