A thermal neutron source for the CERN radiation Calibration Laboratory

This work presents the design, construction and experimental characterisation of a lightweight and low-cost thermal neutron assembly, to be used with the existing Am-Be source irradiator of CERN radiation Calibration Laboratory (Cal Lab). The assembly consists of a cylindrical moderator (18 cm diameter, 25.5 cm height and 5.5 kg weight) and an optional reflector box (5 cm thick walls, 20 kg weight). The moderator is tailored to fit on the Am-Be source in its irradiation position, while the box encloses the detector under test during the irradiation. The exposure volume delimited by the box is 30 x 30 x 30 cm(3). The thermal neutron fluence at the exposure location, i.e., 30 cm from the source, was optimized by FLUKA Monte Carlo (MC) simulations. The simulations were validated with measurements performed with a bare He-3 proportional counter. The thermal neutron fluence at the nominal irradiation position is 7.43 x 10(2) cm(-2)s(-1)with the cylindrical moderator only, and 5.75 x 10(3) cm(-2)s(-1)with the cylinder and the reflector box, with the detector placed at the centre of the box. The thermal neutron fluence inside the box is rather uniform (variation <5%).