Monte-Carlo Analysis of the Ge(Li) Detector Used in the Sum-Coincidence Mode

A Monte-Carlo model for investigating sequential gamma scattering and for predicting sum-coincidence and sum-only detector performance has been developed in order to assess the Compton suppression capabilities of Ge(Li) detectors which accept only multiple gamma interaction sequences. Calculated performance for a concentric cylindrical detector shows good agreement with experimentally observed sum-coincidence spectrometer (SCS) behavior. Calculations were made for hypothetical germanium detectors of cylindrical volumes from 7. 1 cc. to 28.3 cc., and for gamma energies of 320 keV to 2754 keV. The requirement of multiple scattering is shown to eliminate only a minor portion of the total-energy gamma events. Effects of detector volume are characterized, and the location and thickness of dead layers separating active detector regions are shown to be important to SCS performance. Calculations of the energy partitioning between detector volumes for coincidence events indicate that the "backscattering" mode is not the dominent mechanism for good event sequences as has been postulated, and that good and bad event sequences are highly localized on the two-parameter spectrum of the energy output from each detector for coincidence events. The relationship of the calculations to detector performance is given.