Nuclear Instruments and Methods in Physics Research A 592 (2008) 80–87 Calibration of closed-end HPGe detectors using bar (Parallelepiped) sources Sherif S. Nafee, Mahmoud I. Abbas à Faculty of Science, Department of Physics, Alexandria University, 21121 Mohrem Bek, Alexandria 21121, Egypt Received 20 November 2007; received in revised form 28 February 2008; accepted 20 March 2008 Available online 29 March 2008 Abstract The National Institute of Standards and Technology (NIST, Gaithersburg, MD 20878, USA) uses different extended sources, such as bar (parallelepiped) sources of high activity to calibrate the hyper pure germanium (HPGe) cylindrical detectors. The absolute full-energy peak efficiency and the source self-absorption have been investigated in this work by the use of compact analytical expressions for closed- end HPGe detectors and bar sources. The validity of these expressions is tested through extensive comparisons with the experimental values carried out in the NIST. The comparisons indicate that the direct mathematical method used in the present work is useful in the efficiency calibration of HPGe detectors with bar sources. r 2008 Elsevier B.V. All rights reserved. Keywords: HPGe detector; Bar sources; Full-energy peak efficiency; Direct mathematical method 1. Introduction The use of extended sources in gamma-ray spectrometry improves the sensitivity of detection, thus enabling the measurement of low-activity samples. To obtain reliable measurements of radionuclide activity, the knowledge of the detector absolute photopeak (full-energy peak) effi- ciency is required [1]. There is no universal method for the efficiency calibration of HPGe detectors; this is mainly because of two major factors, the extended dimensions and the self-absorption of the source [2]. The most accurate method to determine the efficiency for the germanium crystal g-ray spectrometer is the experimental method, where there is no need to make approximations. However, it is often require extensive and delicate laboratory work, both in terms of source preparation and measuring time [3,4]. A complete evaluation of the photopeak efficiency of germanium detectors using the Monte Carlo simulations has been evolving since the 1970s. Different approaches and codes can be found in literatures [5–14], where various source-detector geometries and photon energy rates have been treated. However, discrepancies commonly existing between experimental and simulated data of the efficiency calibration of germanium detectors have been attributed to the uncertain knowledge of the geometric characteristics of the detectors, the approximations or simplifications used in the codes to simulate the physical process of energy dissipation in the detector crystal and photon cross-section data [15]. In the present work, we describe the experimental method employed to calibrate the National Institute of Standards and Technology (NIST) HPGe detectors used to determine the source activity of bar sources consisting of several 232 Th layers. Experimental values are compared with those calculated by a new simplified theoretical approach based on the Selim and Abbas direct mathema- tical method that applied successfully before for obtaining the efficiencies of source–detector systems with different geometries (point source [16,17], disk source [18–20], cylindrical source [21], Marinelli beaker [22], parallelepiped detector [23,24] and NIST gas sphere source [25]). 2. Experimental setup The absolute full-energy peak efficiency values are carried out for two n-type GMX Ortec HPGe detectors ARTICLE IN PRESS www.elsevier.com/locate/nima 0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.03.106 à Corresponding author. Tel.: +20 127431429. E-mail address: mabbas@physicist.net (M.I. Abbas).