Speaker
Description
Low-level gamma-spectrometers have been used in wide range of measurements from environmental radioactivity monitoring to fundamental nuclear physics. HPGe gamma-spectrometers play important role in radioactivity monitoring and material screening for experiments searching for rare nuclear events, especially in underground experiments. In general, three components of background can be observed: a radioactive contamination of detectors construction parts, cosmic-ray component and environmental radioactivity. While in surface laboratories the cosmic ray component plays the main part of the background, in underground laboratories natural radioactivity of construction materials of HPGe spectrometers (e.g. cryostat material, detector holder, cold finger) has been identified as the dominant components of the detector background. In order to understand origin of induced radioactive background, or to evaluate the background before the detector construction, a simulated background spectrum can be obtained with Monte Carlo simulation. In investigation of different radionuclide contaminants, decay events are simulated for different parts of the detector separately. Under investigation are gamma-rays of different origin: radioactivity of laboratory and surrounding rocks that mostly includes the natural radioactive series ($^{238}$U and $^{232}$Th) and $^{40}$K, and neutron capture and muon bremsstrahlung in rocks. The paper will discuss in detail sources of environmental background of a HPGe spectrometer operating in Modane underground laboratory and compare them with Monte Carlo simulations carried out using GEANT4 package.
