Calibration of neutron sensitive spherical devices with bare and D2O-moderated 252Cf sources in rooms of different sizes

Calibrations of neutron area monitoring devices used in radiation protection are often performed using a 252Cf source. However, if a device is to be used in environments rich in low energy neutrons, then the D2O-moderated 252Cf source (diameter 30 cm) is preferred. The fractional contribution to the device reading due to neutrons scattered by the surroundings and the air may be reduced if the calibration is carried out in close proximity to the source. In this situation, the device readings must be corrected for non-uniform illumination. The necessary geometric correction factor for the calibration of spherical devices with either a bare 252Cf source of very small dimensions or with the physically large spherical D2O-moderated 252Cf source, has been developed. Measurements made with devices of different diameters ranging from 7.62 to 24.40 cm have been evaluated and the computed parameters obtained using the new generalised model, a polynomial fit model, a semi-empirical model and the shadow cone techniques, intercompared. The influence of the scattering effects was investigated by performing the measurements in two differently sized rooms, 7 m x 7 m x 6.5 m and 24 m x 18 m x 18 m. Combined geometric and scatter correction functions as a function of the distance between the centre of the neutron source and the detector are presented. (author)