A low-energy HPGE detector dedicated to radioactivity measurements far below environmental levels

Successive background reduction factors of a low-background high-purity semi-planar Germanium detection system, N type, were achieved with various shielding conditions, at ground level and underground locations. The optimal working conditions, achieved at a depth of 500 m water equivalent (m w.e.) lead to a background reduction factor of about 100 in the energy region from 5 to 1000 keV in comparison with a measurement at sea level. The integral count rate as low as (0.00361 ± 0.00004) s−1 over this energy interval is mainly attributable to the influence of the cosmic rays but also to the residuals of the natural decay chains inside the various constituents of the detection system, the radon and progenies from the environment, the weak level of man-made radionuclides and finally the cosmogenic activation of the shielding materials formerly stored at sea level. About one event per day, or less, corresponding to the the γ-rays of interest, are recorded in energy intervals of four times the FWHM resolution of the detector. Detection limits were deduced for 210Pb and uranium metal activities and for a few radionuclides of interest. Materials studied are; clay standard, uranium metal, biological matrix exposed to radon flux; gold and silver foils, various lead and copper batches, as a contribution to material selection with respect to their low level of radiocontaminants, in particular for the solar neutrino and dark matter experiment BOREXINO.