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The calcium isotopes are an ideal system to investigate the evolution of shell structure and magic numbers. Although the properties of surface nucleons in calcium have been well studied, probing the structure of deeply bound nucleons remains a challenge. Here, we report on the first measurement of unbound states in $^{53}$Ca and $^{55}$Ca, populated from \ts{54,56}Ca($p,pn$) reactions at a beam energy of around 216 MeV/nucleon at the RIKEN Radioactive Isotopes Beam Factory. The resonance properties, partial cross sections, and momentum distributions of these unbound states were analyzed. Orbital angular momentum $l$ assignments were extracted from momentum distributions based on calculations using the distorted wave impulse approximation (DWIA) reaction model. The resonances at excitation energies of 5516(41)\,keV in $^{53}$Ca and 6000(250)\,keV in $^{55}$Ca indicate a significant $l$\, =\,3 component, providing the first experimental evidence for the $\nu 0f_{7/2}$ single-particle strength of unbound hole states in the neutron-rich Ca isotopes. The observed excitation energies and cross-sections point towards extremely localized and well separated strength distributions, with some fragmentation for the $\nu 0f_{7/2}$ orbital in $^{55}$Ca. These results are in good agreement with predictions from shell-model calculations using the effective GXPF1Bs interaction and \textit{ab initio} calculations and diverge markedly from the experimental distributions in the nickel isotones at $Z=28$., Comment: 13 pages, 7 figures

The availability of high-intensity, heavy-ion beams coupled to sensitive, large solid-angleacceptance spectrometers has enabled a detailed examination of the fission fragments produced in induced-fission reactions. The abrasion-fission process involves the formation of projectile-like prefragments in violent nuclear collisions at relative energies in excess of 100 MeV/u. At intermediate energies below this threshold, experiments suggest a change in the prefragment kinematic qualities. Information regarding the influence of this transitional phase upon the evolution of nuclei approaching the point of scission is scarce. In this article, data are presented for over 200 nuclei from nickel to palladium produced in abrasion-fission reactions of a 80 MeV/u 238U beam. Cross sections were obtained following yield measurements performed for the principal charge states of the identified fission fragments and a detailed analysis of the ion transmission. A full kinematic analysis of the fission fragments has been performed using the LISE++ software package, where the trajectory of an ion passing through a spectrometer can be reconstructed based upon measurements at the focal plane. The results obtained at the S800 spectrograph are compared with predictions obtained with a three-fission progenitor (3EER) model. Systematic studies of fission-fragment properties continue to provide a valuable experimental benchmark for theoretical efforts directed toward describing this complex decay channel, that is important in the context of planning experiments to explore the neutron-rich region of the nuclear chart at rare-isotope beam facilities., Comment: 14 pages, 18 figures

Excited states in the $T_z = 0, -1$ nuclei $^{62}$Ga and $^{62}$Ge were populated in direct reactions of relativistic radioactive ion beams at the RIBF. Coincident \grays were measured with the DALI2$^+$ array and uniquely assigned to the $A=62$ isobars. In addition, $^{62}$Ge was also studied independently at JYFL-ACCLAB using the ${}^{24}$Mg(${}^{40}$Ca,$2n$)${}^{62}$Ge fusion-evaporation reaction. The first excited $T=1, J^\pi =2^+$ states in $^{62}$Ga and $^{62}$Ge were identified at $979(1)$ and $965(1)$~keV, respectively, resolving discrepant interpretations in the literature. States beyond the first $2^+$ state in $^{62}$Ge were also identified for the first time in the present work. The results are compared with shell-model calculations in the $fp$ model space. Mirror and triplet energy differences are analyzed in terms of individual charge-symmetry and charge-independence breaking contributions. The MED results confirm the shrinkage of the $p$-orbits' radii when they are occupied by at least one nucleon on average., Comment: aceepted Phys. Lett. B

