  Electron tracking based Compton imaging is a key technique to improve the
sensitivity of Compton cameras by measuring the initial direction of recoiled
electrons. To realize this technique in semiconductor Compton cameras, we
propose a new detector concept, Si-CMOS hybrid detector. It is a Si detector
bump-bonded to a CMOS readout integrated circuit to obtain electron trajectory
images. To acquire the energy and the event timing, signals from N-side are
also read out in this concept. By using an ASIC for the N-side readout, the
timing resolution of few us is achieved. In this paper, we present the results
of two prototypes with 20 um pitch pixels. The images of the recoiled electron
trajectories are obtained with them successfully. The energy resolutions (FWHM)
are 4.1 keV (CMOS) and 1.4 keV (N-side) at 59.5 keV. In addition, we confirmed
that the initial direction of the electron is determined using the
reconstruction algorithm based on the graph theory approach. These results show
that Si-CMOS hybrid detectors can be used for electron tracking based Compton
imaging.
