Study on Variation of HPGE Detector Dead Layer Thickness and its Effect on the Measurements of the Detector Response and Samples Characterization Using Monte Carlo Simulation

The main objective of this work is to produce an optimal modeling for our aged Planar-HPGe detector using Monte Carlo method (MC). That optimization included the analysis of the germanium dead (inactive) layer thickness for our old detection system (planar-HPGe detector). DL is one of the important parameters needed in order to obtain the smallest discrepancy between simulated and experimental measurements of detector efficiency. Also, precise determination of 235U enrichment for UO2 samples which is necessary for purposes of nuclear materials verification in the field of nuclear safeguards. The thickness of Germanium dead layer (DL) can be vary by time as it is not well known due to the existence of a transition zone where photons are strongly attenuated and absorbed, that cannot contribute to the total photon energy absorption which causes a significant decrease in efficiency. Therefore, using data provided by manufacturers since long years (manufacture date) in the detector simulation model is not convenient. As a result, some strong discrepancies appear between measured and simulated efficiency, in addition to that non-accurate results for 235U enrichment determination. The Monte Carlo method applied to overcome this difficulty was to vary the thickness of dead layer step by step in simulation, a good agreement (minimum deviation) between estimated and experimental efficiency was reached when a suitable germanium dead layer thickness was chosen. Calculations and measurements were performed for radioactive nuclear material samples in the form of UO2 powder with different sizes and enrichments at different locations, under different gamma-lines emitted after a-decay of the 235U nuclei. Results indicated that a good agreement between simulated and measured efficiencies is obtained using a value for the germanium dead layer thickness approximately (2.45 mm) six in comparison with (0.389 mm) provided by the detector manufacturer.

Stochastic simulation study of HPGe detector response and the effect of detector aging using Geant4

In this study the effect of detector aging in terms of increased dead layer thickness on detector efficiency has been studied using the Geant4 toolkit. Variation of energy deposition in the detector dead layer with the dead layer thickness has been quantified for various values of incident g-ray energy considering point isotropic as well as extended sources including the circular disk source and cylindrical volume sources. For the point isotropic source, the Geant4 computed values of energy loss per particle in the dead layer are found in good agreement with the corresponding published results with maximum deviation remaining below 2 %. New results for dependence of geometric, full-energy peak and total efficiency on dead layer thickness have been studied using Geant4 simulations for various values of g-ray energy, and for point isotropic and extended sources at various axial and lateral positions. These simulations yield an exponentially decreasing profile of detector aging sensitivity with an increase in g-ray energy for point isotropic, circular disk and cylindrical volume sources highlighting a larger decrease in efficiency due to aging for low energy photons.