MCNPX simulation of influence of cosmic rays on low-activity spectrometric measurements

Germanium gamma spectrometers are effective instruments for low-activity measurement of a mixture of radionuclides in environmental samples, food samples, in materials released from nuclear facilities to the environment, etc. In such measurements cosmic rays have a significant contribution to the background signal. A Monte Carlo code MCNPXTM was used to calculate coaxial high-purity germanium (HPGe) detector pulse-height spectra caused by cosmic rays penetrating through shielding made of concrete and lead. Simulations were compared to two different measurements, one performed inside a 10 cm thick lead shielding and another done inside a larger chamber made of low-activity concrete and with several ceiling thicknesses. In the first experiment, a discrepancy was found between simulated and measured spectra by up to the factor of 4 at 2.62 MeV and slowly decreasing to unity at 13 MeV. It is assumed that the discrepancy between the measured and simulated spectra is caused by the simplification of muon energy losses treatment resulting in the underestimation of count rate in simulated pulse-height spectrum. Good agreement was obtained between simulation and measurement of differences of detector count rates in 662 keV and 1332 keV energy windows inside a concrete chamber with varying ceiling thickness. It is assumed that due to lower effective Z of concrete, delta electron bremsstrahlung has lower yield and the muon radiation energy losses start to be important at higher energies than in lead. As a result, the total contribution of these effects to the outputs of MCNPXTM simulations of concrete chamber is not dominant in the investigated energy windows and the simulation results are in a close agreement with the measurement.

► Detector pulse-height spectrum of cosmic rays simulated by MCNPX can be incorrect.
► MCNPX muon energy losses treatment may be simplified.
► CRY and EXPACS data results in similar simulated pulse-height detector spectra.