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The Zirconium (Z = 40) isotopic chain has attracted interest for more than four decades. The abrupt lowering of the energy of the first $2^+$ state and the increase in the transition strength B(E2; $2_1^\rightarrow 0_1^+$ going from $^{98}$Zr to $^{100}$Zr has been the first example of "quantum phase transition" in nuclear shapes, which has few equivalents in the nuclear chart. Although a multitude of experiments have been performed to measure nuclear properties related to nuclear shapes and collectivity in the region, none of the measured lifetimes were obtained using the Recoil Distance Doppler Shift method in the $\gamma\gamma$-coincidence mode where a gate on the direct feeding transition of the state of interest allows a strict control of systematical errors. This work reports the results of lifetime measurements for the first yrast excited states in $^{98-104}$Zr carried out to extract reduced transition probabilities. The new lifetime values in $\gamma\gamma$-coincidence and $\gamma$-single mode are compared with the results of former experiments. Recent predictions of the Interacting Boson Model with Configuration Mixing, the Symmetry Conserving Configuration Mixing model based on the Hartree-Fock-Bogoliubov approach and the Monte Carlo Shell Model are presented and compared with the experimental data.

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 radiative branching ratio of the Hoyle state is crucial to estimate the triple-$\alpha$ reaction rate in stellar environments at medium temperatures. Knowledge of the $\gamma$-decay channel is critical as this is the dominant radiative decay channel for the Hoyle state. A recent study by Kib\'edi et al. [Phys. Rev. Lett. 125, 182701 (2020)] has challenged our understanding of this astrophysically significant branching ratio and its constraints. The objective of this work was to perform a new measurement of the $\gamma$-decay branching ratio of the Hoyle state to deduce the radiative branching ratio of the Hoyle state. An additional objective was to independently verify aspects of the aforementioned measurement conducted by Kib\'edi et al. For the main experiment of this work, the Hoyle state was populated by the $^{12}\textrm{C}(p,p')$ reaction at 10.8 MeV at the Oslo Cyclotron Laboratory. The $\gamma$-decay branching ratio was deduced through triple-coincidence events, each consisting of a proton ejectile corresponding to the Hoyle state, and the subsequent $\gamma$-ray cascade. In the main experiment of this work, a $\gamma$-decay branching ratio of the Hoyle state of $\Gamma_{\gamma}/\Gamma=4.0(4)\times 10^{-4}$ was determined, yielding a corresponding radiative branching ratio of $\Gamma_{\textrm{rad}}/\Gamma=4.1(4) \times 10^{-4}$, which is in agreement with several recent studies, as well as the previously adopted ENSDF average of $\Gamma_{\textrm{rad}}/\Gamma=4.16(11)\times 10^{-4}$. Aspects of the analysis performed by Kib\'edi et al. were verified in this work and the source of discrepancy between the results of this work and that of Kib\'edi et al. could not be determined. Further independent and innovative studies for the radiative width of the Hoyle state will substantiate whether the discrepant result by Kib\'edi et al. should be excluded from future evaluations.

The $^{111,112,113}$Sn isotopes have been studied with ($p,d \gamma$), ($p,p^{\prime} \gamma$), and ($d,p \gamma$) reactions to extract the nuclear level densities (NLDs) and $\gamma$-ray strength functions (GSFs) of these nuclei below the neutron separation energy by means of the Oslo method. The experimental NLDs for all three nuclei demonstrate a trend compatible with the constant-temperature model below the neutron separation energy while also being in good agreement with the NLDs of neighboring Sn isotopes, obtained previously with the Oslo-type and neutron evaporation experiments. The extracted microcanonical entropies yield $\approx 1.5$ $k_B$ entropy of a valence neutron in both $^{111}$Sn and $^{113}$Sn. Moreover, the deduced microcanonical temperatures indeed suggest a clear constant-temperature behavior above $\approx$ 3 MeV in $^{111,113}$Sn and above $\approx$ 4.5 MeV in $^{112}$Sn. We observe signatures for the first broken neutron pairs between 2 and 4 MeV in all three nuclei. The GSFs obtained with the Oslo method are found to be in good agreement below the neutron threshold with the strengths of $^{112,114}$Sn extracted in the ($p,p^{\prime}$) Coulomb excitation experiments., Comment: 13 pages, 9 figures

