Characterization of NIS neutron irradiation facility for calibration and metrological application

Abstract

In this study, the Neutron Irradiation Facility (NIF) of the National Institute of Standards (NIS) was characterized for metrological applications to improve the accuracy of the calibration process. The NIS neutron irradiation facility consists of a 5 Ci Am-Be and 0.1 μg Cf-252 sources. The flux and dose rate of the Am-Be source was calculated by using MCNP5 code simulation at different distances from the source. The dose rate delivered by the source was determined using NM2-neutron monitor at different source-to-detector distances. A comparison between the measured and the calculated dose rate was performed and the deviation between them was explained in the skeletal arrangement of room scattering contribution. A shadow cone was designed and constructed to determine the scattering contribution at different source-to-detector distances. The optimum source-distance used for calibration was specified. It was found that the Am-Be calculated flux vary with distances from about 107–104 (n/cm2.S−1). The measured and the calculated dose rates were in agreement up to 150 cm distance from the source center after which the measured dose was greater than that calculated. The determined neutron scattering calculated from the measured-to-calculate dose ratio increased from 7% to 25% with increased distances from 150 to 300 cm. Moreover, the standard dose used in the calibration should be measured by a standard neutron monitor at each distance due to the higher value of the room scattering contribution where the optimum distance for calibration was 150 cm. The combined uncertainty of the measured neutron dose was 4.04%.