Romanian Reports in Physics, Vol. 61, No. 2, P. 203–213, 2009 NUCLEAR PHYSICS A METHOD OF EFFICIENCY CALIBRATION FOR DISK SOURCES IN GAMMA-RAY SPECTROMETRY DANIELA RADU 1 , DORU STANGA 1 , OCTAVIAN SIMA 2 1 “Horia Hulubei” National Institute of Physics and Nuclear Engineering, Bucharest, Romania radu.daniela@yahoo.co.uk 2 University of Bucharest, Department of Atomic and Nuclear Physics, Bucharest, Romania osima@olimp.fiz.infim.ro (Received June 18, 2008) Abstract. The quality of the results of gamma spectrometry measurement depends directly on the accuracy of the detection efficiency in the specific measurement conditions. Experimental efficiency calibration is restricted to several measurement geometries and can not be applied directly to all measurement configurations. In this work an approach using efficiencies measured with point sources combined with theoretical procedures is applied for obtaining the peak efficiency ( ) E ε for disk sources measured with a NaI(Tl) detector. Coincidence summing effects are evaluated using GESPECOR and are used to correct the experimental values of the efficiencies. The resulting efficiencies were compared with predictions of the ETNA (Efficiency Transfer for Nuclide Activity measurements) software. The efficiency curves obtained in this way will be applied to the measurement of samples from the decommissioning of the VVR-S reactor. Key words: gamma-ray spectrometry, detector characterization, efficiency calibration. 1. INTRODUCTION An important step in the implementation of the decommissioning plan of the VVR-S nuclear reactor belonging to the National Institute for Physics and Nuclear Engineering “Horia-Hulubei” (NIPNE H-H), Bucharest, Romania, is represented by the radiological characterization of the reactor. The Radiological Characterization Laboratory from NIPNE H-H has the principal task to achieve the radiological characterization in different phases of the decommissioning process. This task is very important because it provides the basis for the correct classification of various types of wastes, which in turn affects the decommissioning solution and the associated costs. The measurement method