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 U-238, Th-232 or Cs-134. 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 Cs-134. (C) 2002 Elsevier Science B.V. All rights reserved.