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

In this work, we present new data on the $^{182,183,184}$W($\gamma,n$) cross sections, utilizing a quasi-monochromatic photon beam produced at the NewSUBARU synchrotron radiation facility. Further, we have extracted the nuclear level density and $\gamma$-ray strength function of $^{186}$W from data on the $^{186}$W($\alpha,\alpha^\prime\gamma$)$^{186}$W reaction measured at the Oslo Cyclotron Laboratory. Combining previous measurements on the $^{186}$W($\gamma,n$) cross section with our new $^{182,183,184}$W($\gamma,n$) and ($\alpha,\alpha^\prime\gamma$)$^{186}$W data sets, we have deduced the $^{186}$W $\gamma$-ray strength function in the range of $1 < E_\gamma < 6$ MeV and $7 < E_\gamma < 14$ MeV. Our data are used to extract the level density and $\gamma$-ray strength functions needed as input to the nuclear-reaction code \textsf{TALYS}, providing an indirect, experimental constraint for the $^{185}$W($n,\gamma$)$^{186}$W cross section and reaction rate. Compared to the recommended Maxwellian-averaged cross section (MACS) in the KADoNiS-1.0 data base, our results are on average lower for the relevant energy range $k_B T \in [5,100]$ keV, and we provide a smaller uncertainty for the MACS. The theoretical values of Bao \textit{et al.} and the cross section experimentally constrained on photoneutron data of Sonnabend \textit{et al.} are significantly higher than our result. The lower value by Mohr \textit{et al.} is in very good agreement with our deduced MACS. Our new results could have implications for the $s$-process and in particular the predicted $s$-process production of $^{186,187}$Os nuclei., Comment: 17 pages, 15 figures; to be submitted to Phys. Rev. C

Nuclear level densities (NLDs) and $\gamma$-ray strength functions (GSFs) of $^{120,124}$Sn have been extracted with the Oslo method from proton-$\gamma$ coincidences in the ($p,p^{\prime}\gamma)$ reaction. The functional forms of the GSFs and NLDs have been further constrained with the Shape method by studying primary $\gamma$-transitions to the ground and first excited states.The NLDs demonstrate good agreement with the NLDs of $^{116,118,122}$Sn isotopes measured previously. Moreover, the extracted partial NLD of 1$^{-}$ levels in $^{124}$Sn is shown to be in fair agreement with those deduced from spectra of relativistic Coulomb excitation in forward-angle inelastic proton scattering. The experimental NLDs have been applied to estimate the magnitude of the Porter-Thomas (PT) fluctuations. Within the PT fluctuations, we conclude that the GSFs for both isotopes can be considered to be independent of initial and final excitation energies, in accordance with the generalized Brink-Axel hypothesis. Particularly large fluctuations observed in the Shape-method GSFs present a considerable contribution to the uncertainty of the method, and may be one of the reasons for deviations from the Oslo-method strength at low $\gamma$-ray energies and low values of the NLD (below $\approx1\cdot10^{3}-2\cdot10^{3}$ MeV$^{-1}$)., Comment: 17 pages, 12 figures

The experimental gamma-ray strength functions (gamma-SFs) of 142,144-151Nd have been studied for gamma-ray energies up to the neutron separation energy. The results represent a unique set of gamma-SFs for an isotopic chain with increasing nuclear deformation. The data reveal how the low-energy enhancement, the scissors mode and the pygmy dipole resonance evolve with nuclear deformation and mass number. The data indicate that the mechanisms behind the low-energy enhancement and the scissors mode are decoupled from each other., Comment: 14 pages and 10 figures

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.

