Nuclear Science and Techniques 19 (2008) 358–364 ———————————— * Corresponding author. E-mail address: askaribakir@ureach.com Received date: 2008-09-17 NUCLEAR SCIENCE AND TECHNIQUES 1 Unité de Physique Nucléaire et des Hautes Énergies, Faculté des Sciences de Tunis, 2092 Tunis, Tunisia 2 Centre National de Sciences et de Technologies Nucl´eaires, Sidi Thabet, Tunisia Abstract A matrix stripping method for the conversion of in-situ gamma ray spectrum, obtained with portable Ge detector, to photon flux energy distribution is proposed. The detector response is fully described by its stripping matrix and full absorption efficiency curve. A charge collection efficiency function is introduced in the simulation to take into account the existence of a transition zone of increasing charge collection after the inactive Ge layer. Good agreement is obtained between simulated and experimental full absorption efficiencies. The characteristic stripping matrix is determined by Monte Carlo simulation for different incident photon energies using the Geant4 toolkit system. The photon flux energy distribution is deduced by stripping the measured spectrum of the partial absorption and cosmic ray events and then applying the full absorption efficiency curve. The stripping method is applied to a measured in-situ spectrum. The value of the absorbed dose rate in air deduced from the corresponding flux energy distribution agrees well with the value measured directly in-situ. Key words Monte Carlo simulation, Ge detector, Gamma radiation, Geant4, Stripping CLC numbers TL817, TL814, O242.2 1 Introduction In-situ gamma spectrometry is a powerful tool to study indoor and outdoor gamma dose rates [1-3] . An important tool in environmental applications is the portable germanium detector used for in-situ measure- ments to collect information from an enormous sample. But generally, the source geometries are of inhomo- geneities and complex configurations. Experimental approaches and theoretical calculations are employed to obtain adequate calibration efficiency curves which permit expression of the measurement results in the quantity of interest [3,4] . Methods based on Monte Carlo simulation have been widely used in radiation field calculation [5-7] and for full-energy peak efficiency calibration of germanium detector [8-10] in particular. For these methods, the main component of uncertainty in the detector response is incomplete knowledge of the dimensions of the detector parts. In particular, the calculated efficiency is very sensitive to thickness of the germanium dead layer [11, 12] . When the thickness provided by detector manufacturers is used in a simulation, strong discrepancies arise between calculated and measured efficiencies. The thickness of this inactive layer is not well known due to the existence of a transition zone where photons are increasingly absorbed. In previous works, sensitivity analysis on effective thickness of the inactive Ge layer was performed by varying this thickness in the detector model to obtain the highest accordance between experimental and simulated efficiencies [13] . A model has been proposed by Clouvas et al. [11] for Method for converting in-situ gamma ray spectra of a portable Ge detector to an incident photon flux energy distribution based on Monte Carlo simulation Boubaker Askri 1, ∗ Adel Trabelsi 1, 2 Brahim Baccari 1