Unfolding the response of a Ge detector used for in-situ gamma-ray spectrometry

In environmental radiation protection portable Ge detectors are used for in-situ gamma-ray spectrometry. In order to determine the complete photon fluence rate spectra including the continuum component due to photons scattered in the environment from measured pulse height distributions one needs to apply unfolding methods. A prerequisite of the unfolding is the knowledge of the response functions of the detector to monoenergetic photons. s work it is investigated whether response functions for modern portable high purity Ge detectors can be calculated by Monte Carlo simulation of photon histories with sufficient accuracy for a reliable determination of the incident photon fluence spectrum. The codes MCNP and EGS4 were used for calculating detector response functions for photon energies ranging from 20 keV to 2.6 MeV. It is demonstrated that a detector response to photons from a 152Eu source can be unfolded with good precision. A comparison of measured and calculated responses shows that for applications in in-situ spectrometry response functions calculated by Monte Carlo simulations are more appropriate for unfolding than those determined experimentally since there is no need to separate the contribution to the response of photons scattered in the laboratory. The dependence of the response functions on the angular distribution of the incident photons was investigated by calculations for various irradiation geometries. Systematic errors in the photon fluence spectra resulting from applying for the unfolding response functions which do not correspond to the real irradiation geometry are estimated.