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A method is presented whereby the intrinsic efficiency of Ge is utilized to calculate the expected peak efficiency of detectors having a wide range of sizes. The intrinsic efficiency of Ge, which is the probability for total absorption, was measured at 122 and 136 keV in Ge(Li) coaxial detectors and HPGe planar detectors having an effective thickness ranging from 5 to 50 mm. At 136 keV it is 64% for a thickness of 10 mm and 82% for 20 mm, after which it levels off reaching 89% at 50 mm. It is shown that the peak efficiency of a detector is a product of only the intrinsic efficiency and the solid angle, once losses due to edge escape and detector imperfections (surface channels and high dislocation densities) are determined. The absolute and relative [to NaI(T1)] peak efficiency of a sample detector, calculated on the basis of intrinsic efficiency, are in good agreement with measured values. This method should find applications in the design of new detector systems particularly those for diagnostic imaging with 99Tc (140 keV).