Development of Background Model for a Shielded HPGe Detector

The development of new gamma-ray detector concepts to optimize sensitivity for radionuclide monitoring requires a computationally efficient simulation framework capable of guiding future detector designs, particularly for coincidence systems. In this work, a background source model is developed to simulate the response of a shielded detector under realistic laboratory conditions. The model is based on background spectra measured with a single-crystal HPGe detector in the 30–2930 keV energy range, corresponding to unexposed and exposed particulate filters, as well as the detector environment. The spectra were analyzed to extract the background components associated with air exposure, filter material, and the detector environment. Corresponding Geant4-based models were developed for each component. The air-exposure and filter-material backgrounds arise from a limited number of dominant radionuclides located within the filter specimen volume and are reproduced by simulating selected radionuclides and decay chains, with their activities determined quantitatively. The detector-environment spectrum is dominated by a continuum background and is modeled using a phenomenological source that generates a spatially uniform and directionally isotropic radiation field. The source energy distribution is modeled using an analytical form with a few fitting parameters, consisting of an exponential continuum component combined with a set of discrete gamma-ray lines. The individual components were combined to reconstruct the measured spectra, showing good agreement with the measurements. The source was reconstructed reliably up to an energy of 3.5 MeV, which is essential for simulating coincidence background contributions of interest. Overall, this work provides a simple and computationally efficient background model for shielded gamma-ray detectors used in radionuclide monitoring systems.