Low level gamma-ray spectrometry at the PTB underground laboratory UDO

A new ultra-low background (ULB) gamma-ray spectrometry system has been installed 925 m below ground at the underground laboratory 'UDO' of the Physikalisch-Technische Bundesanstalt, Braunschweig. This new system, consisting of a 95% coaxial HPGe-detector and a passive shielding (20 cm of low-activity lead, 10 cm electrolytic copper and radon suppression by N2-flushing), will be described. Most of the peak count rates are less than 1 count per day and the integral background count rate in the energy range from 40 keV to 2750 keV is about 20 h-1. This system was especially used to determine very small activity concentrations of 60Co in environmental samples from Hiroshima and Tokai-mura and very small concentrations of long-lived radionuclides in meteorites. Results from these applications will be reported. (author)