New method for measuring the time integral of neutron flux in a reactor

Abstract A new method for measuring the time integral of thermal neutron flux of a nuclear reactor is proposed. This method utilizes two consecutive (n, γ ) reactions on the 58 Ni isotope of a natural nickel sample placed near the core of a reactor, 58 Ni(n, γ ) → 59 Ni(n, γ ) → 60 Ni(n, γ ). Natural nickel contains 68% 58 Ni producing an intense 8.999 MeV γ -line leading to the 59 Ni ground state via the 58 Ni(n, γ ) reaction. The 59 Ni isotope produced in this manner undergoes another (n, γ ) reaction, emitting a γ -line at 11.388 MeV leading to the 60 Ni ground state. By measuring the intensity ratio of the two γ -lines at 8.999 and 11.388 MeV using a HPGe detector, it is possible to deduce the integral over time of the neutron flux necessary for producing the 59 Ni isotope. Good accuracy can be obtained with this method due to the low gamma background at such high energies.