Gamma decays were observed in $^{56}$Ca and $^{58}$Ca following quasi-free one-proton knockout reactions from $^{57,59}$Sc beams at $\approx 200$ MeV/nucleon. For $^{56}$Ca, a $\gamma$ ray transition was measured to be 1456(12) keV, while for $^{58}$Ca an indication for a transition was observed at 1115(34) keV. Both transitions were tentatively assigned as the $2^+_1 \rightarrow 0^+_{gs}$ decays, and were compared to results from ab initio and conventional shell-model approaches. A shell-model calculation in a wide model space with a marginally modified effective nucleon-nucleon interaction depicts excellent agreement with experiment for $2^+_1$ level energies, two-neutron separation energies, and reaction cross sections, corroborating the formation of a new nuclear shell above the $N$ = 34 shell. Its constituents, the $0f_{5/2}$ and $0g_{9/2}$ orbitals, are almost degenerate. This degeneracy precludes the possibility for a doubly magic $^{60}$Ca and potentially drives the dripline of Ca isotopes to $^{70}$Ca or even beyond., Comment: 8 pages, 3 figures

The atomic masses of $^{55}$Sc, $^{56,58}$Ti, and $^{56-59}$V have been determined using the high-precision multi-reflection time-of-flight technique. The radioisotopes have been produced at RIKEN's RIBF facility and delivered to the novel designed gas cell and multi-reflection system (ZD MRTOF), which has been recently commissioned downstream of the ZeroDegree spectrometer following the BigRIPS separator. For $^{56,58}$Ti and $^{56-59}$V the mass uncertainties have been reduced down to the order of $10\,\mathrm{keV}$, shedding new light on the $N=34$ shell effect in Ti and V isotopes by the first high-precision mass measurements of the critical species $^{58}$Ti and $^{59}$V. With the new precision achieved, we reveal the non-existence of the $N=34$ empirical two-neutron shell gaps for Ti and V, and the enhanced energy gap above the occupied $\nu p_{3/2}$ orbit is identified as a feature unique to Ca. We perform new Monte Carlo shell model calculations including the $\nu d_{5/2}$ and $\nu g_{9/2}$ orbits and compare the results with conventional shell model calculations, which exclude the $\nu g_{9/2}$ and the $\nu d_{5/2}$ orbits. The comparison indicates that the shell gap reduction in Ti is related to a partial occupation of the higher orbitals for the outer two valence neutrons at $N=34$.

Background: The nucleus $^{32}$Mg ($N=20$ and $Z=12$) plays a central role in the so-called "island of inversion" where in the ground states $sd$-shell neutrons are promoted to the $fp$-shell orbitals across the shell gap, resulting in the disappearance of the canonical neutron magic number $N=20$. Purpose: The primary goals of this work are to extend the level scheme of $^{32}$Mg, provide spin-parity assignments to excited states, and discuss the microscopic structure of each state through comparisons with theoretical calculations. Method: In-beam $\gamma$-ray spectroscopy of $^{32}$Mg was performed using two direct-reaction probes, one-neutron (two-proton) knockout reactions on $^{33}$Mg ($^{34}$Si). Final-state exclusive cross sections and parallel momentum distributions were extracted from the experimental data and compared with eikonal-based reaction model calculations combined with shell-model overlap functions. Results: Owing to the remarkable selectivity of the one-neutron and two-proton knockout reactions, a significantly updated level scheme for $^{32}$Mg, which exhibits negative-parity intruder and positive-parity normal states, was constructed. The experimental results were confronted with four different nuclear structure models. Conclusions: In some of these models, different aspects of $^{32}$Mg and the transition into the island of inversion are well described. However, unexplained discrepancies remain, and even with the help of these state-of-the-art theoretical approaches, the structure of this key nucleus is not yet fully captured.

States in the $N=35$ and 37 isotopes $^{55,57}$Ca have been populated by direct proton-induced nucleon removal reactions from $^{56,58}$Sc and $^{56}$Ca beams at the RIBF. In addition, the $(p,2p)$ quasi-free single-proton removal reaction from $^{56}$Ca was studied. Excited states in $^{55}$K, $^{55}$Ca, and $^{57}$Ca were established for the first time via in-beam $\gamma$-ray spectroscopy. Results for the proton and neutron removal reactions from $^{56}$Ca to states in $^{55}$K and $^{55}$Ca for the level energies, excited state lifetimes, and exclusive cross sections agree well with state-of-the-art theoretical calculations using different approaches. The observation of a short-lived state in $^{57}$Ca suggests a transition in the calcium isotopic chain from single-particle dominated states at $N=35$ to collective excitations at $N=37$., Comment: accepted Phys. Lett. B

A first gamma-ray study of 47,49Cl spectroscopy was performed at the Radioactive Isotope Beam Factory with 50Ar projectiles at 217 MeV/nucleon, impinging on the liquid hydrogen target of the MINOS device. Prompt de-excitation gamma-rays were measured with the NaI(Tl) array DALI2+. Through the one-proton knockout reaction 50Ar(p,2p), a spin assignment could be determined for the low-lying states of 49Cl from the momentum distribution obtained with the SAMURAI spectrometer. A spin-parity J = 3/2+ is deduced for the ground state of 49Cl, similar to the recently studied N = 32 isotope 51K.

