THE CONTRIBUTION OF COSMIC-RAY MUONS TO THE BACKGROUND SPECTRUM OF GAMMA-RAY SPECTROMETERS

The experimental setup of a veto shield offers not only the possibility to exclude cosmic ray induced background events but also the chance to separate these background contributions by operating in coincidence mode. In this article the most important muonic component is investigated experimentally and by computer simulations. Cosmic muon energy loss spectra were measured with a Ge- and a NaI(Tl)-detector under various shielding and coincidence conditions in order to understand the influence of primary and secondary effects. In the case of the NaI(Tl)-detector, computer simulations were done to identify the contribution of direct muonic events. The results show good agreement with the measured spectra with respect to the so-called muon peak. The deviations at lower energies are explained by the effect of secondary electron-photon showers being mainly responsible for the cosmic ray induced background continuum below a few MeV, which restricts the detection limit of only passively shielded detectors working above ground.