- The dependence of the HPGe detector efficiency when collecting full-energy peaks vs. the distance of the point source from the detector cap was studied by Monte-Carlo methods for 49 cylindrical detectors, having different radii and thicknesses. It was shown that in all cases, interpolation and extrapolation can be done using the model of the virtual point-detector. The dependence of the distance of the virtual point-detector position h 0 from the detector cap was studied as a function of the geometrical dimension of the detector. Assigning r and h to the radius and the thickness of the detector, a , b , c to parameters depending on the photon energy and μ / ρ to the mass attenuation coefficient, a general formula was found: h 0 = ar ( 1 - b exp [ - hc μ / ρ ] ) . The parameter c was found to change very little with the photon energy (±1%), while a and b depend more strongly on the photon energy (±5%).