Situated in the so-called "island of inversion," the nucleus $^{32}$Mg is considered as an archetypal example of the disappearance of magicity at $N=20$. We report on high statistics in-beam spectroscopy of $^{32}$Mg with a unique approach, in that two direct reaction probes with different sensitivities to the underlying nuclear structure are employed at the same time. More specifically, states in $^{32}$Mg were populated by knockout reactions starting from $^{33}$Mg and $^{34}$Si, lying inside and outside the island of inversion, respectively. The momentum distributions of the reaction residues and the cross sections leading to the individual final states were confronted with eikonal-based reaction calculations, yielding a significantly updated level scheme for $^{32}$Mg and spin-parity assignments. By fully exploiting observables obtained in this measurement, a variety of structures coexisting in 32Mg was unraveled. Comparisons with theoretical predictions based on shell-model overlaps allowed for clear discrimination between different structural models, revealing that the complete theoretical description of this key nucleus is yet to be achieved.

Neutron-rich nuclei in the vicinity of the $N=40$ island of inversion are characterized by shell evolution and exhibit deformed ground states. In several nuclei isomeric states have been observed and attributed to excitations to the intruder neutron $1g_{9/2}$ orbital. In the present study we searched for isomeric states in nuclei around $N=40$, $Z=22$ produced by projectile fragmentation at RIBF. Delayed $\gamma$ rays were detected by the EURICA germanium detector array. High statistics data allowed for an updated decay scheme of $^{60}$V. The lifetime of an isomeric state in $^{64}$V was measured for the first time in the present experiment. A previously unobserved isomeric state was discovered in $^{58}$Sc. The measured lifetime suggests a parity changing transition, originating from an odd number of neutrons in the $1g_{9/2}$ orbital. The nature of the isomeric state in $^{58}$Sc is thus different from isomers in the less exotic V and Sc nuclei., Comment: PRC accepted

We studied the proton-rich $T_z=-1$ nucleus $^{70}$Kr through inelastic scattering at intermediate energies in order to extract the reduced transition probability, $B(E2;\;0^+ \rightarrow 2^+)$. Comparison with the other members of the $A=70$ isospin triplet, $^{70}$Br and $^{70}$Se, studied in the same experiment, shows a $3\sigma$ deviation from the expected linearity of the electromagnetic matrix elements as a function of $T_z$. At present, no established nuclear structure theory can describe this observed deviation quantitatively. This is the first violation of isospin symmetry at this level observed in the transition matrix elements. A heuristic approach may explain the anomaly by a shape change between the mirror nuclei $^{70}$Kr and $^{70}$Se contrary to the model predictions., Comment: accepted PRL

Low-lying excited states in the $N=32$ isotope $^{50}$Ar were investigated by in-beam $\gamma$-ray spectroscopy following proton- and neutron-knockout, multi-nucleon removal, and proton inelastic scattering at the RIKEN Radioactive Isotope Beam Factory. The energies of the two previously reported transitions have been confirmed, and five additional states are presented for the first time, including a candidate for a 3$^-$ state. The level scheme built using $\gamma\gamma$ coincidences was compared to shell-model calculations in the $sd-pf$ model space, and to ab initio predictions based on chiral two- and three-nucleon interactions. Theoretical proton- and neutron-knockout cross sections suggest that two of the new transitions correspond to $2^+$ states, while the previously proposed $4^+$ state could also correspond to a $2^+$ state., Comment: Accepted for Phys. Rev. C

