Abstract
The structure of low-spin states originating from shape-coexisting configurations in $^{68}_{40}$Ni$_{28}$ was directly probed via the two-neutron transfer reaction $^{66}$Ni$(t,p)^{68}$Ni in inverse kinematics using a radioactive ion beam on a radioactive target. The direct feeding to the first excited 0$^+$ state was measured for center-of-mass angles 4°–16° and amounts to an integral of 4.2(16)% relative to the ground state. The observed difference in feeding of the 0$^+$ states is explained by the transfer of neutrons, mainly in the $pf$ shell below $N=40$ for the ground state, and across $N=40$ in the $g_{9/2}$ orbital for the 0$_2^+$, based on second-order distorted-wave Born approximation calculations combined with state-of-the-art shell-model two-nucleon amplitudes. However, the direct feeding to the 2$_1^+$ state [29(3)%] is incompatible with these calculations.
Highlights
INTRODUCTIONAs finite many-body quantum systems, atomic nuclei are unique in the way that single-particle and collective degrees
For which the 0+ spin parity is a direct consequence of pairing, excited 0+ states of different character are often present in such systems at low excitation energy
Angular distributions were measured for the ground state and first excited 0+ and 2+ states and are shown in Fig. 3 together with two-step distorted-wave Born approximation (DWBA) calculations performed with the FRESCO code [32] including both direct and sequential transfer
Summary
As finite many-body quantum systems, atomic nuclei are unique in the way that single-particle and collective degrees. Since the shape-coexistence phenomenon is identified in several known regions with closed-proton shell and midshell neutrons [1] and could induce sudden changes of low-lying states properties in unexplored regions, unraveling precisely the microscopic configurations involved in these states has become one of the main challenges of current nuclear physics studies. The possible presence of rotational bands built on top of the 02+ and 03+ states, suggested by calculated quadrupole moments [14,15,18], and by observed and calculated relative transition probabilities between the 2+ and 0+ states [8,19], support the shape-coexistence picture This macroscopic picture, similar to the notable case of 186Pb [2], has found in 68Ni a microscopic interpretation thanks to shell-model calculations, which are still out of reach for the lead region.
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