MeV gamma-ray astronomy at the energy range of 0.1-100 MeV is an unique window for
observing nucleosynthesis, and it also provides the information on particle acceleration
in active galactic nuclei or gamma-ray bursts, the strong gravity potential of black
holes, the interaction of cosmic-ray and interstellar matter in the Galactic
plane. However, the observation in this energy band is not advanced in comparison with
other bands, because there is no imaging method having a sharp point spread function
(PSF) and a powerful background rejection. For the next generation MeV gamma-ray
telescope, we are developing an electron-tracking Compton camera (ETCC), which consists
of a gaseous time projection chamber as an electron tracker and pixel scintillator
arrays as an absorber. The track of Compton-recoil electron restricts the gamma-ray
incident direction with powerful background rejection based on particle identification
and Compton-scattering kinematic test. Because an ETCC detect the incident direction
event by event, we can define a PSF of an ETCC as similar to usual telescopes. Moreover,
we confirmed that the PSF of Compton camera strongly depends on the accuracy of
Compton-recoil direction. For the future observation with the sensitivity of 1 mCrab, an
ETCC will require the accuracy of Compton scattering angle (angular resolution measure:
ARM) of ~2 degrees and the accuracy of Compton-recoil direction (scatter plane
deviation: SPD) of ~5 degrees.Now we are constructing a 30 cm-cubic ETCC for the second
balloon experiment: Sub-MeV gamma ray Imaging Loaded-on-balloon Experiment:
SMILE-II. The current ETCC has an effective area of ~1cm2 at 300 keV, a PSF of 15
degrees (ARM and SPD are ~5 degrees and ~100 degrees, respectively) at FWHM for 662 keV,
and a large field of view of ~3 sr. We will upgrade this ETCC to an effective area of
several cm2 and a PSF of ~5 degrees with the detection of high-energy
recoil-electron. Using the upgraded ETCC, our observation plan of SMILE-II+/III is the
mapping of electron-positron annihilation line and 1.8 MeV from Al-26. In this
presentation, we will report our detector and our observation plan.