Background: The evolution of shell structure around doubly magic exotic nuclei is of great interest in nuclear physics and astrophysics. In the `southwest' region of $^{78}$Ni, the development of deformation might trigger a major shift in our understanding of explosive nucleosynthesis. To this end, new spectroscopic information on key close-lying nuclei is very valuable. Purpose: We intend to measure the isomeric and $\beta$ decay of $^{75}$Co, with one-proton and two-neutron holes relative to $^{78}$Ni, to access new nuclear structure information in $^{75}$Co and its $\beta$-decay daughters $^{75}$Ni and $^{74}$Ni. Methods: The nucleus $^{75}$Co is produced in relativistic in-flight fission reactions of $^{238}$U at the Radioactive Ion Beam Factory in the RIKEN Nishina Center. Its isomeric and $\beta$ decay are studied exploiting the BigRIPS and EURICA setups. Results: We obtain partial $\beta$-decay spectra for $^{75}$Ni and $^{74}$Ni, and report a new isomeric transition in $^{75}$Co. The energy [$E_{\gamma}=1914(2)$ keV] and half-life [$t_{1/2}=13(6)$ $\mu$s] of the delayed $\gamma$ ray lend support for the existence of a $J^{\pi}=(1/2^-)$ isomeric state at 1914(2) keV. A comparison with PFSDG-U shell-model calculations provides a good account for the observed states in $^{75}$Ni, but the first calculated $1/2^-$ level in $^{75}$Co, a prolate $K=1/2$ state, is predicted about 1 MeV below the observed $(1/2^-)$ level. Conclusions: The spherical-like structure of the lowest-lying excited states in $^{75}$Ni is proved. In the case of $^{75}$Co, the results suggest that the dominance of the spherical configurations over the deformed ones might be stronger than expected below $^{78}$Ni. Further experimental efforts to discern the nature of the $J^{\pi}=(1/2^-)$ isomer are necessary., Comment: 15 pages, 6 figures, 3 tables. Physical Review C

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

The validity of the Brink-Axel hypothesis, which is especially important for numerous astrophysical calculations, is addressed for 116,120,124Sn below the neutron separation energy by means of three independent experimental methods. The $\gamma$-ray strength functions (GSFs) extracted from primary $\gamma$-decay spectra following charged-particle reactions with the Oslo method and with the Shape method demonstrate excellent agreement with those deduced from forward-angle inelastic proton scattering at relativistic beam energies. In addition, the GSFs are shown to be independent of excitation energies and spins of the initial and final states. The results provide a critical test of the generalized Brink-Axel hypothesis in heavy nuclei, demonstrating its applicability in the energy region of the pygmy dipole resonance., Comment: 6 pages, 4 figures, Phys. Rev. Lett. (in press)

Understanding the evolution of level densities in the crossover from spherical to well-deformed nuclei has been a long-standing problem in nuclear physics. We measure nuclear level densities for a chain of neodymium isotopes $^{142,144-151}$Nd which exhibit such a crossover. These results represent to date the most complete data set of nuclear level densities for an isotopic chain between neutron shell-closure and towards mid-shell. We observe a strong increase of the level densities along the chain with an overall increase by a factor of $\approx 170$ at an excitation energy of 7.5 MeV and saturation around mass 150. Level densities calculated by the shell model Monte Carlo (SMMC) are in excellent agreement with these experimental results. Based on our experimental and theoretical findings, we offer an explanation of the observed mass dependence of the level densities in terms of the intrinsic single-particle level density and the collective enhancement., Comment: 7 pages, 4 figures

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

The neutron-rich $^{213}$Pb isotope was produced in the fragmentation of a primary 1 GeV $A$ $^{238}$U beam, separated in FRS in mass and atomic number, and then implanted for isomer decay $\gamma$-ray spectroscopy with the RISING setup at GSI. A newly observed isomer and its measured decay properties indicate that states in $^{213}$Pb are characterized by the seniority quantum number that counts the nucleons not in pairs coupled to angular momentum $J=0$. The conservation of seniority is a consequence of the Berry phase associated with particle-hole conjugation, which becomes gauge invariant and therefore observable in semi-magic nuclei where nucleons half-fill the valence shell. The $\gamma$-ray spectroscopic observables in $^{213}$Pb are thus found to be driven by two mechanisms, particle-hole conjugation and seniority conservation, which are intertwined through the Berry phase.

The cascading 3.21 MeV and 4.44 MeV electric quadrupole transitions have been observed from the Hoyle state at 7.65 MeV excitation energy in $^{12}$C, excited by the $^{12}$C(p,p$^{\prime}$) reaction at 10.7 MeV proton energy. From the proton-$\gamma$-$\gamma$ triple coincidence data, a value of ${\Gamma_{\rm rad}}/{\Gamma}=6.2(6) \times 10^{-4}$ was obtained for the radiative branching ratio. Using our results, together with ${\Gamma_{\pi}^{E0}}/{\Gamma}$ from Eriksen et al., Phys. Rev. C 102, 024320 and the currently adopted $\Gamma_{\pi}(E0)$ values, the radiative width of the Hoyle state is determined as $\Gamma_{\rm rad}=5.1(6) \times 10^{-3}$ eV. This value is about 34% higher than the currently adopted value and will impact on models of stellar evolution and nucleosynthesis., Comment: 6 pages, 6 figures

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

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.

