The Dortmund Low Background Facility (DLB) is a non-underground low background germanium facility under heavy shielding. Low background germanium spectroscopy is a vital tool for radiation protection and study of rare nuclear physics processes. Germanium detectors are usually operated under either a fairly low overburden (O(1m) water equivalent (mwe)), e.g. in usual buildings, or a high (O(>100mwe)) overburden, e.g. in specialised underground laboratories. In between only few facilities exist although the weak component of cosmic rays can already be shielded with few meters water equivalent which reduces the cosmic background substantially.
The facility was set-up with an overburden of 10mwe with the advantage of good accessibility of an overground set-up on the university campus in combination with a cosmics veto and a state-of-the-art neutron shielding. The DLB was primarily built to support and speed up the material screening activities for the COBRA experiment, a highly granular 0νββ R&D experiment located at LNGS, Italy.
325t shielding elements made of barite concrete were available at TU Dortmund. The goal for the DLB was to rearrange these blocks so as to maximise the overburden of the germanium detector. Due to the lack of space onsite and the given shape of the shielding blocks the maximum height of shielding material would have been about 1.5m. To achieve an acceptable overburden above the detector, only materials like iron or steel were considered. Finally some blocks of cast iron with a total mass of 43t were kindly provided by DESY, Hamburg.
During the design phase for the overburdens geometry the total shielding of the germanium detector was calculated analytically to be able to
compare different set-ups. The angular distribution of the final set-up can be seen in the left figure. Only below an angle of 45º the overburden decreases below 8mwe. As the intensity of cosmics itself decreases by cos²θ, less shielding is necessary at these angles. The whole outer shielding was sealed. It is constantly kept under overpressure to prevent dust from entering and permanently flushed with fresh air to reduce the radon concentration in the air.
The next layer of the germanium facility is an active veto consisting of plastic scintillators equipped with photomultipliers. Particles which penetrate the outer shielding are detected by the veto and coincident events in the HPGe are rejected. The current stage of completion has the form of a tunnel.
The so called lead castle is a shielding against γ-rays and neutrons. It consists of five screening layers of different materials:
The outermost layers which shield against γ-rays consist of 100mm standard shielding lead followed by 30mm of certified lead with less than 39Bq/kg concerning 210 Pb content.
The third layer is made of 100mm borated polyethylene, which both moderates and captures neutrons produced by muons.
Boron is used because of its high apture cross section for fast and slow neutrons and PE due to its high amount of hydrogen
which has a good moderating capacity for neutrons.
The next layer consists of 20mm lead with less than 3Bq/kg to stop the γ-quanta which were produced in the outer layers.
The innermost layer is made up of 8mm pure electrolyte copper to stop X-ray radiation emitted upon trapping in the lead.
A special tube in the lead castle allows for flushing of the measurement chamber with evaporated nitrogen from the dewar toeliminate the radioactive 222Rn in the air. Another pipe allows electrical access to the measurement chamber.
The detector is a coaxial high purity germanium diode in an ultra low background version by Canberra. Therefore the pre-amplifier is installed outside the detector diode and materials used near the crystal are carefully selected. Its relative efficiency is 60% with a mass of approx. 1.24kg and a volume of 234cm³. The peak to compton ratio is 67:1 and its FWHM is 1.84keV or 0.14% at 1332keV.
Investigations with a set-up of two plastic scintillator paddles revealed a reduction of cosmics rate by as much as 55% inside the overburden in comparison to the experimental hall where the facility is located. With the current veto 89% of cosmics hitting the detector are recorded.
Measurements with the finalised lead castle with and without the veto have shown that the background signals which stem from cosmic muons can be rejected by a factor of 7 which can be further improved with a completion of the veto.
We monitored the radon activity in the air of the compartment for two weeks under several conditions. These measurements revealed that by ventilating the outer shielding the activity is reduced from 32 Bq/m³ to a really low value of 13 Bq/m³.
The data for the left plot document and confirm the achievements during the construction of the heavy shielding in the surface laboratory and were gathered during the different stages of completion of the DLB. They show the background produced by natural radiation sources.
The strong reduction of several γ-peaks of primordial nuclides and the decrease of the overall background is clearly visible. With the final lead castle and the veto in its current stage of completion an integral counting rate (40-2700keV) of 4.26 counts/kg/min is achieved.
Currently an online monitoring system for the LN2 level in the dewar is installed. In the next steps the cosmics veto will be completed and the readout improved to reduce the background further.
Furthermore a Monte Carlo simulation of the system will be mplemented and is currently in preparation by special measurements. Later we will participate in proficiency tests organized by the IAEA or NPL to test our methods.