Neutron Capture in Iron

Abstract

A search for resonances responsible for neutron capture in iron below 3 keV has been made with the fast choppers at the Atomic Energy Commission Laboratory, Chalk River, Canada, and at Brookhaven National Laboratory. Capture area, self-indication, and transmission techniques have been employed. The only resonance observed was that at 1200 eV. The complete set of parameters for the resonance has been derived: $E_0=1200\mathrm{}\pm 30$ eV, ${\sigma }_0=165\mathrm{}\pm 12$ b, ${\Gamma }_{\gamma }=0.673\mathrm{}\pm 0.074$ eV, and ${\Gamma }_n=0.056\mathrm{}\pm 0.06$ eV. Analysis of the capture gamma-ray spectrum shows the resonance to be due to ${\mathrm{Fe}}^{56}$ and establishes the compound state as ${\mathrm{½}}^{+}$. The variation of total cross section with energy from 0 to 3 keV is ascribed to a neutron bound level with the following parameters: $E_0=-4390\mathrm{}\pm 1700$ eV, ${{\Gamma }_n}^0=12.5\mathrm{}\pm 8.1$ eV, $R^{\prime }=3.9\mathrm{}$ F, and ${\Gamma }_{\gamma }=1.02\mathrm{}\pm 0.46$ eV. It is demonstrated that the previously measured resonance capture integral of Fe cannot be accounted for in terms of known resonances.