A microscopic classification of high-spin states in 56Fe

Abstract

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

Status of the Nuclear Shell Model

“…As a consequence of these dimension issues, calculations in the complete (154,(156)(157)(158), as well as with earlier versions of these approaches (159)(160)(161)(162)(163). Other recent studies in the fp shell include (84) and (164).…”