Monte Carlo calculation of the efficiency calibration curve and coincidence-summing corrections in low-level gamma-ray spectrometry using well-type HPGe detectors

Well-type high-purity germanium (HPGe) detectors are well suited to the ana lysis of small amounts of environmental samples, as they can combine both l ow background and high detection efficiency. A low-background well-type det ector is installed in the Modane underground Laboratory. In the well geomet ry, coincidence-summing effects are high and make the construction of the f ull energy peak efficiency curve a difficult task with an usual calibration standard, especially in the high energy range. Using the GEANT code and ta king into account a detailed description of the detector and the source, ef ficiency curves have been modelled for several filling heights of the vial. With a special routine taking into account the decay schemes of the radion uclides, corrections for coincidence-summing effects that occur when measur ing samples containing U-238, Th-232 or Cs-134 have been computed. The resu lts are found to be in good agreement with the experimental data. It is sho wn that triple coincidences effect on counting losses accounts for 7-15% of pair coincidences effect in the case of 604 and 796 keV lines of Cs-134. ( C) 2000 Elsevier Science Ltd. All rights reserved.