Re-construction of a HPGe detector precise modeling for efficiency calibration

High purity germanium (HPGe) detector is a preferred choice for determining the activity of the radioactive samples for nuclear diagnostics of Inertial Confinement Fusion (ICF) experiments. Although the amounts of the radiochemical sample are limited, activity measurement at a close distance between the detector window and the radioactive source is a feasible method. Efficiency calibration of gamma rays at a close distance from the surface of a HPGe detector is crucial. Considering the problem of the coincidence at a close distance, the alternative way is to construct a precise model of the HPGe detector and then to calculate efficiency accurately at an arbitrary distance from the surface of the HPGe detector using Monte Carlo method. In this paper, internal geometry and structure except for dead layers of the HPGe detector is obtained by X-ray radiography and 3D reconstruction. The optimal dead layers of the germanium crystal are determined by tracing the minimal sum squared residual (SSR) of gamma-ray efficiencies between calculations and measurements for standard planar sources. Nonhomogeneous distribution of the dead layer is supposed according to the inner structure on the Ge crystal. The final results show that the corrected model improves the accuracy of the calculated efficiencies.