Threshold Photoneutron Cross Section forFe56

Individual levels of the compound nucleus ${\mathrm{Fe}}^{56}$ at an excitation energy of 11.2 MeV have been studied by examining the ($\gamma ,n$) cross section for natural iron from 0.7 to 70 keV above threshold with high resolution. The technique is generally applicable to the high-resolution study of photoneutron cross sections for photons whose energy exceeds the photoneutron threshold by an amount of the order of keV. Resonance parameters were obtained for certain favorable cases. The $\gamma$-ray strength function $〈\frac{{\Gamma }_{\gamma 0}}{D}〉$ for electric dipole absorption in ${\mathrm{Fe}}^{56}$ was found to be 3.5×${10}^{-5\mathrm{}}$, which is a factor of 2 lower than the estimates obtained either from the low-energy wing of the giant dipole resonance or from the single-particle model of the nucleus.