Completion of the muon veto for the Dortmund Low Background Facility and proton activation measurements on irradiated metals from proton beam therapy

Abstract

The Dortmund Low Background Facility is a highly sensitive gamma ray spectrometry system, operated in a low-background environment, which enables the measurement of traces of radioactivity with high precision. A HPGe detector is set up within an artificial overburden, equivalent to 10 meters of water. This outer shielding reduces the contribution from cosmic muons during the measurement. Signals induced by cosmic muons that penetrate this shielding are suppressed by an active cosmic muon veto detector. Environmental radioactivity is effectively shielded from the germanium detector with an multi-layered inner shielding, which is featuring a neutron absorber inside a massive lead shielding. During this work, the muon veto detector is completed with newly installed plastic scintillator detectors. The upgraded veto detector in combination with passive shielding techniques results in a remarkably low residual background count rate of 1.855 counts/(kg min) between 40 keV and 2700 keV. Subsequently, detection limits below 1 mBq/kg are achieved. A detailed analysis of the remaining background spectrum is presented within this work, including MC simulations, that are used to estimate the origins of different background contributions. The irradiation of brass apertures wich high energetic protons during clinical proton beam therapy can create a radiation protection risk for personnel and patients. In cooperation with the WPE several metal samples are irradiated. Due to the high sensitivity of the Dortmund Low Background, relatively short-lived radionuclides are identified and analysed within two irradiated brass aperture samples. The results, presented in this work, are also compared with FLUKA simulations and used for the estimation of the ambient dose rate caused by the activated samples.