  An electron-tracking Compton camera (ETCC) is a detector that can determine
the arrival direction and energy of incident sub-MeV/MeV gamma-ray events on an
event-by-event basis. It is a hybrid detector consisting of a gaseous time
projection chamber (TPC), that is the Compton-scattering target and the tracker
of recoil electrons, and a position-sensitive scintillation camera that absorbs
of the scattered gamma rays, to measure gamma rays in the environment from
contaminated soil. To measure of environmental gamma rays from soil
contaminated with radioactive cesium (Cs), we developed a portable
battery-powered ETCC system with a compact readout circuit and data-acquisition
system for the SMILE-II experiment. We checked the gamma-ray imaging ability
and ETCC performance in the laboratory by using several gamma-ray point
sources. The performance test indicates that the field of view (FoV) of the
detector is about 1$\;$sr and that the detection efficiency and angular
resolution for 662$\;$keV gamma rays from the center of the FoV is $(9.31 \pm
0.95) \times 10^{^-5}$ and $5.9^{\circ} \pm 0.6^{\circ}$, respectively.
Furthermore, the ETCC can detect 0.15$\;\mu\rm{Sv/h}$ from a $^{137}$Cs
gamma-ray source with a significance of 5$\sigma$ in 13 min in the laboratory.
In this paper, we report the specifications of the ETCC and the results of the
performance tests. Furthermore, we discuss its potential use for environmental
gamma-ray measurements.
