Material screening by means of low-level gamma ray spectrometry with the Dortmund Low Background HPGe Facility
Creators: name: Neddermann, Till
Publisher: name: Technische Universität Dortmund
Year: 2014
Subjects: subject: Dortmund Low Background Facility
subject: DLB
subject: Low background
subject: Gamma spectrometry
subject: Material screening
subject: Material assaying
subject: HPGe detector
subject: High puritiy germanium detector
subject: Radioactivity
subject: Activity determination
subject: Geant4 simulation
subject: Monte Carlo simulation
subject: Cobra experiment
subject: 530

description: Many experiments try to investigate the nature of dark matter by searching for rare interactions of hypothetical WIMPs or probe the nature of the neutrino by hunting for the neutrinoless double beta decay. The expected signals lie in the range of approximately 1 keV to 5 MeV, which is the same as the decay energies of radioactive nuclides and other nuclear reactions. Therefore, any energy deposition in the used detectors by particles emitted in a radioactive decay occurring in or near the detectors has to be avoided to lower the background as much as possible. To support the construction of the COBRA experiment - a second generation project searching for the 0νββ decay - the Dortmund Low Background HPGe Facility (DLB) was designed and set-up. It is a low-level gamma ray spectrometry system intended for the detection of traces of radioactivity and therefore enabling the selection of construction materials. The necessary low background environment for the germanium detector was achieved by setting up a massive graded shielding containing different qualities of lead, copper and a neutron moderating as well as absorbing layer. To reduce the contribution of cosmic rays to the background spectrum an artificial overburden of up to 10m of water equivalent was constructed, which attenuates the soft component of the cosmic rays effectively. To suppress signals induced by muons penetrating the outer shielding an active veto detector using plastic scintillator sheets was set-up. With these measures a remarkably low integral background level of 4.06cts/(min kg) (40keV - 2700keV) has been achieved. Analysing the background spectrum only very small remaining gamma ray peaks can be found. Based on this a thermal neutron flux at the HPGe detector was determined, which is nearly two orders of magnitude below the value of conventional detector systems. The calculated upper limit on the flux of fast neutrons is nearly equal to the upper limit for a system with 500mwe of overburden. To allow the reliable determination of nuclide activities from the acquired low background gamma ray spectra a data analysis software based on the DIN ISO 11929:2011 standard was developed. With the software the detection efficiency between sample and germanium detector can be determined on the basis of Monte Carlo simulations. The Monte Carlo model of the detector system was optimized in several steps by comparing spectra of different nuclides in certain measurement geometries with the Monte Carlo output. By this the discrepancies between simulation and measurement of up to 28% with the initial detector model could be reduced to maximum 3%. With these efforts a detector system was set-up which is able to detertmine radioimpurities as low as a few millibequerel per kilogram sample mass and therefore is able to provide valuable data for ultra low background experiments although the system was set-up at the surface.