First3−excited state ofFe56

Abstract

There is no reliable evidence for the existence of the $3.076$ MeV (${3}^{\ensuremath{-}}$) level adopted in the ENSDF evaluation for $^{56}\mathrm{Fe}$ although it has been reported in a few experiments. Previous reports of the observation of this level appear to be based on an incorrect assignment in early ($e,{e}^{\ensuremath{'}}$) work. Recent neutron inelastic scattering measurements by Demidov et al. [Phys. At. Nucl. 67, 1884, (2004)] show that the assigned $\ensuremath{\gamma}$-ray decay of this state does not occur at a level consistent with known properties of inelastic scattering. In the present work the $^{56}\mathrm{Fe}$($n,{n}^{\ensuremath{'}}\ensuremath{\gamma}$) reaction was used to populate excited states in $^{56}\mathrm{Fe}$. Neutrons in the energy range from 1 to 250 MeV were provided by the pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility. Deexciting $\ensuremath{\gamma}$ rays were detected with the GEANIE spectrometer, a Compton suppressed array of 26 Ge detectors. The $\ensuremath{\gamma}$-$\ensuremath{\gamma}$ data obtained with GEANIE were used to establish coincidence relations between transitions. All previously reported levels up to ${E}_{x}=3.6$ MeV excitation energy were observed except for the 3.076 MeV (${3}^{\ensuremath{-}}$) level. The $991$- and $2229$-keV transitions, previously reported to deexcite this level, were not observed in the $\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidence data obtained in the present experiment. The present work supports the assignment of the $4509.6$ keV level as the first ${3}^{\ensuremath{-}}$ excited state in $^{56}\mathrm{Fe}$ by observation of two previously known transitions deexciting this state.