Background contributions in the electron-tracking Compton camera aboard SMILE-2+

Mega electron volt (MeV) gamma-ray observations are promising diagnostic tools for observing the Universe. However, the sensitivity of MeV gamma-ray telescopes is limited by peculiar backgrounds, restricting the applicability of MeV gamma-ray observations. Thus, background identification is crucial in the design of next-generation telescopes. Here, we assessed the background contributions of the electrontracking Compton camera (ETCC) onboard SMILE-2+ in balloon experiments. This assessment was performed using Monte Carlo simulations. The results revealed that a background below 400 keV existed due to the atmospheric gamma-ray background, cosmic-ray/secondary-particle background, and accidental background. Moreover, an unresolved background component that was not related to direct Comptonscattering events in the ETCC was confirmed above 400 keV. Overall, this study demonstrated that the Compton-kinematics test is a powerful tool for removing backgrounds and principally improves the signalto-noise ratio at 400 keV by an order of magnitude.