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Purpose: Single-neutron adding data was collected in order to determine the distribution of the single-neutron strength of the $0f_{7/2}$, $1p_{3/2}$, $1p_{1/2}$ and $0f_{5/2}$ orbitals outside of $Z=16, N=18$, $^{34}$S. Methods: The $^{34}$S($d$,$p$)$^{35}$S reaction has been measured at 8 MeV/u to investigate cross sections to excited states in $^{35}$S. Outgoing proton yields and momenta were analyzed by the Super-Enge Split-Pole Spectrograph in conjunction with the CeBrA demonstrator located at the John D. Fox Laboratory at Florida State University. Angular distributions were compared with Distorted Wave Born Approximation calculations in order to extract single-neutron spectroscopic overlaps. Results: Spectroscopic overlaps and strengths were determined for states in $^{35}$S up through 6 MeV in excitation energy. Each orbital was observed to have fragmented strength where a single level carried the majority. The single-neutron centroids of the $0f_{7/2}$, $1p_{3/2}$, $1p_{1/2}$ and $0f_{5/2}$ orbitals were determined to be $2360^{+90}_{-40}$ keV, $3280^{+80}_{-50}$ keV, $4780^{+60}_{-40}$ keV, and $\gtrsim7500$ keV, respectively. Conclusion: A previous discrepancy in the literature with respect to distribution of the neutron $1p_{1/2}$ strength was resolved. The integration of the normalized spectroscopic strengths, up to 5.1 MeV in excitation energy, revealed fully-vacant occupancies for the $0f_{7/2}$, $1p_{3/2}$, and $1p_{1/2}$ orbitals, as expected. The spacing in the single-neutron energies highlighted a reduction in the traditional $N=28$ shell-gap, relative to both the $1p$ spin-orbit energy difference ($N=32$) and the lower limit on the $N=34$ shell spacing., Comment: Updated draft

We report on a highly selective experimental setup for particle-$\gamma$ coincidence experiments at the Super-Enge Split-Pole Spectrograph (SE-SPS) of the John D. Fox Superconducting Linear Accelerator Laboratory at Florida State University (FSU) using fast CeBr$_3$ scintillators for $\gamma$-ray detection. Specifically, we report on the results of characterization tests for the first five CeBr$_3$ scintillation detectors of the CeBr$_3$ Array (CeBrA) with respect to energy resolution and timing characteristics. We also present results from the first particle-$\gamma$ coincidence experiments successfully performed with the CeBrA demonstrator and the FSU SE-SPS. We show that with the new setup, $\gamma$-decay branching ratios and particle-$\gamma$ angular correlations can be measured very selectively using narrow excitation energy gates, which are possible thanks to the excellent particle energy resolution of the SE-SPS. In addition, we highlight that nuclear level lifetimes in the nanoseconds regime can be determined by measuring the time difference between particle detection with the SE-SPS focal-plane scintillator and $\gamma$-ray detection with the fast CeBrA detectors. Selective excitation energy gates with the SE-SPS exclude any feeding contributions to these lifetimes.

We performed a measurement of the $^{52}$Cr$(d,p)^{53}$Cr reaction at 16 MeV using the Florida State University Super-Enge Split-Pole Spectrograph (SE-SPS) and observed 26 states. While all of the states observed here had been seen in previous $(d,p)$ experiments, we changed five $L$ assignments from those reported previously and determined $L$ values for nine states that had not had such assignments made previously. The $g_{9/2}$ neutron strength observed in $^{53}$Cr in the present work and in the $N=29$ isotones $^{49}$Ca, $^{51}$Ti, and $^{55}$Fe via $(d,p)$ reactions is much smaller than the sum rule for this strength. Most of the observed $L=4$ strength in these nuclei is located in states near 4 MeV excitation energy. The remaining $g_{9/2}$ strength may be located in the continuum or may be fragmented among many bound states. A covariant density functional theory calculation provides support for the hypothesis that the $g_{9/2}$ neutron orbit is unbound in $^{53}$Cr. The ($\alpha,^3$He) reaction may provide a more sensitive probe for the missing $g_{9/2}$ neutron strength. In addition, particle-$\gamma$ coincidence experiments may help resolve some remaining questions in this nucleus., Comment: 15 pages, 5 figures. arXiv admin note: text overlap with arXiv:2212.04384

In the Ge-Sr mass region, isotopes with neutron number $N \leq 40$ are known to feature rapid shape changes with both nucleon number and angular momentum. To gain new insights into their structure, inelastic proton scattering experiments in inverse kinematics were performed on the rare isotopes $^{74,76}$Kr. This work focuses on observables related to the $J^{\pi} = 4^+_1$ states of the Kr isotopes and, in particular, on the hexadecapole degree of freedom. By performing coupled-channels calculations, hexadecapole deformation parameters $\beta_4$ were determined for the $J^{\pi} = 4^+_1$ states of $^{74,76}$Kr from inelastic proton scattering cross sections. Two possible coupled-channels solutions were found. A comparison to predictions from nuclear energy density functional theory, employing both non-relativistic and relativistic functionals, clearly favors the large, positive $\beta_4$ solutions. These $\beta_4$ values are unambiguously linked to the well deformed prolate configuration. Given the $\beta_2 - \beta_4$ trend, established in this work, it appears that $\beta_4$ values could provide a sensitive measure of the nuclear shell structure., Comment: 7 pages, 4 figures, accepted for publication in Physics Letters B

A measurement of the $^{54}$Fe($d$,$p$)$^{55}$Fe reaction at 16 MeV was performed using the Florida State University Super-Enge Split-Pole Spectrograph to determine single-neutron energies for the $2p_{3/2}$, $2p_{1/2}$, $1f_{5/2}$, $1g_{9/2}$ and $2d_{5/2}$ orbits. Two states were observed that had not been observed in previous (d, p) measurements. In addition, we made angular momentum transfer, \textit{L}, assignments to four states and changed \textit{L} assignments from previous ($d$, $p$) measurements for nine more states. The spin-orbit splitting between the $2p_{3/2}$ and $2p_{1/2}$ orbits is similar to that in the other $N=29$ isotones and not close to zero as a previous measurement suggested. While the $1f_{5/2}$ single neutron energy is significantly lower in $^{55}$Fe than in $^{51}$Ti, as predicted by a covariant density functional theory calculation, the single-neutron energy for this orbit in $^{55}$Fe is more than 1 MeV higher than the calculation suggests, although it is only 400 keV above the $2p_{1/2}$ orbit. The summed spectroscopic strength we observed for the $1g_{9/2}$ orbit up to the single-neutron separation energy of 9.3 MeV is only 0.3. This is surprising because the $1g_{9/2}$ orbit is predicted by Togashi \textit{et al.} to be located only 5.5 MeV above the $2p_{3/2}$ orbit., Comment: 16 pages, 6 figures. arXiv admin note: text overlap with arXiv:2106.05781

Enhanced octupole collectivity is expected in the neutron-deficient Ge, Se and Kr isotopes with neutron number $N \approx 40$ and has indeed been observed for $^{70,72}$Ge. Shape coexistence and configuration mixing are, however, a notorious challenge for theoretical models trying to reliably predict octupole collectivity in this mass region, which is known to feature rapid shape changes with changing nucleon number and spin of the system. To further investigate the microscopic configurations causing the prolate-oblate-triaxial shape transition at $A \approx 72$ and their influence on octupole collectivity, the rare isotopes $^{72}$Se and $^{74,76}$Kr were studied via inelastic proton scattering in inverse kinematics. While significantly enhanced octupole strength of $\sim 32$ Weisskopf units (W.u.) was observed for $^{72}$Se, only strengths of $\sim 15$ W.u. were observed for $^{74,76}$Kr. In combination with existing data, the new data clearly question a simple origin of enhanced octupole strengths around $N = 40$. The present work establishes two regions of distinct octupole strengths with a sudden strength increase around the $A=72$ shape transitional point.

A measurement of the $^{50}$Ti($d$,$p$)$^{51}$Ti reaction at 16 MeV was performed using a Super Enge Split Pole Spectrograph to measure the magnitude of the $N=32$ subshell gap in Ti. Seven states were observed that had not been observed in previous ($d$,$p$) measurements, and the \textit{L} transfer values for six previously measured states were either changed or measured for the first time. The results were used to determine single neutron energies for the $p_{3/2}$, $p_{1/2}$ and $f_{5/2}$ orbitals. The resulting single neutron energies in $^{51}$Ti confirm the existence of the $N=32$ gap in Ti. These single neutron energies and those from previous measurements in $^{49}$Ca, $^{53}$Cr and $^{55}$Fe are compared to values from a covariant density functional theory calculation., Comment: 17 pages, 6 figures

UCGretina, a GEANT4 simulation of the GRETINA gamma-ray tracking array of highly-segmented high-purity germanium detectors is described. We have developed a model of the array, in particular of the Quad Module and the capsules, that gives good agreement between simulated and measured photopeak efficiencies over a broad range of gamma-ray energies and reproduces the shape of the measured Compton continuum. Both of these features are needed in order to accurately extract gamma-ray yields from spectra collected in in-beam gamma-ray spectroscopy measurements with beams traveling at $v/c \gtrsim 0.3$ at the National Superconducting Cyclotron Laboratory and the Facility for Rare Isotope Beams. In the process of developing the model, we determined that millimeter-scale layers of passive germanium surrounding the active volumes of the simulated crystals must be included in order to reproduce measured photopeak efficiencies. We adopted a simple model of effective passive layers and developed heuristic methods of determining passive-layer thicknesses by comparison of simulations and measurements for a single crystal and for the full array. Prospects for future development of the model are discussed., Comment: 26 pages, 12 figures

