Measurement of the Zr-90(n, 2n) Zr-89 reaction cross section using fast neutron spectrum at the Tehran Research reactor (TRR)

Accurate determination of threshold neutron-induced reaction cross sections at relatively high threshold energies is of fundamental importance for nuclear model development and practical applications in physics. In addition, the evaluation and re-evaluation of such cross sections are relevant to ongoing efforts to improve the prompt fission neutron spectra of U-235 and U-238 within the IAEA CIELO project. In this work, the reaction cross sections of natural zirconium isotopes were investigated, with particular emphasis on the threshold reaction Zr-90(n,2n) Zr-89, which has a threshold energy above 10 MeV, in the neutron spectrum of the Tehran Research Reactor. Zirconium and monitor foils were irradiated in an irradiation box located in the reactor core. After an appropriate cooling period, the induced activities of the irradiated samples were measured using a high-purity germanium detector. To accurately estimate the neutron flux on the foils, the reactor core was modeled using the MCNPX code while accounting for burnup effects. The calculated foil reaction rates were then compared with the experimental values to validate the computed neutron flux. The spectrum-averaged cross section of the Zr-90(n,2n) Zr-89 reaction was measured to be 1.73 × 10 −5 barn with an uncertainty of 11%. The corresponding spectrum-averaged cross sections for the Zr-94(n,γ) Zr-95 and Zr-96(n,γ)Zr-97 reactions were 4.18 × 10 −2 and 8.04 × 10 −2 barn, respectively.