Background: In the "island of inversion", ground states of neutron-rich $sd$-shell nuclei exhibit strong admixtures of intruder configurations from the $fp$ shell. The nucleus $^{30}$Mg, located at the boundary of the island of inversion, serves as a cornerstone to track the structural evolution as one approaches this region. Purpose: Spin-parity assignments for excited states in $^{30}$Mg, especially negative-parity levels, have yet to be established. In the present work, the nuclear structure of $^{30}$Mg was investigated by in-beam $\gamma$-ray spectroscopy mainly focusing on firm spin-parity determinations. Method: High-intensity rare-isotope beams of $^{31}$Mg, $^{32}$Mg, $^{34}$Si, and $^{35}$P bombarded a Be target to induce nucleon removal reactions populating states in $^{30}$Mg. $\gamma$ rays were detected by the state-of-the-art $\gamma$-ray tracking array GRETINA. For the direct one-neutron removal reaction, final-state exclusive cross sections and parallel momentum distributions were deduced. Multi-nucleon removal reactions from different projectiles were exploited to gain complementary information. Results: With the aid of the parallel momentum distributions, an updated level scheme with revised spin-parity assignments was constructed. Spectroscopic factors associated with each state were also deduced. Conclusions: Results were confronted with large-scale shell-model calculations using two different effective interactions, showing excellent agreement with the present level scheme. However, a marked difference in the spectroscopic factors indicates that the full delineation of the transition into the island of inversion remains a challenge for theoretical models.

The single-particle structure of the $N=27$ isotones provides insights into the shell evolution of neutron-rich nuclei from the doubly-magic $^{48}$Ca toward the drip line. $^{43}$S was studied employing the one-neutron knockout reaction from a radioactive $^{44}$S beam. Using a combination of prompt and delayed $\gamma$-ray spectroscopy the level structure of $^{43}$S was clarified. Momentum distributions were analyzed and allowed for spin and parity assignments. The deduced spectroscopic factors show that the $^{44}$S ground-state configuration has a strong intruder component. The results were confronted with shell model calculations using two effective interactions. General agreement was found between the calculations, but strong population of states originating from the removal of neutrons from the $2p_{3/2}$ orbital in the experiment indicates that the breakdown of the $N=28$ magic number is more rapid than the theoretical calculations suggest., Comment: accepted for publication, Physical Review C

The structure of $^{33}$Si was studied by a one-neutron knockout reaction from a $^{34}$Si beam at 98.5 MeV/u incident on a $^{9}$Be target. The prompt $\gamma$-rays following the de-excitation of $^{33}$Si were detected using the GRETINA $\gamma$-ray tracking array while the reaction residues were identified on an event-by-event basis in the focal plane of the S800 spectrometer at NSCL (National Superconducting Cyclotron Laboratory). The presently derived spectroscopic factor values, $C^2S$, for the 3/2$^+$ and 1/2$^+$ states, corresponding to a neutron removal from the $0d_{3/2}$ and $1s_{1/2}$ orbitals, agree with shell model calculations and point to a strong $N=20$ shell closure. Three states arising from the more bound $0d_{5/2}$ orbital are proposed, one of which is unbound by about 930 keV. The sensitivity of this experiment has also confirmed a weak population of 9/2$^-$ and 11/2$_{1,2}^-$ final states, which originate from a higher-order process. This mechanism may also have populated, to some fraction, the 3/2$^-$ and 7/2$^-$ negative-parity states, which hinders a determination of the $C^2S$ values for knockout from the normally unoccupied $1p_{3/2}$ and $0f_{7/2}$ orbits.

Background: Neutron-rich nuclei around neutron number N = 60 show a dramatic shape transition from spherical ground states to prolate deformation in 98Sr and heavier nuclei. Purpose: The purpose of this study is to investigate the single-particle structure approaching the shape transitional region. Method: The level structures of neutron-rich 93,94,95Sr were studied via the d(94,95,96Sr,t) one-neutron stripping reactions at TRIUMF using a beam energy of 5.5 AMeV. {\gamma}-rays emitted from excited states and recoiling charged particles were detected by using the TIGRESS and SHARC arrays, respectively. States were identified by gating on the excitation energy and, if possible, the coincident {\gamma} radiation. Results: Triton angular distributions for the reactions populating states in ejectile nuclei 93,94,95Sr were compared with distorted wave Born approximation calculations to assign and revise spin and parity quantum numbers and extract spectroscopic factors. The results were compared with shell model calculations and the reverse (d,p) reactions and good agreement was obtained. Conclusions: The results for the d(94Sr,t)93Sr and d(95Sr,t)94Sr reactions are in good agreement with shell model calculations. A two level mixing analysis for the 0+ states in 94Sr suggest strong mixing of two shapes. For the d(96Sr,t)95Sr reaction the agreement with the shell model is less good. The configuration of the ground state of 96Sr is already more complex than predicted, and therefore indications for the shape transition can already be observed before N = 60., Comment: accepted for publication in PRC

