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Characterisation of plastic scintillators used as an active background shield for neutron detection

This work is part of a JRC Exploratory Research project to develop an active shield that is used to reduce the background due to cosmic radiation in a low-level nuclear waste detection system. The shield consists of an array of plastic scintillators surrounding the detection system. Commercially available plastic scintillation detectors with different thicknesses were characterized for their response to gamma rays, neutrons and muons. Response functions to gamma rays were determined by measurements with radionuclide sources in the energy range from 0.6 MeV to 6.0 MeV. Neutron response functions were measured at mono-energetic neutron beams produced at the Van de Graaff accelerator of the JRC Geel (B) and were derived from results of time-of-flight measurements at the Van de Graaff accelerator of the INFN Legnaro (I). From the response functions for gamma rays and neutrons, light output and resolution functions for protons and electrons were derived. Experimental response functions for muons were determined with the detectors positioned at different orientations. The muon peak is more pronounced in horizontally oriented detectors. Using a scintillator with a minimum thickness of 20 mm a signal caused by the detection of a muon can be separated from events due to natural gamma ray background. For detectors that are stacked, signals caused by the detection of muons can be identified based on a coincidence pattern. Hence, requirements on such a coincidence pattern together with requirements on the light production are effective as parameters for the veto system to be designed.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard