The PTB underground laboratory for dosimetry and spectrometry

In 1991, the Physikalisch-Technische Bundesanstalt established an undergrou nd laboratory for dosimetry and spectrometry (UDO) at the Asse salt mine, n ear Braunschweig. Due to the depth of 925 m below ground (equivalent to abo ut 2100 m of water), the cosmic ray muon intensity in this facility is redu ced by more than 5 orders of magnitude. In addition, the low specific activ ity of the pure rock salt and a low concentration of radon lead to an extre mely low ambient dose equivalent rate of less than 1 nSv/h. The UDO facilit y 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 electrolyti c copper, N-2-flushing) was installed at UDO; the count rate per mass of ge rmanium, 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 applica tions are reported. The design of a ULB gamma-detector system, presently un der construction, is described. (C) 2000 Elsevier Science Ltd. All rights r eserved.