The $N=Z=36$ nucleus $^{72}$Kr has been studied by inelastic scattering at intermediate energies. Two targets, $^{9}$Be and $^{197}$Au, were used to extract the nuclear deformation length, $\delta_\text{N}$, and the reduced $E2$ transition probability, $B(E2)$. The previously unknown non-yrast $2^+$ and $4^+$ states as well as a new candidate for the octupole $3^-$ state have been observed in the scattering on the Be target and placed in the level scheme based on $\gamma-\gamma$ coincidences. The second $2^+$ state was also observed in the scattering on the Au target and the $B(E2;\;2^+_2 \rightarrow 0^+_1)$ value could be determined for the first time. Analyzing the results in terms of a two-band mixing model shows clear evidence for a oblate-prolate shape coexistence and can be explained by a shape change from an oblate ground state to prolate deformed yrast band from the first $2^+$ state. This interpretation is corroborated by beyond mean field calculations using the Gogny D1S interaction., Comment: 12 pages, 11 figures, accepted European Physical Journal A

We report the measurement of reaction cross sections ($\sigma_R^{\rm ex}$) of $^{27,29}$F with a carbon target at RIKEN. The unexpectedly large $\sigma_R^{\rm ex}$ and derived matter radius identify $^{29}$F as the heaviest two-neutron Borromean halo to date. The halo is attributed to neutrons occupying the $2p_{3/2}$ orbital, thereby vanishing the shell closure associated with the neutron number $N = 20$. The results are explained by state-of-the-art shell model calculations. Coupled-cluster computations based on effective field theories of the strong nuclear force describe the matter radius of $^{27}$F but are challenged for $^{29}$F., Comment: 8 pages, 4 figures

We present mass excesses (ME) of neutron-rich isotopes of Ar through Fe, obtained via TOF-$B\rho$ mass spectrometry at the National Superconducting Cyclotron Laboratory. Our new results have significantly reduced systematic uncertainties relative to a prior analysis, enabling the first determination of ME for $^{58,59}{\rm Ti}$, $^{62}{\rm V}$, $^{65}{\rm Cr}$, $^{67,68}{\rm Mn}$, and $^{69,70}{\rm Fe}$. Our results show the $N=34$ subshell weaken at Sc and vanish at Ti, along with the absence of an $N=40$ subshell at Mn. This leads to a cooler accreted neutron star crust, highlighting the connection between the structure of nuclei and neutron stars., Comment: Accepted for publication as a Rapid Communication in Physical Review C

Excited states in the $N=40$ isotone $^{62}$Ti were populated via the $^{63}$V$(p,2p)$$^{62}$Ti reaction at $\sim$200~MeV/u at the Radioactive Isotope Beam Factory and studied using $\gamma$-ray spectroscopy. The energies of the $2^+_1 \rightarrow 0^{+}_{\mathrm{gs}}$ and $4^+_1 \rightarrow 2^+_1$ transitions, observed here for the first time, indicate a deformed $^{62}$Ti ground state. These energies are increased compared to the neighboring $^{64}$Cr and $^{66}$Fe isotones, suggesting a small decrease of quadrupole collectivity. The present measurement is well reproduced by large-scale shell-model calculations based on effective interactions, while ab initio and beyond mean-field calculations do not yet reproduce our findings. The shell-model calculations for $^{62}$Ti show a dominant configuration with four neutrons excited across the $N=40$ gap. Likewise, they indicate that the $N=40$ island of inversion extends down to $Z=20$, disfavoring a possible doubly magic character of the elusive $^{60}$Ca.

