Charge Collection Efficiencies for Lithium-Drifted Silicon and Germanium Detectors in the X-Ray Energy Region

Theoretical calculations showing the charge collection efficiencies of Ge(Li) and Si(Li) planar detectors as a function of trapping lengths and incident X-ray energies are presented. It is demonstrated that there should be an intrinsic energy dependence of the charge collection efficiency for radiation lengths comparable to the detector depletion depth. The nonlinear behavior is generally greater for unequal hole and electron trapping lengths. Experimental results confirm this in part, but surface effects appear to accentuate the effect. It is concluded that, neglecting surface effects, the nonlinearities should be eliminated if the radiation is incident in a direction perpendicular to the field. Highest charge collection efficiency, best resolution and linearity are obtained by injecting the photons through the detector side (perpendicular to the field) along a unique plane which is defined by the electron to hole trapping ratio.