Single particle configurations in ${}^{61}\mathrm{Ni}$

Excited states of the ${}^{61}\mathrm{Ni}$ ($Z=28,N=33$) nucleus have been probed using heavy-ion-induced fusion evaporation reaction and an array of Compton suppressed germanium (clover) detectors as detection system for the emitted $\gamma$ rays. Seventeen new transitions have been identified and placement of six transitions have been modified with respect to the previous measurements, following which the level scheme of the nucleus has been extended up to an excitation energy $E_x\approx 7$  MeV and spin $\approx 10\hslash$. Higher excitations involving the $g_{9/2}$ orbital in the $fpg$ model space have been established. The experimental results on the level structure of the nucleus have been interpreted in the light of large basis shell model calculations that lead to an understanding of the single particle configurations underlying the level structure of the nucleus. The comparison can be suggestive of further refinements in the shell model interactions for better overlap of the theoretical results with the experimental data.

Figure 1

$R_{\mathrm{ADO}}$ values for different transitions of ${}^{61}\mathrm{Ni}$ along with those of selected transitions of previously known multipolarities from other nuclei populated in the present experiment. The latter is used to fix the reference values used in the current analysis as well as for validation of the same.

Figure 2

(Top) Plot of the geometrical asymmetry ($a$) against $\gamma$-ray energies along with the fit to the data points using the equation $a_0+a_1*E_{\gamma }$. (Bottom) Plot of polarization asymmetry $\mathrm{\Delta }$ [defined in Eq. (3)] for different $\gamma$-ray transitions of the ${}^{61}\mathrm{Ni}$ nucleus along with those from other nuclei produced in the same experiment.

Figure 3

(Top) Plot of polarization sensitivity as a function of $\gamma$-ray energy, determined from the observed $\gamma$ rays of previously known multipole mixing ratio, along with the fit using the equation $Q\left(E_{\gamma }\right)=Q_0\left(E_{\gamma }\right)(C{E}_{\gamma }+D)$ ($Q_0\left(E_{\gamma }\right)=\frac{\alpha +1}{{\alpha }^2+\alpha +1}$, $\alpha =E_{\gamma }/m_e{c}^2,m_e{c}^2$ being electron rest mass energy). (Bottom) Plot of polarization $P$, defined by $P=\mathrm{\Delta }/Q$, for different $\gamma$-ray transitions of ${}^{61}\mathrm{Ni}$ and other nuclei populated in the present experiment. The latter are of previously known multipole mixing that was used to calculate their theoretical polarization, included in the plot, for reference and validation of the current analysis.

Figure 4

Level scheme of the ${}^{61}\mathrm{Ni}$ nucleus from the present work. The new $\gamma$-ray transitions identified from the current study are marked in red. The transitions labeled in blue were observed in the previous studies but were either not placed in the level scheme or had different placement with respect to the energy and/or $J^{\pi }$ values of the deexciting states.

Figure 5

Representative spectra with gate on $\gamma$-ray transitions of ${}^{61}\mathrm{Ni}$, generated from $\gamma \text{−}\gamma$ coincidence matrix. The new transitions, first observed in the present study, are labeled with *.

Figure 6

Sum spectrum with $\gamma \text{−}\gamma$ gate on 948-, 1106-, and 1114-keV transitions in ${}^{61}\mathrm{Ni}$, constructed from the $\gamma \text{−}\gamma \text{−}\gamma$ cube. The new transitions, first observed in the present study, are labeled with *.

Figure 7

Comparison of experimental level energies in ${}^{61}\mathrm{Ni}$ with those from large basis shell model calculations using JJ44BPN and JUN45 interactions for (a) negative parity states and (b) positive parity states.