Investigation of the 56Fe(n,γ)57Fe and 58Fe(n,γ)59Fe reactions

The γ-radiation has been investigated produced by thermal neutron capture in a natural Fe sample and in a sample enriched in 58Fe. Of the 253 γ-rays ascribed to the 56Fe(n,γ)57Fe reaction. 191 have been placed in the 57Fe level scheme (of which 124 placements are new); of the 138 γ-rays observed in the 58Fe(n,γ)59Fe reaction 68 have been placed (with 55 new placements). Excitation energies (with 0.02-0.7 keV error) and branching ratios have been determined of 60 bound levels in 57Fe and of 27 in 59Fe. The Q-values of the sup(56,58)Fe(n,γ) reactions amount to 7646.0 +- 0.2 and 6581.0 +- 0.2 keV, respectively. One can conclude that J = 3/2 for the 2836 keV level in 57Fe, and that Jsup(π) = 3/2- for the 2570, 3072, 3104, 3160, 3160 and 3384 keV levels in 59Fe. A shell-model calculation has been performed with up to two holes in the 1fsub(7/2) sub-shell and particles in the 2psub(3/2), 1fsub(5/2) and 2psub(1/2) sub-shells. The energies of low-lying sup(57,58,59)Fe states are reproduced with a mean deviation of 0.17 MeV. The surface-delta interaction appears superior over the Kuo-Brown interaction in explaining the electromagnetic properties of the lowest six states of 57Fe. (orig.)