Determination of Fast-Neutron Dose by Nickel Activation

Fast-neutron dose measurement by the activation of nickel foils involves a correction for thermal-neutron burnup of Co⁵⁸, the daughter product of the (n,p) reaction. Fast-neutron irradiation of nickel produces Co⁵⁸ in its ground and excited isomeric states, and recently the isomer has been shown to have a high thermal-neutron-absorption cross section. This paper considers how the determination of fast-neutron dose by nickel activation should be corrected for thermal-neutron burn-up of both ground and isomeric states of Co⁵⁸. Results, which have been fully corrected, are compared with results obtained at low reactor power where the thermal-neutron burn-up of Co⁵⁸ and Co^58m is negligible. All the data considered were obtained from foils irradiated in rigs in hollow fuel elements in reactors of the DIDO type. The data demonstrate that accurate fast-neutron dose measurements, using nickel activation, in high-flux facilities can only be made if the thermal-neutron cross sections of Co⁵⁸ and Co^{58 m} and the branching ratio of the Ni⁵⁸ (n,p) reaction have previously been determined in the neutron spectrum being utilised.