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5 pags., 4 figs., Excited states in Zn were populated following one-neutron removal from a Zn beam impinging on a Be target at intermediate energies in an experiment conducted at the Radioactive Isotope Beam Factory at RIKEN. Three γ rays were observed and tentatively assigned to the 6→4→2→0 yrast sequence. This turns Zn into the heaviest T=−2 nucleus in which excited states are known. The excitation-energy differences between these levels and the isobaric analogue states in the T=+2 mirror partner, Fe, are compared with large-scale shell-model calculations considering the full pf valence space and various isospin-breaking contributions. This comparison, together with an analysis of the mirror energy differences in the A=58, T=±1 pair Zn and Ni, provides valuable information with respect to the size of the monopole radial and the isovector multipole isospin-breaking terms in the region above doubly-magic Ni., We thank the RIKEN Nishina Center accelerator staff and the BigRIPS team for providing excellent beams to the experiment. This work was supported by the Spanish Ministerio de Ciencia e Inno- vación under contracts FPA2017-84756-C4-2-P, PGC-2018-94583, and SEV-2016-0597, the Swedish Research Council (Vetenskap- srådet, VR 2016-3969), and the UK Science and Technology Facil- ities Council (STFC) under grants ST/L005727/1 and ST/P003885/1. FB is supported by the RIKEN Special Postdoctoral Researcher Pro- gram

The $B(E2,{2}^{+}\ensuremath{\rightarrow}{0}^{+})$ transition strengths of the $T=1$ isobaric triplet $^{70}\mathrm{Kr},$ $^{70}\mathrm{Br},$ $^{70}\mathrm{Se},$ recently measured at the RIKEN Radioactive Isotope Beam Factory (RIBF), are discussed in terms of state-of-the-art large scale shell model calculations using the JUN45 and $\mathrm{JUN}45+\mathrm{LNPS}$ plus Coulomb interactions. In this Letter we argue that, depending on the effective charges used, the calculations are either in line with the experimental data within statistical uncertainties, or the anomaly happens in $^{70}\mathrm{Br},$ rather than $^{70}\mathrm{Kr}$. In the latter case, we suggest that it can be due to the presence of a hitherto undetected ${1}^{+}$ $T=0$ state below the yrast ${2}^{+}$ $T=1$ state. Our results do not support a shape change of $^{70}\mathrm{Kr}$ with respect to the other members of the isobaric multiplet.

Background: The evolution of shell structure around doubly-magic exotic nuclei is of great interest in nuclear physics and astrophysics. In the `south-west' 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 (RIBF) 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 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.

The region of neutron-rich Cr isotopes has garnered much attention in recent years due to a rapid onset of collectivity near neutron number $N=40$. We report here on the first $\ensuremath{\gamma}$-ray spectroscopy beyond the $({4}_{1}^{+})$ state in $^{62,64}\mathrm{Cr}$, using nucleon removal reactions from several projectiles within a rare-isotope beam cocktail. A candidate for the ${6}^{+}$ state in $^{64}\mathrm{Cr}$ is presented as well as one for, possibly, the second excited ${0}^{+}$ state in $^{62}\mathrm{Cr}$. The results are discussed in comparison to the LNPS shell-model predictions that allow for neutron excitations across the $N=40$ harmonic oscillator gap into the ${g}_{9/2}$ and ${d}_{5/2}$ orbitals. The calculated level schemes for $^{62,64}\mathrm{Cr}$ reveal intriguing collective structures. From the predicted neutron particle-hole character of the low-lying states in these Cr isotopes, $^{62}\mathrm{Cr}$ emerges as a transitional system on the path to the center of the $N=40$ island of inversion.

