Low-lying states of 56Fe

Low-lying states at E-x=4.4-8.6 MeV of excitation energy in Fe-56 have been studied by means of inelastic deuteron scattering at E-d = 56 MeV. The spin, parity, and isoscalar matrix element of detected levels have been deduced by comparing the measured cross sections with coupled-channel calculations using collective form factors. The present data for higher excitation energies have been analyzed by combining with previous data at lower excitation energies. The low-energy octupole strength in Fe-56 is found to be mostly located in the excitation energy range of E-x=4.4-8.6 MeV. The strength is weakly fragmented, and the 3(1)(-) level exhausts the major part of the octupole energy-weighted-sum-rule value. A similar behavior is observed for the quadrupole strength in Fe-56, which is mostly located below E-x=4.4 MeV. This is in contrast with the behavior of the 3(-) and 2(+) states in Fe-54, which are heavily fragmented despite the weaker quadrupole deformation. The hexadecapole strength is strongly fragmented both in Fe-56 and in Fe-54. The strength distributions for the transitions with multipolarities lambda = 2, 3, and 4 in Fe-54,Fe-56 have been compared with random phase approximation calculations. The theoretical calculations show that the closure of the nu f(7/2) shell is responsible for the observed fragmentation of the 2(+) and 3(-) strengths in Fe-54, and that the collectivity of the 2(1)(+) and 3(1)(-) levels of Fe-56 is restored by the presence of two neutrons outside the nu f(7/2) shell. [S0556-2813(98)00504-4].