Status of vibrational structure inNi62

A. Chakraborty, J. N. Orce, S. F. Ashley, B. A. Brown, B. P. Crider, E. Elhami, M. T. McEllistrem, S. Mukhopadhyay, E. E. Peters, B. Singh, S. W. Yates
2011 Physical Review C  
Measurements consisting of γ-ray excitation functions and angular distributions have been performed using the (n, n ′ γ) reaction on 62 Ni. The excitation function data allowed us to check the consistency of the placement of transitions in the level scheme. From γ-ray angular distributions, the lifetimes of levels up to ∼ 3.8 MeV in excitation energy have been extracted with the Doppler-shift attenuation method. The experimentally deduced values of reduced transition probabilities have been
more » ... ared with the predictions of the quadrupole vibrator model and with large-scale shell model calculations in the f p shell configuration space. Two-phonon states have been found to exist with some notable deviation from the predictions of the quadrupole vibrator model, but no evidence for the existence of three-phonon states could be established. Z = 28 proton core excitations play a major role in understanding the observed structure. PACS numbers: 21.10.Tg, 23.20.Lv, 25.40.Fq, 27.50.+e I. INTRODUCTION The stable nickel isotopes, lying above doubly magic 56 28 Ni 28 , have been the focus of many experimental and theoretical investigations over the past few decades. The closure of both the 1f 7/2 proton and neutron shells in 56 28 Ni 28 leads to a simple shell-model description of the low-lying levels in the nickel isotopes with the proton shells completely inert and the valence neutrons in the 2p 3/2 , 1f 5/2 , and 2p 1/2 orbitals. Shell model calculations with this simple configuration space reproduce the energy spectra of the nickel isotopes up to ∼ 3 MeV [1-3]; however, calculations of electromagnetic transition rates do not reproduce the experimental values. This failure indicates the necessity for excitations of the 56 Ni core. In fact, the excitation of both protons and neutrons from the 1f 7/2 core orbital to the higher orbitals of the f p shell are required to reproduce the experimentally observed systematic trends of both g factors and B(E2)s of the first 2 + states of the even-even nickel isotopes with A= 56 -68 [4, 5]. As seen from Fig. 1 , the B(E2; 2 + 1 → 0 + 1 ) values vary systematically with number of neutrons and ideally should become a maximum at 62 Ni, which lies at the middle of the f p shell (beyond the 56 Ni core). But more recent values adopted in Ref. [6] for the even-even Ni-isotopes in the entire f p shell region indicate the maximum B(E2; 2 + 1 → 0 + 1 ) for 60 Ni. However, this B(E2) is large in 62 Ni and indicates collectivity at low excitation energy. The small value of Q(2 + 1 )(= +0.05(12) barn) [7] reveals a nearly spherical shape and the collectivity is expected to appear in terms of vibrational motion, with the onset of a sequence of quadrupole vibrational phonon states [8] . The experimental value of E 4 + /E 2 + ∼ 2 is suggestive of considering this nucleus as a "spherical harmonic vibrator" and a 0 + 2 , 2 + 2 , and 4 + 1 triplet of levels does exist at approximately twice the energy of the one-phonon 2 + 1 state. Also, the energy level scheme at higher excitation exhibits close-lying levels at approximately thrice the energy of the one-phonon state. For a true categorization, one would need to identify the underlying single-particle and multiphonon contributions by obtaining B(E2) values for the associated transitions. Large E2 decay strengths (> 10 W.u.) are important properties of levels to be identified as multiphonon excitations. Numerous experiments have previously been performed in order to study the low-lying structure of 62 Ni. These studies include Coulomb excitation [4]
doi:10.1103/physrevc.83.034316 fatcat:cramu6uyezbbnd5kxcdeadi7hy