  To explore the sub-MeV/MeV gamma-ray window for astronomy, we have developed
the Electron-Tracking Compton Camera (ETCC), and carried out the first
performance test at room condition using several gamma-ray sources in the
sub-MeV energy band. Using a simple track analysis for a quick first test of
the performance, the gamma-ray imaging capability was demonstrated by clear
images and 5.3 degrees of angular resolution measure (ARM) measured at 662 keV.
As the greatest impact of this work, a gamma-ray detection efficiency on the
order of $10^{-4}$ was achieved at the sub-MeV gamma-ray band, which is one
order of magnitude higher than our previous experiment. This angular resolution
and detection efficiency enables us to detect the Crab Nebula at the 5 sigma
level with several hours observation at balloon altitude in middle latitude.
Furthermore, good consistency of efficiencies between this performance test and
simulation including only physical processes has a large importance; it means
we achieve nearly 100% detection of Compton recoil electrons. Thus, our
estimation of enhancements by upgrades of the detector is more dependable. We
are planning to confirm the imaging capability of the ETCC by observation of
celestial objects in the SMILE-II (Sub-MeV gamma ray Imaging Loaded-on-balloon
Experiment II). The SMILE-II and following SMILE-III project will be an
important key of sub-MeV/MeV gamma-ray astronomy.
