Low-, medium-, and high-spin states in the $N=Z+1$ nucleus ${}^{63}\mathrm{Ga}$

The fusion-evaporation reaction ${}^{28}\mathrm{Si}+{}^{40}\mathrm{Ca}$ at 122 MeV beam energy was used to populate excited states in the $N=Z+1$ nucleus ${}^{63}\mathrm{Ga}$. With the combination of the Gammasphere spectrometer and the Microball CsI(Tl) charged-particle detector array, the level scheme of ${}^{63}\mathrm{Ga}$ was extended by more than a factor of two in terms of number of $\gamma$-ray transitions and excited states, excitation energy $(E_x>30\mathrm{MeV})$, and angular momentum $(I>30\hslash )$. Nine regular sequences of states were newly established in the high-spin part of the level scheme. The majority of these rotational band structures could be connected to the previously known part of the level scheme by high-energy $\gamma$-ray transitions in the energy range $E_{\gamma }=4–6\mathrm{MeV}$. Low-spin states were assessed by shell-model calculations. The high-spin rotational bands were interpreted and classified by means of cranked Nilsson-Strutinsky calculations.

Figure 1

Proposed high-spin level scheme of ${}^{63}\mathrm{Ga}$ 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. Based on intensity pattern and decay-out characteristics, the bands labeled Q7, Q8, and Q9 have been tentatively placed at $X_1=19000\mathrm{keV}$ with $I_1=(43/2)$, $X_2=20300\mathrm{keV}$ with $I_2=(45/2)$, and $X_3=19200\mathrm{keV}$ with $I_3=(43/2)$, respectively.

Figure 2

Gamma-ray spectra related to the main yrast structure of ${}^{63}\mathrm{Ga}$ shown in Fig. 1. All spectra are subject to the preselection for the analysis of ${}^{63}\mathrm{Ga}$ residues outlined in Sec. 3. Panel (a) is the spectrum taken in coincidence with the 625-keV transition and any of the two 894- or 1140-keV transitions. The inset is part of a spectrum in coincidence with the 625- and 649-keV transitions. Panel (b) is the spectrum taken in coincidence with the 1077-keV transition and any of the two 894- or 1140-keV transitions. Panel (c) is the spectrum taken in coincidence with the 649-keV transition and the 1422-/1425-keV doublet. Panel (d) is the spectrum taken in coincidence with the 1546-keV transition and any of the three 649-, 894-, or 1140-keV transitions. Panel (e) is the spectrum taken in coincidence with the 1830-keV transition and any of the five 625-, 649-, 894-, 1140-keV, or 1772-keV transitions. The binning is 2 keV per channel. Energy labels are in keV. Dashed vertical lines are meant to guide the eye.

Figure 3

Gamma-ray reference spectrum for the newly observed high-spin part of the level scheme of ${}^{63}\mathrm{Ga}$. The spectrum is in coincidence with either the 649- or 894-keV transitions, which serve as additional selection of ${}^{63}\mathrm{Ga}$ residues, and any of the other intense transitions found to belong to ${}^{63}\mathrm{Ga}$, namely those at 75, 625, 649, 894, 1077, 1140, 1209, 1330, 1422/1425, 1632/1635, 1772/1773, 1830, 1867/1868, 1925, 1941, and 1963 keV. The binning is 4 keV per channel. Energy labels are in keV. In case the labels are given in parentheses, the transitions are tentatively placed in the level scheme in Fig. 1. In case the labels are given in brackets (mostly gray), the transitions could not be unambiguously placed into the level scheme in Fig. 1, though they are likely to directly connect into one of the states at $10870$, $10908$, $10962$, $12738$, or $12833$ keV. The color code of the labels refers to the band structures introduced in connection with Fig. 1. The note “s.e.” stands for “single escape.”

