MONTE CARLO SIMULATION OF P-TYPE HPGe DETECTORS – THE DEAD LAYER PROBLEM

The Monte Carlo method is increasingly applied for simulation of germanium detectors. It can be successfully used for the evaluation of full energy peak efficiency or of the efficiency corrections due to self-attenuation, coincidence summing and geometry effects. Usually the simulation task is to evaluate the energy deposited in the sensitive volume of the detector, assuming that the detector signal is strictly proportional with this quantity. In this work it is shown that simulations of the energy deposition in a p-type HPGe detector for low energy photons, carried out with GEANT3 and PENELOPE, do not describe correctly experimental data that depend on the interactions taking place in the dead layer. Specifically, the effects of coincidence summing with low energy photons on peak shape and on coincidence summing correction factors, observed in the experimental spectra, are not reproduced by simulations. The reason is that the charge produced in specific regions of the dead layer is partly, but not completely, collected, and this fact is not taken into account in the simulations. A simplified procedure to avoid this problem is described, but the general solution requires the incorporation of the charge collection process into the simulation codes.