Celestial MeV gamma-ray observation using electron-tracking Compton camera loaded on long duration balloons (SMILE-3)

MeV gamma-ray observations provide unique information about nucleosynthesis, diffusion in our galaxy, low-energy cosmic rays, particle acceleration, and other phenomena. However, the detection sensitivity in this band is significantly lower than that in other bands due to a large background contamination. To address this issue, we are developing an electron-tracking Compton camera (ETCC) with powerful background rejection tools based on Compton recoil electron tracks. This will enable future observations to be conducted with greater sensitivity. We have successfully demonstrated the detection technology and performance of the ETCC with two balloon experiments. We are preparing for the next balloon flight, SMILE-3, to observe galactic diffusion gamma rays and some bright celestial objects.