7 pags., 4 figs., 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., N.K. acknowledges support of the Grant-in-Aid for JSPS Fellows (18J12542) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. K.W. acknowledges support from the Ministerio de Ciencia e Innovación (Spain) through the Ramón y Cajal program RYC-2017-22007. A.P. is supported in part by the Ministerio de Ciencia, Innovación y Universidades (Spain), Severo Ochoa program SEV-2016-0597 and grant PGC-2018-94583. The SDPF-M calculations were enabled by the CNS-RIKEN joint project for large-scale nuclear structure calculations and were performed mainly on the Oakforest-PACS supercomputer. N.S. acknowledges support from “Priority Issue on post-K computer” (hp190160) and “Program for Promoting Researches on the Supercomputer Fugaku” (hp200130) by JICFuS and MEXT, Japan. J.A.T. acknowledges support from the U.K. Science and Technology Facilities Council Grant No. ST/L005743/1. This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Nuclear Physics, under Grant No. DE-SC0020451 and by the U.S. National Science Foundation (NSF) under Grant No. PHY-1306297. GRETINA was funded by the U.S. DOE, Office of Science. Operation of the array at NSCL was supported by the U.S. NSF under Cooperative Agreement No. PHY-1102511 (NSCL) and DOE under Grant No. DE-AC02-05CH11231 (LBNL).

The Ge80 structure was investigated in a high-statistics β-decay experiment of Ga80 using the GRIFFIN spectrometer at TRIUMF-ISAC through γ, β-e, e-γ, and γ-γ spectroscopy. No evidence was found for the recently reported 02+ 639-keV level suggested as evidence for low-energy shape coexistence in Ge80. Large-scale shell model calculations performed in Ge78,80,82 place the 02+ level in Ge80 at 2 MeV. The new experimental evidence combined with shell model predictions indicate that low-energy shape coexistence is not present in Ge80.

The recent experimental observation of isospin symmetry breaking (ISB) in the ground states of the $T=3/2$ mirror pair $^{73}\mathrm{Sr}\text{\ensuremath{-}}^{73}\mathrm{Br}$ is theoretically studied using large-scale shell-model calculations. The large valence space and the successful PFSDG-U effective interaction used for the nuclear part of the problem capture possible structural changes and provide a robust basis to treat the ISB effects of both electromagnetic and nonelectromagnetic origin. The calculated shifts and mirror-energy differences are consistent with the inversion of the ${I}^{\ensuremath{\pi}}=1/{2}^{\ensuremath{-}},5/{2}^{\ensuremath{-}}$ states between $^{73}\mathrm{Sr}$ and $^{73}\mathrm{Br}$ and suggest that the role played by the Coulomb interaction is dominant. An isospin breaking contribution of nuclear origin is estimated to be $\ensuremath{\approx}25\phantom{\rule{0.28em}{0ex}}\mathrm{keV}$.

The interpretation of nuclear observables in the laboratory frame in terms of the intrinsic deformation parameters beta and gamma is a classical theme in nuclear structure. Here we use the quadrupole invariants (Kumar), calculated within the framework of the configuration-interaction shell model, to clarify the meaning and limitations of nuclear shapes. We introduce a novel method that enables us to calculate accurately higher-order invariants and, therefore, the fluctuations in both beta and gamma. We find that the shape parameter beta often has a non-negligible degree of softness, and that the angle gamma is usually characterized by large fluctuations, rendering its effective value not meaningful. Contrary to common belief, we conclude that doubly magic nuclei are not spherical because the notion of a well-defined shape does not apply to them., Comment: Accepted in Physical Review C. Title changed. Added sections on triaxiality and sphericity

Accurate mass measurements of neutron-rich iron and cobalt isotopes $^{67}\mathrm{Fe}$ and $^{69,70}\mathrm{Co}$ have been realized with the JYFLTRAP double Penning-trap mass spectrometer. With novel ion-manipulation techniques, the masses of the $^{69,70}\mathrm{Co}$ ground states and the $1/{2}^{\ensuremath{-}}$ isomer in $^{69}\mathrm{Co}$ have been extracted for the first time. The measurements remove ambiguities in the previous mass values and yield a smoother trend on the mass surface, extending it beyond $N=40$. The moderate $N=40$ subshell gap has been found to weaken below $^{68}\mathrm{Ni}$, a region known for shape coexistence and increased collectivity. The excitation energy for the $1/{2}^{\ensuremath{-}}$ intruder state in $^{69}\mathrm{Co}$ has been determined for the first time and is compared to large-scale shell-model calculations. The new mass values also reduce significantly mass-related uncertainties for the astrophysical rapid neutron-capture process calculations.