Nuclear magic numbers, which emerge from the strong nuclear force based on quantum chromodynamics, correspond to fully occupied energy shells of protons, or neutrons inside atomic nuclei. Doubly magic nuclei, with magic numbers for both protons and neutrons, are spherical and extremely rare across the nuclear landscape. While the sequence of magic numbers is well established for stable nuclei, evidence reveals modifications for nuclei with a large proton-to-neutron asymmetry. Here, we provide the first spectroscopic study of the doubly magic nucleus $^{78}$Ni, fourteen neutrons beyond the last stable nickel isotope. We provide direct evidence for its doubly magic nature, which is also predicted by ab initio calculations based on chiral effective field theory interactions and the quasi-particle random-phase approximation. However, our results also provide the first indication of the breakdown of the neutron magic number 50 and proton magic number 28 beyond this stronghold, caused by a competing deformed structure. State-of-the-art phenomenological shell-model calculations reproduce this shape coexistence, predicting further a rapid transition from spherical to deformed ground states with $^{78}$Ni as turning point.

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 Oslo Method has been applied to particle-$\gamma$ coincidences following the $^{239}\mathrm{Pu}$(d,p) reaction to obtain the nuclear level density (NLD) and $\gamma$-ray strength function ($\gamma$SF) of $^{240}\mathrm{Pu}$. The experiment was conducted with a 12 MeV deuteron beam at the Oslo Cyclotron Laboratory. The low spin transfer of this reaction leads to a spin-parity mismatch between populated and intrinsic levels. This is a challenge for the Oslo Method as it can have a significant impact on the extracted NLD and $\gamma$SF. We have developed an iterative approach to ensure consistent results even for cases with a large spin-parity mismatch, in which we couple Green's Function Transfer calculations of the spin-parity dependent population cross-section to the nuclear decay code RAINIER. The resulting $\gamma$SF shows a pronounced enhancement between 2-4 MeV that is consistent with the location of the low-energy orbital $M1$ scissors mode.

The GALILEO $\gamma$-ray spectrometer has been constructed at the Legnaro National Laboratory of INFN (LNL-INFN). It can be coupled to advanced ancillary devices which allows nuclear structure studies employing the variety of in-beam $\gamma$-ray spectroscopy methods. Such studies benefit from reactions induced by the intense stable beams delivered by the Tandem-ALPI-PIAVE accelerator complex and by the radioactive beams which will be provided by the SPES facility. In this paper we outline two experiments performed within the experimental campaign at GALILEO coupled to the EUCLIDES Si-ball and the Neutron Wall array. The first one was aimed at spectroscopic studies in A=31 mirror nuclei and the second one at measurements of lifetimes of excited states in nuclei in the vicinity of $^{100}$Sn., Comment: The IX International Symposium on EXOtic Nuclei (EXON-2018)

In this paper we present the first systematic analysis of the impact of the populated vs. intrinsic spin distribution on the nuclear level density and $\gamma$-ray strength function retrieved through the Oslo Method. We illustrate the effect of the spin distribution on the recently performed $^{239}\mathrm{Pu}$(d,p$\gamma$)$^{240}\mathrm{Pu}$ experiment using a 12 MeV deuteron beam performed at the Oslo Cyclotron Lab. In the analysis we couple state-of-the-art calculations for the populated spin-distributions with the Monte-Carlo nuclear decay code RAINIER to compare Oslo Method results to the known input. We find that good knowledge of the populated spin distribution is crucial and show that the populated distribution has a significant impact on the extracted nuclear level density and $\gamma$-ray strength function for the $^{239}\mathrm{Pu}$(d,p$\gamma$)$^{240}\mathrm{Pu}$ case., Comment: Presented at the 6th International Workshop on Compound-Nuclear Reactions and Related Topics

The lifetimes of first excited 2$^+$, 4$^+$ and 6$^+$ states in $^{98}$Zr were measured with the Recoil-Distance Doppler Shift method in an experiment performed at GANIL. Excited states in $^{98}$Zr were populated using the fission reaction between a 6.2 MeV/u $^{238}$U beam and a $^{9}$Be target. The $\gamma$ rays were detected with the EXOGAM array in correlation with the fission fragments identified in mass and atomic number in the VAMOS++ spectrometer. Our result shows very small B(E2;2$_1^+$ $\rightarrow$ 0$_1^+$) value in $^{98}$Zr thereby confirming the very sudden onset of collectivity at $N=60$. The experimental results are compared to large-scale Monte Carlo Shell model and beyond mean field calculations. The present results indicate coexistence of two additional deformed shapes in this nucleus along with the spherical ground state.

