IRMM operates a laboratory for ultra-sensitive radioactivity measurements inside the 225 m deep underground laboratory HADES, which is operated by EURIDICE (www.euridice.be) and located at the premises of the Belgian Nuclear Research Centre (www.sckcen.be). In HADES, the muon flux (cosmic rays) is a factor of 5000 lower compared to above ground and the flux of protons, neutrons and electrons is reduced to an insignificant level. This reduction of the cosmic ray flux makes the background in gamma-ray spectrometry measurements significantly lower compared to above ground. Therefore it is possible to detect very low amounts of radioactivity, we are talking about levels of some hundred mBq (micro-Becquerel, i.e. a few nuclear disintegrations per day). Twelve specially designed low-background gamma-ray detectors are used for the measurements. The measurements support projects in a wide range of fields, and over 200 projects have been carried out the past 15 years.
Some examples of measurement project are:
Some technical considerations
Measuring underground is not trivial. It is not simply to take an above ground detector and place it underground. In such case one measures only radioactivity from the detector or the shield. The lead shield need to be as old as possible to minimise the influence of 210Pb which has a half life of 22 years. In HADES we use lead from Versailles and Hampton Court (300-500 years old) as well as some 800 years old roof tiles from Het Pand in Gent. The copper in the shield need to be as new as possible. It is transported from the Cu-factory with a truck immediately as it comes out from the factory and taken to an underground storage. This is to avoid activation by the cosmic radiation.