The structures of the neutron-rich Co69,71,73 isotopes were investigated via (p,2p) knockout reactions at the Radioactive Isotope Beam Factory, RIKEN. Isotopes of interest were studied using the DALI2 γ-ray detector array combined with the MINOS target and tracker system. Level schemes were reconstructed using the γ-γ coincidence technique, with tentative spin-parity assignments based on the measured inclusive and exclusive cross sections. Comparison with shell-model calculations using the Lenzi-Nowacki-Poves-Sieja LNPS and PFSDG-U interactions suggests coexistence of spherical and deformed shapes at low excitation energies in the Co69,71,73 isotopes. The distorted-wave impulse approximation (DWIA) framework was used to calculate the single-particle cross sections. These values were compared with the experimental findings.

7 pags., 4 figs., 1 tab., Excited states in the N=40 isotone Ti were populated via the V(p,2p)Ti reaction at ∼200 MeV/nucleon at the Radioactive Isotope Beam Factory and studied using γ-ray spectroscopy. The energies of the 2 →0 and 4 →2 transitions, observed here for the first time, indicate a deformed Ti ground state. These energies are increased compared to the neighboring Cr and 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 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 Ca., We thank the RIKEN Nishina Center accelerator staff and the Bi-gRIPS team for the stable operation of the high-intensity Zn beam and for the preparation of the secondary beam setting. K.O. ac-knowledges the support by Grant-in-Aid for Scientific Research of the Japan Society for the Promotion of Science (JSPS) JP16K05352. A.P. is supported in part by the Ministerio de Ciencia, Innovación y Universidades (Spain), Severo Ochoa Programme SEV-2016-0597 and grant PGC-2018-94583. F.B. is supported by the RIKEN Spe-cial Postdoctoral Researcher Program. L.X.C. and B.D.L. would like to thank the Vietnam Ministry of Science and Technology (MOST) for its support through the Physics Development Program Grant No. ÐTÐLCN.25/18. I.G. has been supported by HIC for FAIR and Croatian Science Foundation under projects no. 1257 and 7194. D. So. was supported by the the European Regional Develop-ment Fund contract No. GINOP-2.3.3-15-2016-00034 and the National Research, Development and Innovation Fund of Hungary via Project No. K128947. V.V. acknowledges support from the Span-ish Ministerio de Economía y Competitividad under Contract No. FPA2017-84756-C4-2-P. K.I.H., D.K. and S.Y.P. acknowledge the sup-port from the National Research Foundation of Korea grant No. 2018R1A5A1025563 and 2019M7A1A1033186. The development of MINOS was supported by the European Research Council through the ERC Grant No. MINOS-258567. This work was also supported by the JSPS KAKENHI Grant No. 18K03639, MEXT as “Priority is-sue on post-K computer” (Elucidation of the fundamental laws and evolution of the universe), the Joint Institute for Computational Fundamental Science (JICFuS), the CNS-RIKEN joint project for large-scale nuclear structure calculations, Natural Sciences and Engineering Research Council (NSERC) of Canada, the Deutsche Forschungsgemeinschaft – Projektnummer 279384907 – SFB 1245, the PRISMA Cluster of Excellence, and the BMBF under Contracts No. 05P18RDFN1 and 05P19RDFN1. TRIUMF receives funding via a contribution through the National Research Council Canada. Com-putations were performed at the Jülich Supercomputing Center (JURECA)

Etienne CaurierEtienne Caurier, directeur of research (emeritus) at Centre National de la Recherche Scientifique (CNRS) and a key protagonist of the renaissance of nuclear structure studies in the ...

We report on the first detailed study of intruder configurations in the ground state of 30 Ne by means of the 12 C( 30 Ne, 29 Ne+ γ )X one-neutron knockout reaction at 228 MeV/nucleon. Using a combined analysis of individual parallel momentum distributions and partial cross sections we find: (a) comparable p - and d -wave removal strength to 29 Ne final states with excitation energies below 200 keV, and (b) significant p -wave removal strength to the 620 keV state of 29 Ne, and (c) no evidence for f -wave intruder strength leading to bound 29 Ne final states. The SDPF-U-MIX shell model calculation in the sd–pf model space provides a better overall agreement with the measured energy levels of 29 Ne and the fp -intruder amplitudes in 30 Ne than the SDPF-M prediction, suggesting that the refinement of the sd–pf cross shell interaction and extension of the model space to include the 2p1/2 and 1f5/2 levels are important for understanding the island of inversion.

