Single-particle and collective excitations in ${}^{62}\mathrm{Ni}$

Background: Level sequences of rotational character have been observed in several nuclei in the $A=60$ mass region. The importance of the deformation-driving $\pi f_{7/2}$ and $\nu g_{9/2}$ orbitals on the onset of nuclear deformation is stressed.

Purpose: A measurement was performed in order to identify collective rotational structures in the relatively neutron-rich ${}^{62}\mathrm{Ni}$ isotope.

Method: The ${}^{26}\mathrm{Mg}\left({}^{48}\mathrm{Ca},2\alpha 4n\gamma \right){}^{62}\mathrm{Ni}$ complex reaction at beam energies between 275 and 320 MeV was utilized. Reaction products were identified in mass $\left(A\right)$ and charge $\left(Z\right)$ with the fragment mass analyzer (FMA) and $\gamma$ rays were detected with the Gammasphere array.

Results: Two collective bands, built upon states of single-particle character, were identified and sizable deformation was assigned to both sequences based on the measured transitional quadrupole moments, herewith quantifying the deformation at high spin.

Conclusions: Based on cranked Nilsson-Strutinsky calculations and comparisons with deformed bands in the $A=60$ mass region, the two rotational bands are understood as being associated with configurations involving multiple $f_{7/2}$ protons and $g_{9/2}$ neutrons, driving the nucleus to sizable prolate deformation.

Figure 1

Representative, background-subtracted, coincidence spectra with gates on ${}^{62}\mathrm{Ni}$ recoils detected in the FMA. (a) Total projection of the full $\gamma \text{−}\gamma$ matrix for ${}^{62}\mathrm{Ni}$; transitions belonging to the low-energy structure (referred to in the text as $ND1)$ are labeled with their respective energies. (b,c) Sum of coincidence gates on in-band $\gamma$-ray transitions in the two collective bands labeled $D1$ (b) and $D2$ (c) in the text. The $\gamma$ rays of interest are indicated by their energies.

Figure 2

Level scheme of ${}^{62}\mathrm{Ni}$ deduced in the present work. The states are labeled with their spin and parity.

Figure 3

Experimental (points) and calculated (lines) values of the fractional Doppler shift $F\left(\tau \right)$ as a function of $E_{\gamma }$ for band $D1$ (a) and band $D2$ (b) in ${}^{62}\mathrm{Ni}$. The best fit is represented by a solid black line, while the dashed blue lines indicate the statistical errors. Note that the $\left(\sim 15\%\right)$ systematic errors associated with the stopping powers are not shown.

Figure 4

Excitation energy versus spin for the two collective bands observed in the present measurement and for the ground-state bands in ${}^{58}\mathrm{Cr}$ [25] and ${}^{60}\mathrm{Fe}$ [27]. See text for details.

Figure 5

Experimental single-particle states in ${}^{62}\mathrm{Ni}$ compared to shell-model calculations using the JUN45 and jj44b effective interactions. The band heads of bands $D1$ and $D2$ are marked in red. Note that, while the parity of $D1$ is tentative, it is adopted as positive for the purposes of this comparison.

Figure 6

Spin along the rotational axis, $I_x$, vs. rotational frequency $\omega$ for bands $D1$ and $D2$ in ${}^{62}\mathrm{Ni}$, bands $D1$ and $D2$ in ${}^{63}\mathrm{Ni}$ [22], band $A$ in ${}^{64}\mathrm{Zn}$ [46], and the yrast bands in ${}^{58}\mathrm{Cr}$ [25] and ${}^{60}\mathrm{Fe}$ [27]. See text for details.

Figure 7

(a) Energies relative to a liquid drop reference $E_{rld}$ drawn as a function of spin $I$ for bands $D1$ and $D2$ and for selected low-spin states, as specified in the left-hand legend. The $8^{-},{10}^{-}$ and $7^{-},9^{-},{11}^{-}$, and ${12}^{-}$ levels refer to the lowest yrast ${12}^{-}$ and the ${11}^{-}–7^{-}$ states fed by it as shown in Fig. 2; (b) same as (a), but for the results of calculations corresponding to the specific configurations found in the right-hand legend. (c) Energy difference between those calculations and data drawn with the same symbols/colors. When some state or band is compared with two different configurations, the notation is given in the lower legend. Note that (calculated) noncollective states associated with an oblate shape are encircled. The levels with the highest angular momenta of most calculated bands are not encircled, although a close inspection reveals most to be close to an oblate deformation, as also illustrated in Fig. 8 below.

Figure 8

The measured transition quadrupole moment for band $D1$ in ${}^{62}\mathrm{Ni}$ is shown in the spin region where it was measured (see text). It is compared with the value calculated as a function of spin for the [20,02] configuration, i.e., the configuration assigned to the $D1$ band with two $f_{7/2}$ proton holes and two $g_{9/2}$ neutrons.