High spin band structure in ⁶¹Ni

With Z=28, Ni nuclei are expected to have spherical shapes near ground state & the low lying states for ⁵⁶Ni (N=28) to ⁶⁸Ni (N=40) are formed involving νp_3/2, νf_5/2, νp_1/2 and νg_9/2 orbitals. The occupancy of νg_9/2 orbital induce deformation in mid-shell Ni isotopes & rotational bands have been reported in ⁵⁶⁻⁵⁹Ni. With moderate excitation energy, magnetic rotational (MR) bands and super-deformed bands were also observed in ⁶⁰Ni and ⁶³Ni, respectively. The MR bands in ⁶⁰Ni has been explained involving a proton hole in f_7/2 and neutron particle in g_9/2. With an extra valance neutron, ⁶¹Ni expected to exhibit the similar collective excitation. Recently, a theoretical study also predicts non axial shapes & possibility of wobbling motion for ^59-61Ni.

The present work populated ⁶¹Ni at high excitation energy & spin using the ⁵⁰Ti(¹⁴C,3n)⁶¹Ni reaction (thin ⁵⁰Ti enriched foil) at 40 MeV beam energy from the 9 MV Tandem accelerator at FSU. The de-excited γ-rays were detected using the FSU array of 6 BGO shielded clover & 3 single crystal HPGe detectors placed at 3 different angles; 90^o, 45^o and 135^o. With γ−γ coincidence analysis, DCO ratio and polarization measurements the level scheme for ⁶¹Ni has been considerably extended up to 13 MeV and 35/2ℏ spin with the observation of 78 new transitions and 31 new levels with definite spin-parity. We have observed 2 dipole (M1) bands and 2 quadrupole (E2) bands for the first time in ⁶¹Ni. The lower states of the ⁶¹Ni are well reproduced by shell model calculation involving the g_9/2 as well as d_5/2 orbital. These dipole bands can be interpreted as magnetic rotational band which is expected in this region. The regular E2 structures are interpreted as rotational bands with small deformation.