Performance of CdTe as a Gamma Spectrometer and Detector

The gamma response of CdTe surface barrier detectors fabricated from semi-insulating material is analyzed. Small devices (10 mm3 to 15 mm3) from vapor grown crystals are capable at room temperature of resolving photopeaks at 90% charge collection efficiency for gamma emitters ranging in energy from 122 keV for 57Co to 1.33 MeV for 60Co. A resolution of 16% (FWHM) has been obtained with 137Cs (662 keV). Incomplete charge collection due to heavy trapping degrades resolution and limits the size of present devices. Larger devices (≈l cm3) prepared from melt crystals, although quite efficient to gammas (80% to 60Co), cannot resolve photopeaks. Their performance for straight counting applications is limited by a temperature dependent count rate, polarization at low temperatures and thermal noise at high temperatures. Detrapping of electrons and holes is necessary to prevent polarization at low temperatures and may be useful in extending the high temperature limit.