Discovery of a new isomeric state in68Ni: Evidence for a highly deformed proton intruder state

Abstract

We report on the observation of a new isomeric state in 68 Ni. We suggest that the newly observed state at 168(1) keV above the first 2 + state is a π (2p-2h) 0 + state across the major Z = 28 shell gap. Comparison with theoretical calculations indicates a pure proton intruder configuration and the deduced low-lying structure of this key nucleus suggests a possible shape coexistence scenario involving a highly deformed state. The atomic nucleus is a complex quantum system consisting of two kinds of strongly interacting fermions. A direct consequence of this fermionic nature, the Pauli principle, is the shell model of the nucleus, one property of which being the existence of magic gaps. Shell structures are present in a number of systems such as atoms, metal clusters, and quantum dots and wires, for instance, and are strongly linked to the symmetries of the mean field. How the shell gaps evolve in nuclei that are further and further away from stability is one of the key questions to which the radioactive beam facilities that are currently under construction hope to bring answers. Already today, the structure of moderately exotic nuclei such as 68 Ni allows one to pave the way toward a general answer to the problem of shell evolution. Unusual configurations which are expected to dominate in the ground-state structure of very exotic nuclei can be identified as excited structures in systems not very far away from stability. The strong contribution of the spin-orbit term in the nucleon-nucleon interaction affects in a major way the single-particle levels with the largest angular momentum, pushing it down in energy. This quenches significantly the N = 40 magic gap from the spherical harmonic oscillator. The intrusion of the 1g 9/2 and the 2d 5/2 neutron orbitals brings collectivity and enhances neutron-pair excitations across N = 40 from the fp shell into the 1g 9/2 . Conversely, however, this parity change hinders quadrupole excitation and mimics some properties usually associated to magicity. In 68 Ni, the observation of a first excited 0 + 2 state at low energy [1] and the high excitation energy of the 2 + 1 state [2] are examples of such properties. These competing consequences of shell quenching make 68 Ni a particularly suited case to study the evolution of shell gaps with isospin.Reactions involving single-proton particle-hole excitations, π (1p-1h), are an ideal tool to learn about the residual interaction. Unfortunately they lie at very high excitation energy. One possibility to circumvent this obstacle is to look for π (2p-2h) states which are lowered in energy thanks to pairing correlations and proton-neutron residual interactions. Studying pair excitation across magic gaps means, therefore, studying these residual interactions. Pair excitations are revealed by the presence of excited 0 + states. In 68 Ni, two such states are reported, mainly of neutron character, originating from the scattering of pairs into the 1νg 9/2 . A state corresponding to the excitation of two protons (2p-2h) has been predicted by ...

βdecay of61Mn to levels in61Fe

“…A super cycle (SC) contains up to 40 proton pulses (PP) with a certain sequence, distributed with a difference of 1.2 s between each of them. In the present case, the super cycle consisted of 33 proton pulses of which ISOLDE received 11, non-consecutive ones with numbers: 4,6,8,10,13,18,21,23,26,28, and 31. After each PP a delay time of 10 ms was introduced before the separator beam gate of 5 ms was opened, to let the beam implant on the tape.…”

“…Especially the harmonic oscillator shell closure for neutrons at N = 40, separating the negative-parity pf and positive-parity g 9/2 orbitals, makes 68 Ni and adjacent nuclei objects of intensive studies, both experimentally [4][5][6] and theoretically [7,8]. It has been suggested that neutron excitations across the N = 40 sub-shell closure, when moving to lighter Z-values below 68 Ni increases, resulting in the development of collective behavior as protons are removed from the f 7/2 single-particle orbital [9], making this region of the nuclear chart suitable for studying the effects of proton-neutron interactions and the onset of deformation [10,11].…”

Low-energy level schemes of66,68Fe and inferred proton and neutron excitations acrossZ=28Liddick1, Abromeit2, Ayres3 et al. 2013Phys. Rev. C217126View full textAdd to dashboardCite

“…The identification of both proton and neutron intruder levels sparked a renewed interest in identifying corresponding states in the even-even 68 Ni nucleus and theoretical predications were put forward for the excitation energy of the proton 2-particle 2-hole 0 + state in 68 Ni [18]. Despite initial indications [19] the state has not yet been identified [20].…”

Search for shape-coexisting 0+ states in Ni66 from lifetime measurements

“…Large-scale Shell Model calculations using the Lenzi-Nowacki-Poves-Sieja (LNPS) interaction [3,[5][6][7] Thanks to the great effort put into studying this particular nucleus over the recent decades shape coexistence in 68 Ni is well established both from an experimental [3,[9][10][11][12] and a theoretical point of view [3,5,8]. But the situation is not that clear for nuclei around it.…”