Lifetimes of the $T=1$ isobaric analog ${I}^{\ensuremath{\pi}}={2}^{+}$ states in $^{50}\mathrm{Mn}$ and $^{50}\mathrm{Cr}$ were measured simultaneously by employing the recoil distance Doppler-shift (RDDS) technique. The states were populated in a fusion-evaporation reaction with a $^{12}\mathrm{C}$ beam on a $^{40}\mathrm{Ca}$ target. An analysis of the data and the calculations from the present work, together with the available $B(E2:{2}^{+}\ensuremath{\rightarrow}{0}^{+})$ data of the isobaric analog states in $^{50}\mathrm{Fe}$ and $^{50}\mathrm{Cr}$, were used to study isospin symmetry in the three $A=50$ isobaric nuclei. Shell-model calculations reproduce the magnitudes as well as the increasing trend of the $B(E2)$ data with increasing $Z$. To draw a firm conclusion on the level of isospin mixing in the triplet, a new precision measurement of the ${2}^{+}\ensuremath{\rightarrow}{0}^{+}$ transition rate in $^{50}\mathrm{Fe}$ will be required.

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., Phys. Lett. B accepted

Excited states in the neutron-rich 70 Fe nucleus were populated in a one-proton removal reaction from 71 Co projectiles at 87 MeV/nucleon. A new transition was observed with the γ -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 γ ray possibly originates from the 6+ 1 state. It is further shown that the Doppler-reconstructed γ -ray spectra are sensitive to the very different lifetimes of the 2+ and 4+ states, enabling their approximate measurement. The emerging structure of 70 Fe is discussed in comparison to LNPS-new large-scale shell-model calculations.

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.

Shape coexistence is a ubiquitous phenomenon in the neutron-rich nuclei belonging to (or sitting at the shores of) the $N=20$ island of inversion (IoI). Exact isospin symmetry predicts the same behavior for their mirrors and the existence of a proton-rich IoI around $Z=20$, centered in the (surely unbound) nucleus $^{32}\mathrm{Ca}$. In this article we show that in $^{36}\mathrm{Ca}$ and $^{36}\mathrm{S}$, Coulomb effects break dramatically the mirror symmetry in the excitation energies due to the different structures of the intruder and normal states. The mirror energy difference (MED) of their ${2}^{+}$ states is known to be very large at $\ensuremath{-}246$ keV. We reproduce this value and predict the first excited state in $^{36}\mathrm{Ca}$ to be a ${0}^{+}$ at 2.7 MeV, 250 keV below the first ${2}^{+}$. In its mirror $^{36}\mathrm{S}$ the ${0}^{+}$ lies at 55 keV above the ${2}^{+}$ measured at 3.291 MeV. Our calculations predict a huge MED of $\ensuremath{-}720$ keV, that we dub the ``colossal'' mirror energy difference. A possible reaction mechanism to access the ${0}_{2}^{+}$ in $^{36}\mathrm{Ca}$ will be discussed. In addition, we theoretically address the MEDs of the $A=34,\phantom{\rule{4pt}{0ex}}T=3$ and $A=32,\phantom{\rule{4pt}{0ex}}T=4$ mirrors.

The nuclear g factor and quadrupole moment ( Q s ) have been measured for the low-lying isomeric 1 + state of 34Al, using the β-detected nuclear magnetic resonance (β-NMR) and quadrupole resonance (β-NQR) methods. Spin-polarized 34Al isotopes were produced in a one-neutron pickup reaction induced by a 36S beam on a 9Be target at a kinetic energy of 77.5 MeV/nucleon, and were collected with the LISE fragment separator at GANIL. The measured g factor ( | g | in a range of 1.757 ± 0.014 ) and quadrupole moment ( | Q s | = 38 ( 5 ) mb) are in good agreement with shell-model calculations using effective interactions. The similarity between the g factor of 34 m Al and that of 32Al confirms a dominant neutron 2 p 1 h configuration π ( d 5 / 2 ) − 1 ⊗ ν ( d 3 / 2 ) − 1 ( f 7 / 2 ) 2 of 34 m Al. In addition, the enhanced quadrupole moment of 34 m Al relative to that of 32Al suggests that the neutron 1 p 1 h excitation across the N = 20 shell gap introduces extra nucleon–nucleon correlation and deformation.