Particle-$\gamma$ coincidence experiments were performed at the Oslo Cyclotron Laboratory with the $^{181}$Ta(d,X) and $^{181}$Ta($^{3}$He,X) reactions, to measure the nuclear level densities (NLDs) and $\gamma$-ray strength functions ($\gamma$SFs) of $^{180, 181, 182}$Ta using the Oslo method. The Back-shifted Fermi-Gas, Constant Temperature plus Fermi Gas, and Hartree-Fock-Bogoliubov plus Combinatorial models where used for the absolute normalisations of the experimental NLDs at the neutron separation energies. The NLDs and $\gamma$SFs are used to calculate the corresponding $^{181}$Ta(n,$\gamma$) cross sections and these are compared to results from other techniques. The energy region of the scissors resonance strength is investigated and from the data and comparison to prior work it is concluded that the scissors strength splits into two distinct parts. This splitting may allow for the determination of triaxiality and a $\gamma$ deformation of $14.9^{\circ} \pm 1.8^{\circ}$ was determined for $^{181}$Ta.

The nuclear level density and $\gamma$-decay strength of $^{192}$Os have been extracted using particle-$\gamma$ coincidence data from the $^{192}$Os($\alpha,\alpha^\prime\gamma$)$^{192}$Os reaction by means of the Oslo method. The level density is found to be a rather smooth function of excitation energy, approximately following the constant temperature model. The $\gamma$-decay strength is compared to photoneutron cross-section data above the neutron separation energy, and to $E1$ and $M1$ strengths for nuclei in this mass region derived from primary transitions following neutron capture. Our results are in good agreement with these previous data and draw a consistent picture of the $\gamma$-strength function in the range $E_\gamma \approx 1.5-6 $ MeV. Using the measured nuclear level density and $\gamma$-decay strength as input to the nuclear-reaction code TALYS, we provide the first experimentally constrained Maxwellian-averaged cross section (MACS) for the $^{191}$Os($n,\gamma$)$^{192}$Os reaction relevant to $s$-process nucleosynthesis. The systematic uncertainties introduced by the normalization procedure of the level density and $\gamma$-strength function were investigated and propagated to the calculated Maxwellian-averaged cross section. The obtained result of the Maxwellian-averaged cross section at $k_BT=30$ keV, $\langle \sigma \rangle_{n,\gamma}=1134\pm 375$ mb, is in very good agreement with the theoretical estimate provided by the KADoNiS project, giving experimental support to the adopted KADoNiS value. Good agreement is also found with MACS values obtained from other libraries, such as TENDL-2017, ENDF/B-VII.0, and JEFF., Comment: 14 pages, 11 figures, accepted to Phys. Rev C

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.

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

The $\gamma$-ray strength function ($\gamma$SF) and nuclear level density (NLD) have been extracted for the first time from inverse kinematic reactions with the Oslo Method. This novel technique allows measurements of these properties across a wide range of previously inaccessible nuclei. Proton-$\gamma$ coincidence events from the $\mathrm{d}(^{86}\mathrm{Kr}, \mathrm{p}\gamma)^{87}\mathrm{Kr}$ reaction were measured at iThemba LABS and the $\gamma$SF and NLD in $^{87}\mathrm{Kr}$ obtained. The low-energy region of the $\gamma$SF is compared to Shell Model calculations which suggest this region to be dominated by M1 strength. The $\gamma$SF and NLD are used as input parameters to Hauser-Feshbach calculations to constrain $(\mathrm{n},\gamma)$ cross sections of nuclei using the TALYS reaction code. These results are compared to $^{86}\mathrm{Kr}(n,\gamma)$ data from direct measurements., Comment: Submitted to EpJ A Letters

