European Network of Underground Laboratories

Measurements of very low count rates from radioactive decays are necessary for basic and applied physics, technology and life sciences. They require an almost "radiation free" environment. At ground level these measurements are often hampered by the background of cosmic radiation. Best possible suppression of cosmic radiation, especially of muons, can only be achieved in experiments performed far below ground level. For this purpose, PTB operates an underground laboratory at the Asse salt mine near Braunschweig. Located at 925 m under the surface in a pure rock salt surrounding there are very low specific activities of primordial radio-nuclides such as 40K present in the lab. Hence, the ambient dose rate at this facility is only about 1 % of that at ground level, the muon flux density is reduced by even more than five orders of magnitude and negligible for most applications. PTBÕs laboratory has successfully participated in several international comparative measurements of sensitive detector systems. It is equipped with a facility for calibrating dose and dose-rate meters at low dose rates and with shielded low-background germanium detector systems dedicated to measure low radioactivities by gamma-ray spectrometry.

PTB and six other institutes, IRMM (Belgium), IAEA (Austria), LN Gran Sasso (Italy), University of Iceland, LSCE (France), MPI Heidelberg and VKTA Rossendorf (Germany) have recently founded a Collaboration of European Underground Laboratories ("CELLAR"). This collaboration aims to stimulate investigations in applied physics, to co-ordinate measurement programs and comparative measurements as well as to provide easier access to underground facilities for science and industry.