Collective and noncollective states in ${}^{66}\mathrm{Zn}$

Excited states in the ${}^{66}\mathrm{Zn}$ nucleus were populated via a ${}^{56}\mathrm{Fe}({}^{12}\mathrm{C},2p\gamma$) fusion-evaporation reaction at a beam energy of $\approx 62$ MeV. The deexciting $\gamma$ rays were detected using the Indian National Gamma Array (INGA). The level scheme of the ${}^{66}\mathrm{Zn}$ nucleus has been updated by placing several new $\gamma$ rays as well as by assigning the spin and parity of various excited states from the present spectroscopic results. The microscopic structure of the observed states have been investigated in the light of large shell-model calculations. The shape of this nucleus in the low-spin regime has been studied under the framework of total Routhian surface (TRS) calculations. The lifetime of first $3^{-}$ state at 2826 keV is experimentally measured using the Doppler-shift attenuation method and the deduced $B\left(E1\right)$ value indicates the presence of octupole collectivity in this nucleus.

Figure 1

Asymmetry factor “$a$” vs $\gamma$-ray energy for a ${90}^{\circ }$ clover detector in the present setup. Solid red line shows linear fit to the data.

Figure 2

The projected spectrum of $E_{\gamma }-E_{\gamma }$ matrix gated by the 1039 keV $2^{+}\to 0^{+}$ transitions. Known transitions belonging to ${}^{66}\mathrm{Zn}$ nucleus are labeled with respective energies using black and the newly observed transitions are indicated by blue, the indigo peak is from ${}^{65}\mathrm{Ga}$, the green peaks are from ${}^{61}\mathrm{Cu}$, while the peaks which could not be identified or placed in the level scheme are shown in red.

Figure 3

Spectra of prompt $\gamma$ rays observed in coincidence with the (a) 1917, (b) 1508, (c) 2024, and (d) 1898 keV $\gamma$ rays belonging to ${}^{66}\mathrm{Zn}$. Newly observed transitions are indicated by the blue color. The green $\gamma$ ray [in panel (b)] is from the other channel ${}^{66}\mathrm{Ge}$ populated in this experiment. The indigo $\gamma$ rays [in panel (d)] are from ${}^{63}\mathrm{Zn}$, the turquoise $\gamma$ rays are from ${}^{51}\mathrm{Co}$, and the red $\gamma$ rays [in panels (b) and (d)] are unknown and could not be placed in the level scheme.

Figure 4

The one-dimensional projection spectra of the $E_{\gamma }-E_{\gamma }-E_{\gamma }$ cube gated by (a) 1039 keV on one axis and 834 keV on the other axis, (b) 1039 keV on one axis and 1333 keV on the other axis. The $\gamma$ lines of 2422, 1917, 1508, 1356, 1232, and 1418 keV are clearly visible now.

Figure 5

Level scheme of ${}^{66}\mathrm{Zn}$ nucleus deduced from this work. Newly observed $\gamma$ transitions and energy levels are shown in red.

Figure 6

The lineshape fits of 1787 keV $\gamma$ rays at different angles. The blue line is the background, the red lines are the extra stopped peaks present in the fitted area, the green line is the lineshape fit for the attenuated Doppler shift and the brown line is the total fit of the spectrum.

Figure 7

Plot of ${\omega }_{\mathrm{rot}}(\pi =-)/{\omega }_{\mathrm{rot}}(\pi =+)$ versus spin for ${}^{66}\mathrm{Zn}$ nuclei. This ratio should be equal to unity for the rotating rigidly reflection-asymmetric systems. The dashed blue line shows how it varies in the case of an aligned octupole phonon.

Figure 8

The $B\left(E1\right)/B\left(E2\right)$ ratio of the yrast band at different spin levels of the yrast positive band states and the lowest negative-parity states. One data point (higher value) in the graph at the spin $5\hslash$ is the ratio of $B\left(E1\right)$ ($5^{-}\to 4_2^{+}$) to $B\left(E2\right)$ ($5^{-}\to 3^{-}$). The arrows $\uparrow$ and $\downarrow$ show the lower limit and the higher limit of the quantity, respectively.

Figure 9

The energy trend of the $2^{+}$, $4^{+}$, and $3^{-}$ states in the Zn isotopes.

Figure 10

Comparison of the energy levels of ${}^{66}\mathrm{Zn}$ observed in the present data (denoted by EXPT) with the shell-model calculations using the $jj44bpn$ [40] effective interaction (denoted SM) for the yrast positive-parity states and the lowest-energy negative-parity states.

Figure 11

Experimental alignments of the yrast positive-parity band as a function of rotational frequency in ${}^{66}\mathrm{Zn}$ and ${}^{68}\mathrm{Ge}$. Harris parameters used for the calculation are $J_0=6.0{\hslash }^2$ ${\mathrm{MeV}}^{-1}$ and $J_1=3.5{\hslash }^4$ ${\mathrm{MeV}}^{-3}$ [42].

Figure 12

Calculated quasiparticle energies for proton and neutron for ${}^{66}\mathrm{Zn}$ at deformation ${\beta }_2=0.22$, $\gamma =0^{\circ }$. The green lines denote the positive parity, positive signature; the blue lines denote the positive parity, negative signature; the red lines denote the negative parity, positive signature; and the magenta lines denote the negative parity, negative signature.

Figure 13

TRS plot for zero quasiparticle configuration for ${}^{66}\mathrm{Zn}$ at $\hslash \omega =0.15$ MeV. The energy separation between the two consecutive surface contours is 250 keV.

Figure 14

Same as Fig. 13 but at $\hslash \omega =0.35$ MeV.