Silicon semiconductor detectors described in the previous chapter have depletion depths
less than 1 mm, which are sufficient for charged particle spectroscopy or soft X-ray
detection. For photon spectroscopy in the energy region of hundred keV or several MeV,
much thicker semiconductor detectors are required. Fig. 8.1 shows the mean free path of
a photon in silicon and germanium. It is obvious that a depletion depth of at least several
cm is required. Due to its higher atomic number, Ge has a much lager linear attenuation
coefficient, which leads to a shorter mean free path. Thus, Ge is preferred for hard X-ray
or gamma-ray detection to achieve higher detection efficiency