Monte Carlo calculation of the efficiency response of a low-background well-type HPGe detector

Well-type HPGe detectors are well suited for the analysis of small amounts of environmental samples, as they can combine both low background and high detection efficiency. A low-background well-type detector is installed in the Modane underground Laboratory. In the well geometry, coincidence-summing effects are high and make construction of the full energy peak efficiency curve difficult with the usual calibration standard, especially in the high energy range. Using the GEANT code, taking into account a detailed description of the detector and the source, efficiency curves have been modelled for several filling heights of the vial. With a special routine, taking into account the decay schemes of the radionuclides, corrections have been computed for coincidence-summing effects that occur when measuring samples containing 238U, 232Th or 134Cs. The results are found to be in good agreement with experimental data. It is shown that triple coincidences effect on counting losses accounts for 7–15% of pair coincidences effect in the case of the 604 and 796 keV lines of 134Cs.