Extensive shell-model calculations have been performed on 5'*'55'56Fe. The results obtained in a model space with two and with up to three f7/2 holes in the 56Ni core are compared with experiment. The Surface Delta Interaction (SDI) and the Kuo Brown interaction (KB) have been used to calculate energy levels, spectroscopic factors and electromagnetic properties resulting especially for 56Fe in a remarkably good agreement with experiment. Admixtures of three-hole components in the wave functions are significant and increase with mass number. Properties of high-spin states with J <15 are discussed. Pronounced collective features derived microscopically are expected in 56Fe. Finally some suggestions for interesting experiments are given.

Abstract:The y-radiation has been investigated produced by thermal neutron capture in a natural Fe sample and in a sample enriched in 5sFe. Of the 253 y-rays ascribed to the 56Fe(n, y)"Fe reaction, 19 1 have been placed in the 57Fe level scheme (of which 124 placements are new); of the 138 y-rays observed in the 58Fe(n, y)s9Fe 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 S7Fe and of 27 in 59Fe The Q-values of the s6,"Fe(n, y) reactions amount to 7646.OkO.2 and 6581.0+0.2 keV, respectively. One can conclude that J = 3 for the 2836 keV level in 57Fe, and that J" = +-for the 2570, 3072, 3104, 3160 and 3384 keV levels in 59Fe.A shell-model calculation has been performed with up to two holes in the If,,, sub-shell and particles in the 2p,,,, If,,, and 2p,,, sub-shells. The energies of low-lying ". 58, s9Fe 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 "Fe. E NUCLEAR REACTIONS s6. s*Fe(n, y), E = thermal; measured E,, I?; deduced Q. 57, 59Fe levels deduced E,, y-branching, J". Enriched and natural targets.

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A large-scale shell-model calculation on S6Fe including positive parity states with spins up to J = 15 shows that several states in the yrast region may be of a particular nature. These states can be arranged m groups of which the gamma decay and quadrupole moments show a collective behavlour. The signature of each group is the f7/2 hole structure. This structure is coupled to a definite Jhole and Thole with the spin Jhole being as large as possible. The level density above the yrast region turns out to be largely independent of J.Nuclei near A = 56 are interesting for further theoretical and experimental investigation since they are light enough to be treated microscopically in quite some detail, while these nuclei probably are also heavy enough to exhibit some collective properties. A nice example for a microscopic treatment is the nucleus 56Fe for which some collective properties have been the subject of experimental [1-3] and theoretical [2][3][4] investigations.Very recently large-scale shell-model calculations on positive-parity states in 54-56Fe have been performed [5]. It has been demonstrated that many properties of these isotopes can be described well in a (fT/2)m(fs/2P3/2Pl/2) n model space with m >~ 13 and n ~< 3, i.e. up to three holes in the closed f7/2 shell have been taken into account. Two quite different effective interactions have been investigated (i) a realistic interaction derived from slightly modified [5] Kuo-Brown matrix elements (denoted by KB) and (ii) two-body matrix elements from the schematic Surface Delta Interaction [6] (denoted by SDI). In particular for 56Fe a nice one-to-one correspondence exasts between theory and experiment for all states below 4 MeV excitation energy. For KB the average deviation between experimental and theoretical excitation energies turns out to be less than 100 keV. Also spectroscopic factors for single-particle transfer as well as electromagnetic transition strengths and moments can be well explained with only very few deviations. In general KB is found to be superior to SDI. For SDI the fs/2 occupation numbers are considerably larger than for KB. The experimental S-factor for pick-up on 56Fe favours the SDI value [5].With the success of the detailed interpretation of the properties of low-lying states in 56Fe in mind, a shell-model investigation of other observables in this nucleus becomes more meaningful. Presently we would like to discuss some remarkable properties of high-spin states (J= 8-15) in 56Fe which have been obtained with the same interactions (KB and SDI) and model space as used in ref. [5]. These states are interesting because one can investigate the possible appearance of collective features in the yrast region from a microscopic point of view.The calculated excitation energies obtained with KB for the ten lowest eigenstates of each J are presented in fig. 1. The excitation energies obtained with the very different SDI matrix elements show a nearly 171