Nuclear Inst. and Methods in Physics Research, A 908 (2018) 206–214 Contents lists available at ScienceDirect Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima Modeling of a HPGe well detector using PENELOPE for the calculation of full energy peak efficiencies for environmental samples J. G.Guerra a,b,∗ , J. G.Rubiano a,b , G. Winter c , A. G.Guerra a,b , H. Alonso a,b , M.A. Arnedo a,b , A. Tejera a,b , P. Martel a,b , J.P. Bolivar d a Departamento de Física, Universidad de Las Palmas de Gran Canaria, 35001 Las Palmas de Gran Canaria, Spain b Instituto Universitario de Estudios Ambientales y Recursos naturales, Universidad de Las Palmas de Gran Canaria, 35001 Las Palmas de Gran Canaria, Spain c Instituto Universitario de Sistemas Inteligentes y Aplicaciones Numéricas en la Ingeniería, Universidad de Las Palmas de Gran Canaria, 35001 Las Palmas de Gran Canaria, Spain d Departamento de Física Aplicada, Universidad de Huelva, 21071 Huelva, Spain ARTICLE INFO Keywords: HPGe well detector Efficiency calibration Reference materials Monte Carlo simulation Gamma-ray spectrometry ABSTRACT When determining the activity concentration of radionuclides using gamma-ray spectrometry the Full Energy Peak Efficiency (FEPE) for the energies of interest must be known. Determination of the FEPE can be made by means of either experimental calibration or theoretical–mathematical methods, such as Monte Carlo simulations. Given the difficulties related to experimental calibration and improvements in the capabilities of modern computers, Monte Carlo simulation is an increasingly widely used alternative, but requires an accurate model of the detector. The purpose of this work is to generate and validate a computational model, based on Monte Carlo simulation, of an HPGe well detector that permits the performance of FEPE calculations with appropriate precision and accuracy for the measurement of environmental samples. To achieve this, an optimization methodology is applied to the model that minimizes the differences between a set of computational FEPEs and a set of experimental FEPEs used as a benchmark. The resulting optimized model is used to calculate computational FEPEs for 25 different samples with different reference materials and sample heights, which are measured by means of the well detector modeled here. To validate the optimized model, the abovementioned computational FEPEs are used during the calibration of the corresponding spectra, to enable the subsequent comparison of the results of the analyses with the expected values. The measured activities differ from the reference values by less than 10% in most cases and are compatible with these considering the uncertainties involved, thus confirming the validity of the model. 1. Introduction Gamma-ray spectrometry is a technique commonly used for the identification and quantification of a wide range of radionuclides by means of their direct gamma emissions or those of their progeny. HPGe detectors are currently the most widely used gamma-ray detectors due to their various advantages, of which high energy resolution is the most important; this permits high-precision during the identification of the peaks. For measurement of the radioactive content of environmental samples of low volume and activity in laboratory, HPGe well detectors are the best option due to their high detection efficiency, as the counting geometry approaches 4 [1]; this implies improvements in both the uncertainty of the results and the Minimum Detectable Activity. More specifically, the use of this type of detector is recommended when samples of only a few grams of organic matter are to be analyzed for multidisciplinary studies, including radiological techniques [2,3]. These ∗ Corresponding author at: Departamento de Física, Universidad de Las Palmas de Gran Canaria, 35001 Las Palmas de Gran Canaria, Spain. E-mail address: jglezg2002@gmail.com (J. G.Guerra). aspects motivated us to acquire and setup a spectrometry system of this type. When the activity concentration of radionuclides in a sample is intended to be quantified by gamma-ray spectrometry, the Full Energy Peak Efficiency (FEPE) for the energies of interest must be determined. FEPE determination can be made either by means of experimental cal- ibration, or by theoretical–mathematical methods such as Monte Carlo simulations. The experimental efficiency calibration [4–8] requires the preparation of standard sources using reference materials with known activities and with the same measuring geometry as the sample to be measured. Materials with very similar densities and chemical compo- sitions need to be used during the experimental calibration if self- attenuation corrections are to be avoided, especially at low energies [1]. Moreover, when environmental samples are studied, the preparation of standard sources that are very similar may be a process of considerable complexity, involving appreciable economic and time costs. https://doi.org/10.1016/j.nima.2018.08.048 Received 18 April 2018; Received in revised form 16 August 2018; Accepted 17 August 2018 Available online xxxx 0168-9002/© 2018 Elsevier B.V. All rights reserved.