The PTB underground laboratory for dosimetry and spectrometry

In 1991, the Physikalisch-Technische Bundesanstalt established an underground laboratory for dosimetry and spectrometry (UDO) at the Asse salt mine, near Braunschweig. Due to the depth of 925 m below ground (equivalent to about 2100 m of water), the cosmic ray muon intensity in this facility is reduced by more than 5 orders of magnitude. In addition, the low specific activity of the pure rock salt and a low concentration of radon lead to an extremely low ambient dose equivalent rate of less than 1 nSv/h. The UDO facility is therefore well suited for dosimetry at very low dose rates, as well as for Ultra-Low-Background (ULB) gamma-ray spectrometry. In 1998, a coaxial low-background HPGe-detector (88% relative efficiency, FWHM 2.0 keV at 1.33 MeV) with an extended shielding (20 cm low-activity lead, 1 cm electrolytic copper, N2-flushing) was installed at UDO; the count rate per mass of germanium, integrated over the energy range from 40 to 2750 keV, was measured to be 0.012 s(-1) kg(-1). Results from test measurements and first applications are reported. The design of a ULB gamma-detector system, presently under construction, is described.