A LOW-ENERGY HPGE DETECTOR DEDICATED TO RADIOACTIVITY MEASUREMENTS FAR BELOW ENVIRONMENTAL LEVELS

Successive background reduction factors of a low-background high-purit y semi-planar Germanium detection system, N type, were achieved with v arious shielding conditions, at ground level and underground locations . The optimal working conditions, achieved at a depth of 500 m water e quivalent (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 measureme nt at sea level. The integral count rate as low as (0.00361 +/- 0.0000 4) s(-1) over this energy interval is mainly attributable to the influ ence of the cosmic rays but also to the residuals of the natural decay chains inside the various constituents of the detection system, the r adon and progenies from the environment, the weak level of man-made ra dionuclides and finally the cosmogenic activation;of the shielding mat erials formerly stored at sea level. About one event per day, or less, corresponding to the the gamma-rays of interest, are recorded in ener gy intervals of four times the FWHM resolution of the detector. Detect ion limits were deduced for Pb-210 and uranium metal activities and fo r a few radionuclides of interest. Materials studied are: clay standar d, uranium metal, biological matrix exposed to radon flux; gold and si lver foils, various lead and copper batches, as a contribution to mate rial selection with respect to their low level of radiocontaminants, i n particular for the solar neutrino and dark matter experiment BOREXIN O. Copyright (C) 1996 Elsevier Science Ltd