The use of modeling programs to predict the response of HPGe detectors is increasing in importance due to the extensive laboratory work, both in term of source preparations and measuring time. MCNP code is a powerful and useful tool for the simulation of Ge-detector efficiency calibration. The experimental efficiency data and MCNP calculations based only on the known physical measurements of the HPGe crystal do not agree well in some detectors. Detector construction materials and surface dead layers must be well specified. The dead layer of Ge detector is one of the most important factors that affect the calculations. In addition, and if provided by the manufacturer, the dead layer may changes with time. Consequently, it is necessary to optimize the thickness of the detector's dead layer in order to obtain more accurate results for the efficiency of the detector using Monte Carlo calculations. Our approach consists of employing hybrid UCODE-MCNP codes to optimize the dead layer of the Ge-crystal aiming at decreasing discrepancies between experimental and simulated data of the Ge detector efficiency. UCODE has two attributes that are not jointly available in other inverse models: (1) the ability to work with any mathematically based model or pre- or post processor with ASCII or text only input and output files, and (2) the inclusion of more informative statistics.
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