Development of a Detector System for Detection of Thermal Neutrons from Spent Nuclear Fuel

Abstract: The nuclear reaction 10B(n,alpha)7Li' was utilised to develop a neutron detection system. This system has the purpose of being used to detect the thermal neutron radiation emitted by spent nuclear fuel, before it is transported to a final repository. This to investigate the content of the fuel-containing capsules, primarily to ensure non-proliferation of radioactive materials in the nuclear waste management process. The investigated detector system utilises two scintillators mounted on one photomultiplier tube (PMT). One of them boron loaded to be sensitive to neutrons, and the other sensitive to gamma radiation to detect the de-excitation quanta released by the lithium ion, often left in an excited state after the 10B(n,alpha)7Li' reaction. The detection of these signals together reduces background noise, and those signals of interest are extracted from the others by a pulse shape discrimination scheme. The possibility to read out the scintillator signals using an avalanche photodiode (APD) was also investigated. Inherent noise difficulties and poorly matched properties of scintillators and APDs made read out with APDs difficult. Studies of boron carbide (B4C) coated plastic scintillators and diodes were conducted as well. A neutron detection method using boron carbide coated diodes was tested. Proof of concept measurements were made using such coated diodes. The effect on the properties of plastic scintillators when coated with B4C were qualitatively assessed.