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

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Abstract
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.

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