Noise, Trapping and Energy Resolution in Semiconductor Gamma-Ray Spectrometers

We consider in this paper the relative importance of noise, trapping and recombination of carriers in determining the overall energy resolution of semiconductor gamma-ray spectrometers. In particular we present a detailed analysis of the effects of trapping. Our model derives from an earlier study by Northrop and Simpson1, but we have been able to eliminate some approximations made in their analysis which are quite important under practical conditions. Relationships are derived between the total signal charge, its mean square deviation, and the drift lengths of electrons and holes in a planar detector. The results show that very stringent conditions are necessary in order to achieve the energy resolution which has been reported in some instances. It is shown that electron and hole drift lengths should be as closely equal as possible in a good detector. A uniform geometry, either wholly planar or wholly coaxial, is shown to be desirable.