The region around neutron number N = 60 in the neutron-rich Sr and Zr nuclei is one of the most dramatic examples of a ground state shape transition from (near) spherical below N = 60 to strongly deformed shapes in the heavier isotopes. The single-particle structure of 95-97Sr approaching the ground state shape transition at 98 Sr has been investigated via single-neutron transfer reactions using the (d, p) reaction in inverse kinematics. These reactions selectively populate states with a large overlap of the projectile ground state coupled to a neutron in a single-particle orbital. Radioactive 94,95,96Sr nuclei with energies of 5.5 AMeV were used to bombard a CD 2 target. Recoiling light charged particles and {\gamma} rays were detected using a quasi-4{\pi} silicon strip detector array and a 12 element Ge array. The excitation energy of states populated was reconstructed employing the missing mass method combined with {\gamma}-ray tagging and differential cross sections for final states were extracted. A reaction model analysis of the angular distributions allowed for firm spin assignments to be made for the low-lying 352, 556 and 681 keV excited states in 95Sr and a constraint has been placed on the spin of the higher-lying 1666 keV state. Angular distributions have been extracted for 10 states populated in the d(95Sr,p)96Sr reaction, and constraints have been provided for the spins and parities of several final states. Results are compared to shell model calculations in several model spaces and the structure of low-lying states in 94Sr and 95Sr is well-described. The spectroscopic strength of the 0+ and 2 states in 96Sr is significantly more fragmented than predicted., Comment: PRC accepted

Collective states in cold nuclei are represented by a wave function that assigns coherent phases to the participating nucleons. The degree of coherence decreases with excitation energy above the yrast line because of coupling to the increasingly dense background of quasiparticle excitations. The consequences of decoherence are discussed, starting with the well studied case of rotational damping. In addition to superdeformed bands, a highly excited oblate band is presented as a new example of screening from rotational damping. Suppression of pair correlation leads to incoherent thermal M1 radiation, which appears as an exponential spike (LEMAR) at zero energy in the $\gamma$ strength function of spherical nuclei. In deformed nuclei a Scissors Resonance appears and LEMAR changes to damped magnetic rotation, which is interpreted as partial restoration of coherence., Comment: Proceedings of the conference on Nuclear structure and dynamics 2019

A pronounced spike at low energy in the strength function for magnetic radiation (LEMAR) is found by means of Shell Model calculations, which explains the experimentally observed enhancement of the dipole strength. LEMAR originates from statistical low-energy M1-transitions between many excited complex states. Re-coupling of the proton and neutron high-j orbitals generates the strong magnetic radiation. LEMAR is predicted for nuclides with $A\approx 132$ participating in the r-process of element synthesis. It increases the reaction rates by a factor of 2.5. The spectral function of LEMAR follows Planck's Law. A power law for the size distribution of the $B(M1)$ values are found., Comment: arXiv admin note: text overlap with arXiv:1407.1721

The proton-rich isotope 68Br was discovered in secondary fragmentation reactions of fast radioactive beams. Proton-rich secondary beams of 70,71,72Kr and 70Br, produced at the RIKEN Nishina Center and identified by the BigRIPS fragment separator, impinged on a secondary 9Be target. Unambiguous particle identification behind the secondary target was achieved with the ZeroDegree spectrometer. Based on the expected direct production cross sections from neighboring isotopes, the lifetime of the ground or long-lived isomeric state of 68Br was estimated. The results suggest that secondary fragmentation reactions, where relatively few nucleons are removed from the projectile, offer an alternative way to search for new isotopes, as these reactions populate preferentially low-lying states., Comment: Phys. Lett. B accepted

The tetraneutron has been drawing the attention of the nuclear physics community for decades, but a firm conclusion on its existence and properties is still far from being reached despite many experimental and theoretical efforts. New measurements have recently been performed at RIBF with the SAMURAI spectrometer by applying complementary reaction probes, which will help to pin down the properties of this four-neutron system., Comment: 5 pages, 2 figures, conference proceedings of XXII International Conference on Few-Body Problems in Physics (FB22), Caen, France, July 2018

Isomeric states in 59,61Ti have been populated in the projectile fragmentation of a 345 AMeV 238U beam at the Radioactive Isotope Beam Factory. The decay lifetimes and delayed gamma-ray transitions were measured with the EURICA array. Besides the known isomeric state in 59Ti, two isomeric states in 61Ti are observed for the first time. Based on the measured lifetimes, transition multipolarities as well as tentative spins and parities are assigned. Large-scale shell model calculations based on the modified LNPS interaction show that both 59Ti and 61Ti belong to the Island of Inversion at N=40 with ground state configurations dominated by particle-hole excitations to the g_9/2 and d_5/2 orbits., Comment: Phys. Lett. B accepted

