Applied Radiation and Isotopes 61 (2004) 117–121 Extension of the efficiency calibration of germanium detectors using the GESPECOR software Dirk Arnold a, *, Octavian Sima b a Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany b Physics Department, Bucharest University, RO-76900 Bucharest-Magurele, Romania Abstract To avoid losses due to coincidence summing, the efficiency calibration of gamma-spectrometers is often carried out by means of radionuclides emitting only one photon during a single decay. Calibration with this kind of radionuclides has, however the disadvantage that the energy range of the gamma rays (46 keVoEo1115 keV) is limited. Coincidence- summing corrections can be calculated with the Monte Carlo program Germanium Spectrometry Correction for multi gamma radionuclides which emit photons with energies above this range. The efficiency curve as a function of energy can therefore be extended up to 2.7 MeV with uncertainties below 4%. r 2004 Elsevier Ltd. All rights reserved. Keywords: Gamma-ray spectrometry; Ge detectors; Coincidence-summing effects; Efficiency calibration 1. Introduction In case gamma-ray spectrometry is used to measure sources at a distance close to the detector endcap (which is the common geometry for the measurement of environmental samples), nuclide specific coincidence- summing effects have to be taken into account. This is valid not only for the measurement of common samples but also for the calibration of the detector system. To avoid the problem of coincidence summing for calibra- tion measurements, activity standards with single photon emitting radionuclides (e.g. 210 Pb, 241 Am, 57 Co, 7 Be, 137 Cs, 54 Mn, 65 Zn) are often used. The drawback is that the emission energies E of these radionuclides cover only the range 46keVoEo1115keV and that the uncertainties of the extrapolated efficiency curve outside this energy range are therefore much higher than within the range. With the Germanium Spectrometry Correc- tion (GESPECOR) software it is possible to calculate the coincidence-summing corrections for about 100 radionuclides (Sima and Arnold, 2000; Arnold and Sima, 2001). For the work presented here we use GESPECOR to calculate the corrections for activity standards with multi photon emitting radionuclides which have higher energies than mentioned above (e.g. 60 Co, 88 Y) and to extend the efficiency curve to energies up to 2.7MeV. This extension is helpful for the determination of the activity of all the radionuclides which emit gamma rays with energies higher than 1.2MeV. 2. Experimental method Calculations and measurements were performed for five different geometries (point sources on the detector endcap, boxes with a sample volume of 25, 50, 100 ml, and 1 l Marinelli beaker) and a p-type coaxial HPGe detector with 50% relative efficiency, shielded with electrolytic copper (thickness 7.5 mm) and low-activity lead (thickness 20cm). The measured spectra were analyzed with the INTERWINNER (version 4.1) computer program from Eurisys. For the point source measurements we used single nuclide activity standards (PTB 6–11, 2003) placed on a special holder providing a small spacing ARTICLE IN PRESS *Corresponding author. Tel.: +49-531-592-6120; fax: +49- 531-592-6109. E-mail address: dirk.arnold@ptb.de (D. Arnold). 0969-8043/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2004.03.031