Neutron induced activity in natural and enriched 70Ge detectors

The results of irradiations of isotopically enriched and natural Ge detectors with a neutron beam are presented. The analysis of the gamma-ray lines generated by decay of neutron induced unstable nuclei have provided us with a direct measurement of relevant interaction cross sections. Within a factor of 2, measurements and predictions are in good agreement. These results have important implications for the instrumental background in astrophysical gamma-ray spectrometers using germanium detectors, We confirm the reduction of the beta-background component, which dominates the continuum background in the 0.1-1 MeV energy range, using Ge-70 enriched detectors. We clearly identify beta(+) decays inside the detector as a significant source of continuum background in the 1-4 MeV energy range, This component is about 2 times more intense in Ge-70 enriched detectors than in natural ones. This is mainly due to the enhanced yield of Ge-69 and Ga-68 isotopes. The choice of either natural or Ge-70 enriched as optimum detector material depends on the energies of astrophysical interest. Detectors made of enriched Ge-70 are more appropriate for studies at energies below 1 MeV. For higher energies natural germanium is slightly better. The possibility of rejecting most of beta-background component by applying alternative analysis techniques makes natural Ge an appropriate material for future gamma-ray spectrometers.