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

In the MeV band, we can obtain the unique information about nucleosynthesis, diffusion in our galaxy, low-energy cosmic rays, particle acceleration, and so on. However, the detection sensitivity in this band has a large gap compared to that in the other bands due to a huge background contamination. For the future observations, we are developing an electron-tracking Compton camera (ETCC) with the powerful background rejection tools based on the Compton recoil electron tracks. This ETCC has been demonstrated in the last two balloon experiments, and we are now preparing for the next balloon flight SMILE-3 to observe galactic diffuse gamma rays and some bright celestial objects. In this talk, we will talk about the scientific motivation and the preparation of ETCC for SMILE-3.