High-spin spectroscopy and the onset of quasicollective structures in $^{69}$Ga

TL;DR

This study probes high-spin states in the odd-A nucleus $^{69}$Ga to understand the emergence of collectivity near $N=40$ and the role of $g_{9/2}$ intruder orbitals. It uses a fusion-evaporation reaction $^{26}$Mg($^{48}$Ca, p4nγ) at 195 MeV with Gammasphere and the Fragment Mass Analyzer to build an extended level scheme up to ~15 MeV, employing angular distributions and correlation ratios to assign spins and parities. Shell-model calculations with JUN45 and jj44b describe the low- to mid-spin (negative and positive parity) spectrum, while TAC-CDFT-SLAP provides a successful interpretation of the observed high-spin quadrupole sequences (S I–S III) as deformed, collective structures driven by $g_{9/2}$ proton and neutron occupancy, including an indication of backbending around $I=25/2$. The work highlights the gradual transition from single-particle excitations to collective rotation in $^{69}$Ga and offers robust benchmarks for shell-model and TAC-CDFT methodologies in this mass region.

Abstract

The intermediate- and high-spin level structure of the odd-$A$ $^{69}$Ga nucleus was investigated via the $^{26}$Mg($^{48}$Ca, $p4nγ$) fusion evaporation reaction at a beam energy of 195 MeV. The experiment was performed using the Gammasphere multidetector array in conjunction with the Fragment Mass Analyzer (FMA), with mass and charge identification achieved via an ionization chamber placed at the focal plane of the spectrometer. Coincidence relationships between the $^{69}$Ga reaction products and emitted $γ$ rays were analyzed to establish the level sequences, while angular distribution and angular correlation measurements were used to propose spin and parity assignments. As a result, the level scheme of the nucleus has been considerably extended. Near the ground state, the structure of $^{69}$Ga is well described by single-particle excitations, with shell-model calculations using the JUN45 and jj44b effective interactions providing a satisfactory interpretation of the observed levels. At spins in excess of 21/2 $\hbar$, three sequences of $E2$ transitions have been delineated, suggesting the onset of collectivity. An interpretation within the framework of the tilted-axis-cranking covariant density functional theory is proposed which reveals the role of $g_{9/2}$ protons and neutrons in this angular momentum regime.

Figures (16)

• Figure 1: Angular distributions for some transitions in $^{69}$Ga. Experimental data are shown as black circles, while the angular-distribution fits correspond to the red curves.
• Figure 2: Level scheme of $^{69}$Ga. Previously known transitions are shown in black, newly-placed ones are in red. Tentatively placed transitions are enclosed in parentheses.
• Figure 3: Coincidence spectra resulting from a single gate on the 574- and 1337-keV transitions. The black-, blue- and red-colored energies are the previously known, newly identified and the rotational-like quadrupole transitions, respectively. See text for details.
• Figure 4: Coincidence spectra resulting from double gates on (a) 635- and 746-keV and (b)746- and 916-keV $\gamma$-ray transitions. The previously known transitions are marked in black, while the colored $\gamma$ rays are the newly established ones. Transitions populating the 2719-keV state are marked in red. The doublet $\gamma$ rays in coincidence with the gated transitions are in magenta.
• Figure 5: Coincidence spectra resulting from double coincidence gates on (a) 671- and 746-keV and (b) 746- and 824-keV $\gamma$ rays. The transitions decaying directly to the 3390-keV level are marked in blue color. The blue arrow in (b) marks the position of the 153-keV $\gamma$ rays that was not observed in coincidence with the 824-keV transition. (see text for details).