Cross Sections of Threshold Reactions for Fission Neutrons: Nickel as a Fast Flux Monitor

Measured values of effective cross sections for fission neutrons obtained for n,p reactions on Ni⁵⁸, Fe⁵⁴, Zn⁶⁴, Mg²⁴, Fe⁵⁶, Zn⁶⁷, and Cu⁶⁵ are 92, 54, 28, 1.2, 0.82, 0.57, and 0.36 mb, respectively. All cross sections are based upon the value 0.60 mb for the n,α reaction on Al²⁷. Measurements were made at the exact center of the EBR-I core. This position has been shown by other investigators to have a neutron energy spectrum similar to that of virgin fission neutrons in the region above 2 Mev. The n,p reaction on Ni⁵⁸ is shown to have unusual practical advantages as a fast flux monitor. The chemical and physical stability of nickel metal in most reactor coolants, the absence of radioactivities obscuring the Co⁵⁸, the long half-life of Co⁵⁸ (72 days), and the ease with which its 0.800 Mev gamma can be measured, are some of these advantages. A major but avoidable drawback is the 1650 barn thermal neutron capture cross section of Co⁵⁸. A comparison of nickel and sulfur in measuring the fast neutron flux spectrum in beam hole HB-3 of the MTR is included. Evidence is presented which indicates that the correct value for the fission neutron cross section of S³² is 65 mb.