In this work, we present new data on the $^{89}$Y($\gamma$,n) cross section studied with a quasi-monochromatic photon beam produced at the NewSUBARU synchrotron radiation facility in Japan contributing torwards resolving a long standing discrepancy between existing measurements of this cross section. Results for $\gamma$-ray strength function below threshold obtained by applying the Oslo method to $^{89}$Y($p,p'\gamma$)$^{89}$Y coincidences combined with the $^{89}$Y($\gamma$,n) data this providing experimental data for the $\gamma$-ray strength function of $^{89}$Y for $\gamma$ energies in the range of $\approx 1.6$ Mev to $\approx$ 20 MeV. A low-energy enhancement is seen for $\gamma$-rays below $\approx 2.5$ MeV. Shell-model calculations indicate that this feature is caused by strong, low-energy $M1$ transitions at high excitation energies. The nuclear level density and $\gamma$-ray strength function have been extracted from $^{89}$Y($d,p \gamma$)$^{90}$Y coincidences using the Oslo method. Using the ($\gamma,n$) and ($d,p\gamma$) data as experimental constraints, we have calculated the $^{89}$Y($n,\gamma$)$^{90}$Y cross section with the TALYS reaction code. Our results have been compared with directly measured (n,$\gamma$) cross sections and evaluations. The $N=50$ isotope $^{89}$Y is an important bottleneck in the s-process and the magnitude of the $^{89}$Y(n,$\gamma)$ cross section is key to understanding how s-process stars produce heavy isotopes., Comment: 18 pages (including references) and 16 figures, to be submitted to PRC

The low-lying structures of the midshell $\nu g_{9/2}$ Ni isotopes $^{72}$Ni and $^{74}$Ni have been investigated at the RIBF facility in RIKEN within the EURICA collaboration. Previously unobserved low-lying states were accessed for the first time following $\beta$ decay of the mother nuclei $^{72}$Co and $^{74}$Co. As a result, we provide a complete picture in terms of the seniority scheme up to the first $(8^+)$ levels for both nuclei. The experimental results are compared to shell-model calculations in order to define to what extent the seniority quantum number is preserved in the first neutron $g_{9/2}$ shell. We find that the disappearance of the seniority isomerism in the $(8^+_1)$ states can be explained by a lowering of the seniority-four $(6^+)$ levels as predicted years ago. For $^{74}$Ni, the internal de-excitation pattern of the newly observed $(6^+_2)$ state supports a restoration of the normal seniority ordering up to spin $J=4$. This property, unexplained by the shell-model calculations, is in agreement with a dominance of the single-particle spherical regime near $^{78}$Ni.

Prompt fission $\gamma$-rays are responsible for approximately 5\% of the total energy released in fission, and therefore important to understand when modelling nuclear reactors. In this work we present prompt $\gamma$-ray emission characteristics in fission, for the first time as a function of the nuclear excitation energy of the fissioning system. Emitted $\gamma$-ray spectra were measured, and $\gamma$-ray multiplicities and average and total $\gamma$ energies per fission were determined for the $^{233}$U(d,pf) reaction for excitation energies between 4.8 and 10 MeV, and for the $^{239}$Pu(d,pf) reaction between 4.5 and 9 MeV. The spectral characteristics show no significant change as a function of excitation energy above the fission barrier, despite the fact that an extra $\sim$5 MeV of energy is potentially available in the excited fragments for $\gamma$-decay. The measured results are compared to model calculations made for prompt $\gamma$-ray emission with the fission model code GEF. Further comparison with previously obtained results from thermal neutron induced fission is made to characterize possible differences arising from using the surrogate (d,p) reaction.

One of the greatest questions for modern physics to address is how elements heavier than iron are created in extreme, astrophysical environments. A particularly challenging part of that question is the creation of the so-called p-nuclei, which are believed to be mainly produced in some types of supernovae. The lack of needed nuclear data presents an obstacle in nailing down the precise site and astrophysical conditions. In this work, we present for the first time measurements on the nuclear level density and average strength function of $^{92}$Mo. State-of-the-art p-process calculations systematically underestimate the observed solar abundance of this isotope. Our data provide stringent constraints on the $^{91}$Nb$(p,{\gamma})^{92}$Mo reaction rate, which is the last unmeasured reaction in the nucleosynthesis puzzle of $^{92}$Mo. Based on our results, we conclude that the $^{92}$Mo abundance anomaly is not due to the nuclear physics input to astrophysical model calculations., Comment: Submitted to PRC