Low-background counting using Ge(Li) detectors with anti-muon-shields

Low-intensity photon counting experiments carried out in the energy range between 8 and 20 MeV using Ge(Li) detectors revealed a strong background due to cosmic muons. By using a lead shield to suppress photons and a five-sided 5 cm thick plastic shield to suppress muons, a background reduction of 99.5% was achieved. GEANT computer simulations are carried out to study properties of cosmic muon spectra in different Ge and Si detectors. A comparison of measured and simulated spectra leads to very good agreement. Background reduction by an annular NaI detector is studied in the anticoincidence and pair-spectrometer modes.