  We report on the measurements of the spectra of gamma radiation generated by
855 MeV electrons in bent silicon and germanium crystals at MAMI (MAinzer
MIkrotron). The crystals were 15 {\mu}m thick along the beam direction to
ensure high deflection efficiency. Their (111) crystalline planes were bent by
means of a piezo-actuated mechanical holder, which allowed to remotely change
the crystal curvature. In such a way it was possible to investigate the
radiation emitted under planar channeling and volume reflection as a function
of the curvature of the crystalline planes. We show that using volume
reflection, one can produce intense gamma radiation with comparable intensity
but higher angular acceptance than for channeling. We studied the trade-off
between radiation intensity and angular acceptance at different values of the
crystal curvature. The measurements of radiation spectra have been carried out
for the first time in bent Germanium crystals. In particular, the intensity of
radiation in the Ge crystal is higher than in the Si one due to the higher
atomic number, which is important for the development of the X-ray and gamma
radiation sources based on higher-Z deformed crystals, such as crystalline
undulator.