Background: Shell evolution can impact the structure of the nuclei and lead to effects such as shape coexistence. The nuclei around $^{68}$Ni represent an excellent study case, however, spectroscopic information of the neutron-rich, $Z<28$ nuclei is limited. Purpose: The goal is to measure $\gamma$-ray transitions in $^{66}$Fe, $^{66}$Co and $^{66}$Ni populated in the $\beta^-$ decay of $^{66}$Mn, to determine absolute $\beta$-feedings and relative $\gamma$-decay probabilities and to compare the results with Monte Carlo Shell Model calculations in order to study the influence of the relevant single neutron and proton orbitals occupancies around $Z=28$ and $N=40$. Method: The low-energy structures of $^{65,66}$Fe, $^{66}$Co and $^{66}$Ni were studied in the $\beta^-$ decay of $^{66}$Mn produced at ISOLDE, CERN. The beam was purified by means of laser resonance ionization and mass separation. The $\beta$ and $\gamma$ events detected by three plastic scintillators and two MiniBall cluster germanium detectors, respectively, were correlated in time to build the low-energy excitation schemes and to determine the $\beta$-decay half-lives of the nuclei. Results: The relative small $\beta$-decay ground state feeding of $^{66}$Fe obtained in this work is at variant to the earlier studies. Spin and parity $1^+$ was assigned to the $^{66}$Co ground state based on the strong ground state feeding in the decay of $^{66}$Fe as well as in the decay of $^{66}$Co. Experimental log(ft) values, $\gamma$-ray deexcitation patterns and energies of excited states were compared to Monte Carlo Shell Model calculations. Based on this comparison, spin and parity assignments for the selected number of low-lying states in the $^{66}$Mn to $^{66}$Ni chain were proposed. Conclusions: The $\beta$-decay chain starting $^{66}$Mn towards $^{66}$Ni, crossing $N=40$, evolves from deformed nuclei to sphericity..., Comment: accepted for publication in PRC

The first $\gamma$-ray spectroscopy of $^{52}$Ar, with the neutron number N = 34, was measured using the $^{53}$K(p,2p) one-proton removal reaction at $\sim$210 MeV/u at the RIBF facility. The 2$^{+}_{1}$ excitation energy is found at 1656(18) keV, the highest among the Ar isotopes with N $>$ 20. This result is the first experimental signature of the persistence of the N = 34 subshell closure beyond $^{54}$Ca, i.e., below the magic proton number Z = 20. Shell-model calculations with phenomenological and chiral-effective-field-theory interactions both reproduce the measured 2$^{+}_{1}$ systematics of neutron-rich Ar isotopes, and support a N = 34 subshell closure in $^{52}$Ar.

The low energy excited $0_{2,3}^+$ states in $^{96}$Sr are amongst the most prominent examples of shape coexistence across the nuclear landscape. In this work, the neutron $[2s_{1/2}]^2$ content of the $0_{1,2,3}^+$ states in $^{96}$Sr was determined by means of the d($^{95}$Sr,p) transfer reaction at the TRIUMF-ISAC2 facility using the SHARC and TIGRESS arrays. Spectroscopic factors of 0.19(3) and 0.22(3) were extracted for the $^{96}$Sr ground and 1229~keV $0^+$ states, respectively, by fitting the experimental angular distributions to DWBA reaction model calculations. A detailed analysis of the $\gamma$-decay of the isomeric $0_3^+$ state was used to determine a spectroscopic factor of 0.33(13). The experimental results are compared to shell model calculations, which predict negligible spectroscopic strength for the excited $0^+$ states in $^{96}$Sr. The strengths of the excited $0_{2,3}^+$ states were also analyzed within a two-level mixing model and are consistent with a mixing strength of $a^2$=0.40(14) and a difference in intrinsic deformations of $|\Delta \beta|=0.31(3)$. These results suggest coexistence of three different configurations in $^{96}$Sr and strong shape mixing of the two excited $0^+$ states., Comment: Phys Lett B accepted