Excited states of the neutron-rich isotopes $^{42,44}$S and $^{41,43}$P have been studied via inverse-kinematics proton scattering from a liquid hydrogen target, using the GRETINA $\gamma$-ray tracking array to extract inelastic scattering cross sections. Deformation lengths of the $2^+_1$ excitations in $^{42,44}$S have been determined and, when combined with deformation lengths determined with electromagnetic probes, yield the ratio of neutron-to-proton matrix elements $M_n/M_p$ for the $2^+_1$ excitations in these nuclei. The present results for $^{41,43}$P$(p,p')$ are used to compare two shell model interactions, SDPF-U and SDPF-MU. As in a recent study of $^{42}$Si, the present results on $^{41,43}$P favor the SDPF-MU interaction., Comment: 11 pages, 14 figures

Excited states in the neutron-rich \nuc{70}{Fe} nucleus were populated in a one-proton removal reaction from \nuc{71}{Co} projectiles at 87~MeV/nucleon. A new transition was observed with the $\gamma$-ray tracking array GRETINA and shown to feed the previously assigned $4^+_1$ state. In comparison to reaction theory calculations with shell-model spectroscopic factors, it is argued that the new $\gamma$ ray possibly originates from the $6^+_1$ state. It is further shown that the Doppler-reconstructed $\gamma$-ray spectra are sensitive to the very different lifetimes of the $2^+$ and $4^+$ states, enabling their approximate measurement. The emerging structure of \nuc{70}{Fe} is discussed in comparison to LNPS-new large-scale shell-model calculations., Comment: accepted for publication as PRC rapid communication

Excited states of the $N=32$ nucleus $^{54}$Ti have been studied, via both inverse-kinematics proton scattering and one-neutron knockout from $^{55}$Ti by a liquid hydrogen target, using the GRETINA $\gamma$-ray tracking array. Inelastic proton-scattering cross sections and deformation lengths have been determined. A low-lying octupole state has been tentatively identified in $^{54}$Ti for the first time. A comparison of $(p,p')$ results on low-energy octupole states in the neutron-rich Ca and Ti isotopes with the results of Random Phase Approximation calculations demonstrates that the observed systematic behavior of these states is unexpected., Comment: 7 pages, 8 figures

Low-lying excited states of the neutron-rich calcium isotopes $^{48-52}$Ca have been studied via $\gamma$-ray spectroscopy following inverse-kinematics proton scattering on a liquid hydrogen target using the GRETINA $\gamma$-ray tracking array. The energies and strengths of the octupole states in these isotopes are remarkably constant, indicating that these states are dominated by proton excitations., Comment: 15 pages, 3 figures

We have performed measurements of the $0_\mathrm{g.s.}^+ \rightarrow 2_1^+$ excitations in the neutron-rich isotopes $^{48,50}$Ca via inelastic proton scattering on a liquid hydrogen target, using the GRETINA $\gamma$-ray tracking array. A comparison of the present results with those from previous measurements of the lifetimes of the $2_1^+$ states provides us the ratio of the neutron and proton matrix elements for the $0_\mathrm{g.s.}^+ \rightarrow 2_1^+$ transitions. These results allow the determination of the ratio of the proton and neutron effective charges to be used in shell model calculations of neutron-rich isotopes in the vicinity of $^{48}$Ca., Comment: Phys. Rev. C, accepted

We report on the first in-beam $\gamma$-ray spectroscopy of \nuc{23}{Al} using two different reactions at intermediate beam energies: inelastic scattering off \nuc{9}{Be} and heavy-ion induced one-proton pickup, \nuc{9}{Be}(\nuc{22}{Mg},\nuc{23}{Al}$+\gamma$)X, at 75.1 MeV/nucleon. A $\gamma$-ray transition at 1616(8) keV -- exceeding the proton separation energy by 1494 keV -- was observed in both reactions. From shell model and proton decay calculations we argue that this $\gamma$-ray decay proceeds from the core-excited $7/2^+$ state to the $5/2^+$ ground state of \nuc{23}{Al}. The proposed nature of this state, $[\nuc{22}{Mg}(2^+_1) \otimes \pi d_{5/2}]_{7/2+}$, is consistent with the presence of a gamma-branch and with the population of this state in the two reactions., Comment: Physics Letters B, accepted

The energy of the first excited state in the neutron-rich N=28 nucleus 45Cl has been established via in-beam gamma-ray spectroscopy following proton removal. This energy value completes the systematics of the E(1/2^+_1)-E(3/2^+_1) level spacing in odd-mass K, Cl and P isotopes for N=20-28. The results are discussed in the framework of shell-model calculations in the sd-fp model space. The contribution of the central, spin-orbit and tensor components is discussed from a calculation based on a proton single-hole spectrum from G-matrix and pi + rho meson exchange potentials. A composite model for the proton 0d_{3/2}-1s_{1/2} single-particle energy shift is presented., Comment: Phys. Rev. C, in press