Single-particle and collective excitations in ${}^{63}$Ni

A study of excited states in ${}^{63}$Ni up to an excitation energy of 28 MeV and a probable spin of 57/2 was carried out with the ${}^{26}$Mg(${}^{48}$Ca,2$\alpha$3$n\gamma$)${}^{63}$Ni reaction at beam energies between 275 and 320 MeV. Three collective bands, built upon states of single-particle character, were identified. For two of the three bands, the transition quadrupole moments were extracted, herewith quantifying the deformation at high spin. The results have been compared with shell-model and cranked Nilsson-Strutinsky calculations. Despite the $Z=28$ shell closure and the approach to the purported $N=40$ subshell, the ${}^{63}$Ni isotope is able to sustain collective excitations at moderate and high spin.

Figure 1

Two-dimensional particle identification plots: (a) $\Delta E$ vs $E_{\mathrm{total}}$ histogram with different element groups labeled. The solid contour line indicates the particle gate for $Z$ selection. (b) $E{T}^2$ vs $M/q$ histogram of the detected ions obtained after $Z$ selection. The dashed lines indicate the particle gates for the $M/q$ selection; note the $M/q$ ambiguities. (c) ToF vs $E_{\mathrm{total}}$ plot following $Z$ and $M/q$ selection used to resolve the ambiguity in $M/q$. The dotted lines indicate particles with the same $M/q$ ratio, but different masses $M$. All axes are in arbitrary units.

Figure 2

Background-subtracted coincidence spectra with gates on ${}^{63}$Ni recoils detected in the FMA. (a) Total projection of the full $\gamma$-$\gamma$ matrix for ${}^{63}$Ni; transitions belonging to the low-energy structure (referred to in the text as $ND1$) are labeled with their respective energies. (b)–(d) Sum of coincidence gates on in-band $\gamma$-ray transitions in the three collective bands labeled $D1$ (b), $D2$ (c), and $D3$ (d) in the text. The $\gamma$ rays of interest are indicated by their energies.

Figure 3

Level scheme of ${}^{63}$Ni deduced in the present work. The states are labeled with their spins and parities.

Figure 4

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 ${}^{63}$Ni. The best calculated fit is represented by a solid black line, while the dashed blue lines indicate the statistical errors.

Figure 5

Intensities of the three observed rotational bands in ${}^{63}$Ni vs the $\gamma$-ray transition energy.

Figure 6

Excitation energy vs spin for the three collective bands observed in the present measurement. The figure is drawn under the assumption that the three bands are very close in excitation energy at the highest spins. This approach provides a means to estimate the bandhead energy and spin of band $D3$ which is not linked to the lower-spin level structure. See text for details.

Figure 7

Spin along the rotational axis $I_x$ vs rotational frequency $\omega$ for (a) bands $D1$, $D2$, and $D3$ in ${}^{63}$Ni and the ground-state band in ${}^{60}$Fe (GSB in [21]); (b) selected rotational bands in ${}^{56}$Ni ($SD2$ in [32, 33]), ${}^{57}$Ni ($SD1$ in [34, 35]), ${}^{58}$Ni ($B3$ in [37]), ${}^{59}$Ni ($B2$ in [39]), and ${}^{60}$Ni ($WD2$ in [41]) together with the trajectory of the yrast band in ${}^{60}$Fe in the $I=8$–20 range, where this sequence is associated with a configuration with two aligned $g_{9/2}$ neutrons; (c) same as (b), but for selected bands in ${}^{60}$Zn ($SD$ in [53]), ${}^{61}$Zn ($SD$ in [54]), ${}^{62}$Zn ($SD$ in [55] and $SD1$ in [56]), and ${}^{65}$Zn ($SD1$ in [52]). See text for details.

Figure 8

(a) Energies of the three observed deformed bands in ${}^{63}$Ni drawn relative to a rotating liquid drop reference ($E_{\exp }-E_{\mathrm{rld}}$) as a function of spin $I$; (b) same as (a), but with energies calculated for the specific configurations indicated in the figure; (c) difference between calculated and experimental energies again plotted against $I$. See text for details.

Figure 9

Experimental single-particle states in ${}^{63}$Ni compared to shell-model calculations using the JUN45 and jj44b effective interactions. The bandheads of bands $D1$ and $D2$ are marked in red.