  This paper reports on the demonstration of a high-rate energy measurement
technique using a thin depletion layer silicon avalanche photodiode (Si-APD). A
dedicated amplitude-to-time converter is developed to realize simultaneous
energy and timing measurement in a high rate condition. The energy response of
the system is systematically studied by using monochromatic X-ray beam with an
incident energy ranging from 6 to 33 keV. The obtained energy spectra contain
clear peaks and tail distributions. The peak fraction monotonously decreases as
the incident photon energy increases. This phenomenon can be explained by
considering the distribution of the energy deposit in silicon, which is
investigated by using a Monte Carlo simulation.