Excited states in the $T_z=-1$ nucleus $^{70}$Kr have been populated using inelastic scattering of a radioactive $^{70}$Kr beam as well as one- and two-neutron removal reactions from $^{71,72}$Kr at intermediate beam energies. The level scheme of $^{70}$Kr was constructed from the observed $\gamma$-ray transitions and coincidences. Tentative spin and parity assignments were made based on comparison with the mirror nucleus $^{70}$Se. A second $2^+$ state and a candidate for the corresponding $4^+_2$ state suggest shape coexistence in $^{70}$Kr., Comment: accepted Phys. Lett. B

Many properties of the atomic nucleus, such as vibrations, rotations and incompressibility, can be interpreted as due to a two component quantum liquid of protons and neutrons. Electron scattering measurements on stable nuclei demonstrate that their central densities are saturated, as for liquid drops. In exotic nuclei near the limits of mass and charge, with large imbalances in their proton and neutron numbers, the possibility of a depleted central density, or a 'bubble' structure, has been discussed in a recurrent manner since the 1970s. Here we report first experimental evidence that points to a depletion of the central density of protons in the short-lived nucleus 34Si. The proton-to-neutron density asymmetry in 34Si offers the possibility to place constraints on the density and isospin dependence of the spin--orbit force-on which nuclear models have disagreed for decades-and on its stabilizing effect towards limits of nuclear existence., Comment: 6 pages, 3 figures

The low-lying energy levels of proton-rich $^{56}$Cu have been extracted using in-beam $\gamma$-ray spectroscopy with the state-of-the-art $\gamma$-ray tracking array GRETINA in conjunction with the S800 spectrograph at the National Superconducting Cyclotron Laboratory at Michigan State University. Excited states in $^{56}$Cu serve as resonances in the $^{55}$Ni(p,$\gamma$)$^{56}$Cu reaction, which is a part of the rp-process in type I x-ray bursts. To resolve existing ambiguities in the reaction Q-value, a more localized IMME mass fit is used resulting in $Q=639\pm82$~keV. We derive the first experimentally-constrained thermonuclear reaction rate for $^{55}$Ni(p,$\gamma$)$^{56}$Cu. We find that, with this new rate, the rp-process may bypass the $^{56}$Ni waiting point via the $^{55}$Ni(p,$\gamma$) reaction for typical x-ray burst conditions with a branching of up to $\sim$40$\%$. We also identify additional nuclear physics uncertainties that need to be addressed before drawing final conclusions about the rp-process reaction flow in the $^{56}$Ni region., Comment: 8 pages, accepted for Phys. Rev. C

The calcium isotopes have emerged as an important testing ground for new microscopically derived shell-model interactions, and a great deal of focus has been directed toward this region. We investigate the relative spectroscopic strengths associated with $1f_{7/2}$ neutron hole states in $^{47, 49}$Ca following one-neutron knockout reactions from $^{48,50}$Ca. The observed reduction of strength populating the lowest 7/2$^{-}_{1}$ state in $^{49}$Ca, as compared to $^{47}$Ca, is consistent with the description given by shell-model calculations based on two- and three-nucleon forces in the neutron $pf$ model space, implying a fragmentation of the $l$=3 strength to higher-lying states. The experimental result is inconsistent with both the GXPF1 interaction routinely used in this region of the nuclear chart and with microscopic calculations in an extended model space including the $\nu1g_{9/2}$ orbital., Comment: 6 pages, 2 figures

In an experiment with the BigRIPS separator at the RIKEN Nishina Center, the fragmentation of a $^{78}$Kr beam allowed the observation of new neutron-deficient isotopes at the proton drip-line. Clean identification spectra could be produced and $^{63}$Se, $^{67}$Kr, and $^{68}$Kr were identified for the first time. In addition, $^{59}$Ge was also observed. Three of these isotopes, $^{59}$Ge, $^{63}$Se, and $^{67}$Kr, are potential candidates for ground-state two-proton radioactivity. In addition, the isotopes $^{58}$Ge, $^{62}$Se, and $^{66}$Kr were also sought but without success. The present experiment also allowed the determination of production cross sections for some of the most exotic isotopes. These measurements confirm the trend already observed that the empirical parameterization of fragmentation cross sections, EPAX, significantly overestimates experimental cross sections in this mass region., Comment: accepted as Rapid Communication in PRC