Figure 4

Gamma-ray spectra illustrating the nine high-spin rotational bands Q1 through Q9 in panels (a) through (i). The binning is 4 keV per channel. Energy labels are in keV. In case the labels are given in parentheses, the transitions are tentatively placed in the level scheme in Fig. 1. The color code of the labels refers to the band structures introduced in connection with Fig. 1. All spectra are subject to the pre-selection for the analysis of ${}^{63}\mathrm{Ga}$ residues outlined in Sec. 3 and are in coincidence with any of the 75-, 625-, 649-, 894-, 1077-, 1140-, 1209-, 1330-, 1422/1425-, or 1772/1773-keV transitions. In addition, panel (a) is in coincidence with the 2010- and 2512-keV transitions. Panel (b) is in coincidence with the 4019-keV transition. Panel (c) is taken in coincidence with the 5826-keV transition. Panel (d) is in coincidence with the 2270-, 2648-, or 3158-keV transitions. Panel (e) is in coincidence with the 4904- or 4924-keV transitions. Panel (f) is in coincidence with the 2351-keV transition. Panel (g) is in coincidence with the 2915 or 3249-keV transitions. Panel (h) is in coincidence with the 2569-, 2777-, or 3071-keV transitions. Panel (i) is in coincidence with the 2706-, 3032-, or 3409-keV transitions. These coincidence requirements are also indicated by a slanted energy label in the respective panel. Dashed vertical lines are meant to guide the eye.

Figure 5

Evolution of predicted low-energy negative-parity states as a function of parameter $t$, denoting the number of particles allowed to be excited across the shell gap at $N=Z=28$. The different panels relate to different interactions used for the full $fp$ shell-model space. The experimentally observed states (exp) are shown on the right-hand side of each panel. See text for details.

Figure 6

(a) Excitation energies of the experimental low- to medium spin states labeled gb and B1, B2, B3, and B4 in Fig. 1, respectively. (b) Excitation energies of yrast and a few selected yrare states predicted by shell-model calculations in the $f_{5/2}p{g}_{9/2}$ model space and the JUN45 interaction [32]. The energies in both panels are plotted relative to a rotating liquid drop energy, $E_{\mathrm{rld}}$, calculated according to Ref. [51] (cf. Fig. 7). Structures of positive (negative) parity are connected with full (dashed) lines. Structures with positive (negative) signature, $\alpha =\pm 1/2$, are indicated by filled (open) symbols.

Figure 7

Comparison between observed and calculated bands in the valence space of ${}^{63}\mathrm{Ga}$. In panel (a) the energies of the observed bands are drawn relative to a rotating drop reference [51]. The calculated configurations assigned to these bands are drawn relative to the same reference in panel (b). The difference between experiment and calculations is shown in panel (c). The excited configuration, ${\left([{21}_{+};3_{+}{1}_{+}]\right)}_2$, was calculated allowing for all possible excitations. Structures of positive (negative) parity are connected with full (dashed) lines. Structures with positive (negative) signature, $\alpha =\pm 1/2$, are indicated by filled (open) symbols.

Figure 8

Comparison between observed high-spin collective bands Q1–Q6, which were linked into the low- and medium-spin level scheme ${}^{63}\mathrm{Ga}$, and their calculated counterparts. The bands are drawn into panels (a)–(c) in the same way as in Fig. 7.

Figure 9

Comparison between observed bands—Q7, Q8, and Q9—not connected to the main body of the level scheme and the calculated bands in ${}^{63}\mathrm{Ga}$. The bands are drawn into panels (a), (b), and (c) in the same way as in Fig. 7.

Figure 10

Classification of the configurations assigned to the observed bands in ${}^{63}\mathrm{Ga}$ in terms of $f_{7/2}$ holes and $g_{9/2}$ particles. Arrows indicate how the various bands are related through the excitations shown schematically in the inset. The configurations are short labeled as $[p_1{p}_3;n_1{n}_3]$, i.e., by the number of proton and neutron holes in $f_{7/2}$ orbitals and particles in $g_{9/2}$ orbitals, respectively. Boxes with well-established configuration assignments are shaded.