Intermediate energy single-neutron removal from $^{31}$Mg has been employed to investigate the transition into the N=20 island of inversion. Levels up to 5~MeV excitation energy in $^{30}$Mg were populated and spin-parity assignments were inferred from the corresponding longitudinal momentum distributions and $\gamma$-ray decay scheme. Comparison with eikonal-model calculations also permitted spectroscopic factors to be deduced. Surprisingly, the 0$^{+}_{2}$ level in $^{30}$Mg was found to have a strength much weaker than expected in the conventional picture of a predominantly $2p - 2h$ intruder configuration having a large overlap with the deformed $^{31}$Mg ground state. In addition, negative parity levels were identified for the first time in $^{30}$Mg, one of which is located at low excitation energy. The results are discussed in the light of shell-model calculations employing two newly developed approaches with markedly different descriptions of the structure of $^{30}$Mg. It is concluded that the cross-shell effects in the region of the island of inversion at Z=12 are considerably more complex than previously thought and that $np - nh$ configurations play a major role in the structure of $^{30}$Mg., Comment: Physics Letters B, Volume 779, 10 April 2018, Pages 124-129

The lifetime of the 0(3)(+) state in Ni-66, two neutrons below the N = 40 subshell gap, has been measured. The transition B(E2; 0(3)(+) -> 2(1)(+)) is one of the most hindered E2 transitions in the Ni isotopic chain and it implies that, unlike Ni-68, there is a spherical structure at low excitation energy. We have performed extensive shell-model calculations that correctly predict this result, obtaining a spherical 0(+) state at the correct energy and with an extremely low B(E2; 0(3)(+)-> 2(1)(+)) value.

International audience; The N=20 island of inversion is an excellent playground for testing shell model calculations. The Mg chain is a region of shell evolution still far from being well understood. In this paper we present preliminary results of a single-neutron knockout experiment from 31Mg performed at GANIL to study the structure of 31Mg and of the core 30Mg. The level scheme and longitudinal momentum distributions were mesured and spectroscopic factors were deduced. Negative parity states arise at low energy and the spectroscopic factor for the isomeric in 30Mg was determined to be smaller than foreseen in the standard picture. The preliminary experimental results are compared to state-of the art shell model calculations revealing opposed interpretations.

International audience; In this study we propose a common mechanism for the disappearance of shell closures far from stabilty. With the use of Large Scale Shell Model calculations (SM-CI), we predict that the region of deformation which comprises the heaviest Chromium and Iron isotopes at and beyond N=40 will merge with a new one at N=50 in an astonishing parallel to the N=20 and N=28 case in the Neon and Magnesium isotopes. We propose a valence space including the full pf-shell for the protons and the full sdg shell for the neutrons, which represents a come-back of the the harmonic oscillator shells in the very neutron rich regime. Our calculations preserve the doubly magic nature of the ground state of 78Ni, which, however, exhibits a well deformed prolate band at low excitation energy, providing a striking example of shape coexistence far from stability. This new Island of Inversion (IoI) adds to the four well documented ones at N=8, 20, 28 and 40.

