MCNP modelling of scintillation-detector γ-ray spectra from natural radionuclides

γ-ray spectra of natural radionuclides are simulated for a BGO detector in a borehole geometry using the Monte Carlo code MCNP. All γ-ray emissions of the decay of 40K and the series of 232Th and 238U are used to describe the source. A procedure is proposed which excludes the time-consuming electron tracking in less relevant areas of the geometry. The simulated γ-ray spectra are benchmarked against laboratory data.