Evaluation of Neutron Flux Distribution in the Nigeria Research Reactor for Materials Irradiation Applications

The Nigeria Research Reactor, NIRR-1, at Ahmadu Bello University, Zaria, provides neutron irradiation services for neutron activation analysis, radioisotope production, and materials research. Accurate knowledge of the spatial neutron flux distribution within the reactor core and reflector irradiation positions is essential for quantitative activation analysis and sample irradiation planning. This study evaluates the thermal and epithermal neutron flux distributions in the NIRR-1 irradiation channels using the flux-wire activation method and multi-foil activation technique. Thin foils of gold, cobalt, and copper are irradiated at six positions within the reactor core and inner reflector, and the induced activities are measured using a calibrated HPGe gamma spectrometry system. The cadmium ratio method separates thermal from epithermal flux contributions. Results indicate thermal neutron flux values of 4.2 to 5.8 times ten to the eleventh neutrons per square centimetre per second in the inner irradiation channels, consistent with the NIRR-1 design specifications. The epithermal-to-thermal flux ratio ranges from 0.032 to 0.048 across the measured positions. The flux distribution map produced supports sample irradiation scheduling and activation analysis uncertainty quantification. Keywords: neutron flux, NIRR-1, activation analysis, neutron activation, research reactor