Onset of high-spin rotational bands in the $N=Z$ nucleus ${}^{62}\mathrm{Ga}$

The fusion-evaporation reaction ${}^{28}\mathrm{Si}+{}^{40}\mathrm{Ca}$ at 122 MeV beam energy was used to populate high-spin states in the odd-odd $N=Z$ nucleus ${}^{62}\mathrm{Ga}$. With the combination of the Gammasphere spectrometer and the Microball CsI(Tl) charged-particle detector array the decay scheme of ${}^{62}\mathrm{Ga}$ was extended beyond 10 MeV excitation energy. The onset of band structures was observed. These high-spin rotational states are interpreted and classified by means of cranked Nilsson-Strutinsky calculations.

Figure 1

Proposed level scheme of ${}^{62}\mathrm{Ga}$ from the present study. Energy labels are in keV. Tentative transitions and levels are dashed. The widths of the arrows correspond to the relative intensities of the $\gamma$ rays. In the figure, $X=11$ MeV.

Figure 2

$\gamma$-ray spectra related to the main yrast structure of ${}^{62}\mathrm{Ga}$ shown in Fig. 1. Besides the preselection for the analysis of ${}^{62}\mathrm{Ga}$ residues described in Sec. 3, the spectrum in (a) is in coincidence with any of the two 246- or 377-keV transitions, selecting ${}^{62}\mathrm{Ga}$, and, in addition, any of the 246-, 377-, 571-, or 1241-keV transitions known to belong to the low-spin yrast sequence of ${}^{62}\mathrm{Ga}$. The spectrum in (b) is in coincidence with either the 246- or 377-keV transition and the 1387-keV ${17}^{+}\to {15}^{+}$ yrast transition. Energy labels are in keV. The binning is 2 keV per channel. The mark 'C' indicates a transition, which arises from a tiny $\gamma \gamma \gamma$ contamination involving the 1310-1317-keV ground-state sequence in ${}^{61}\mathrm{Cu}$.

Figure 3

High-energy part of a $\gamma$-ray spectrum taken in coincidence with any of the two 246- or 377-keV transitions and in combination with any of the other transitions in the main yrast sequence of ${}^{62}\mathrm{Ga}$ up to the 9976-keV ${17}^{+}$ state, namely $\gamma$ rays at 246, 377, 571, 867, 946, 1107, 1241, 1387, 1487, 1746, or 2355 keV. The binning is 4 keV per channel. Transitions marked with a ‘$★$’ could either not be placed in the decay scheme of ${}^{62}\mathrm{Ga}$ or relate to a small $\gamma \gamma \gamma$ contamination from other, much more intensely populated reaction channels.

Figure 4

(a) shows typical examples for different classes of predicted bands. They are obtained from a scan for yrast configurations for the four combinations of parity and signature for spins up to $I=30\hslash$ in ${}^{62}\mathrm{Ga}$, using standard CNS parameters [40]. They are plotted relative to a rotating liquid drop energy, $E_{rld}$, calculated according to Ref. [44]. Positive (negative) parity is indicated by straight (dashed) lines. Signature $\alpha =0$ ($\alpha =1$) is indicated by filled (open) symbols). Large open circles indicate noncollective states, i.e., where the spin is built from ‘rotation around the symmetry axis’. Large squares are used for states which have reached their $I_{max}$ values though still more or less collective. (b) shows proton-neutron symmetric configurations of ${}^{62}\mathrm{Ga}$ of types $[3_{+}0;3_{+}0]$, $[3_{-}0;3_{-}0]$, $[{21}_{+};{21}_{+}]$, $[1_{+}2;1_{+}2]$, $[1_{-}2;1_{-}2]$, and $[{03}_{+};{03}_{+}]$, i.e., without considering holes in $f_{7/2}$ orbitals. These CNS calculations use the new parameters [43].

Figure 5

(a) shows energies relative to the rotating liquid drop energy [44] of the observed ‘bands’ in ${}^{62}\mathrm{Ga}$ They are compared with the assigned calculated CNS configurations using standard parameters [40] in (b). The difference between experiment and theoretical interpretation is shown in (c). The label ‘gb’ represents the $\pi {\left(fp\right)}^3\nu {\left(fp\right)}^3$ ground band up to spin $9^{+}$, denoted $[3_{+}0;3_{+}0]$ in the CNS description. Moving one proton and one neutron into the lowest $g_{9/2}$ orbitals leads to the $[{21}_{+};{21}_{+}]$ band labeled ‘yb’. High-spin bands Q1, Q2, and Q3 are interpreted to involve the excitation of either one proton or one neutron across the shell gap at particle number 28. Connections of experiment-related data points of Q3 are dotted due to the unknown excitation energy ($X\approx 11$ MeV in Fig. 1). The data point at spin $I=21\hslash$ in Q1 is tentative, hence put in parentheses.

Figure 6

Same as Fig. 5 but using new parameters [43] for the CNS calculations.

Figure 7

Energy surfaces for spins $I=9$, 13, 17, and $21\hslash$ of the $[{21}_{+};\left(1_{-}\right)3_{+}{1}_{+}]$ configuration, which is assigned to the newly observed 1816-2221(-2607)-keV rotational sequence Q1, shown in the upper-left part of Fig. 1. Standard CNS parameters [40] were used. Contour lines correspond to energy differences of 250 keV.