The $(\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{He},t)$ charge-exchange reaction populating ${J}^{\ensuremath{\pi}}={2}^{\ensuremath{-}}$ states has been examined at 420 MeV incident energy for a series of double-$\ensuremath{\beta}$ decaying nuclei, i.e., $^{76}\mathrm{Ge}$, $^{82}\mathrm{Se}$, $^{96}\mathrm{Zr}$, $^{100}\mathrm{Mo}$, $^{128}\mathrm{Te}$, $^{130}\mathrm{Te}$, and $^{136}\mathrm{Xe}$. The measurements were carried out at the Grand Raiden spectrometer of the Research Center for Nuclear Physics at the University Osaka with typical spectral resolution of 30--40 keV. It is found that the charge-exchange reaction leading to ${2}^{\ensuremath{-}}$ spin-dipole states is selective to the $\ensuremath{\sigma}\ensuremath{\tau}$ part of the interaction much similar to the observed selectivity to Gamow-Teller transitions. In the present case, the $\mathrm{\ensuremath{\Delta}}L=1$ peak cross sections at finite momentum transfers are used to extract the spin-isospin part of the low-lying transition strength near the Fermi surface (i.e., ${E}_{x}\ensuremath{\le}5$ MeV). Relative strength values are confronted with various model calculations, i.e., the interacting shell model, the quasiparticle random-phase approximation, and the Fermi surface quasiparticle model. The impact on the nuclear matrix elements for the neutrinoless double-$\ensuremath{\beta}$ decay is discussed.

I will discuss the impact of the research on exotic nuclei carried out at ISOLDE and later on, following its impulse, in many laboratories worldwide, in our present knowledge of the nuclear dynamics. I will put particular emphasis on the occurrence of islands of inversion (IoI) at several magic numbers for the neutrons, far from stability. I will also discuss the appearance of new local shell closures in the calcium and silicon isotopic chains, and qualify their status as new doubly magic nuclei.

Large-scale shell-model calculations predict that the region of deformation which comprises the heaviest chromium and iron isotopes at and beyond $N=40$ will merge with a new one at $N=50$ in an astonishing parallel to the $N=20$ and $N=28$ case in the neon and magnesium isotopes. We propose a valence space including the full $pf$ shell for the protons and the full $sdg$ shell for the neutrons, which represents a comeback of the the harmonic oscillator shells in the very neutron- rich regime. The onset of deformation is understood in the framework of the algebraic SU(3)-like structures linked to quadrupole dominance. Our calculations preserve the doubly magic nature of the ground state of $^{78}\mathrm{Ni}$, which, however, exhibits a well-deformed prolate band at low excitation energy, providing a striking example of shape coexistence far from stability. This new IOI adds to the four well-documented ones at $N=8$, 20, 28, and 40.

Our understanding of the evolution of the shell structure in nuclei far from stability is based on the study of some key nuclei. Nuclei at or next to double shell closures play a special role in this. Presently, a lot of discussion is concentrated on the neutron-rich calcium isotopes, which provide a rich testing ground for various nuclear models with several traditional and new magic numbers. 60 Ca is now almost within reach with the most advanced radioactive beam facilities. In order to investigate the evolution of the shell gap at N = 40, the configuration of states in the odd–even titanium isotopes up to N = 37 ( 59 Ti) have been studied. In order to experimentally access the shell gap at N = 40, it is nowadays within the reach of the most advanced facility the investigation of neutron hole configuration states in odd–even titanium isotopes up to N = 37, in the 59 Ti nucleus. Such states correspond to relatively simple configurations that constitute a challenging testing ground for effective nuclear interactions. The new data obtained in our experiment allows to place the present predictions concerning the shell closure at N = 40 in the calcium region on a more solid ground.

Author(s): Macchiavelli, AO; Crawford, HL; Campbell, CM; Clark, RM; Cromaz, M; Fallon, P; Jones, MD; Lee, IY; Salathe, M; Brown, BA; Poves, A | Abstract: Background: Competing interpretations of the results of a Mg30(t,p)Mg32 measurement populating the ground state and 02+ state in Mg32, both limited to a two-state mixing description, have left an open question regarding the nature of the Mg32 ground state. Purpose: Inspired by recent shell-model calculations, we explore the possibility of a consistent interpretation of the available data for the low-lying 0+ states in Mg32 by expanding the description from two-level to three-level mixing. Methods: A phenomenological three-level mixing model of unperturbed 0p0h, 2p2h, and 4p4h states is applied to describe both the excitation energies in Mg32 and the transfer reaction cross sections. Results: Within this approach, self-consistent solutions exist that provide good agreement with the available experimental information obtained from the Mg30(t,p)Mg32 reaction. Conclusion: The inclusion of the third state, namely the 4p4h configuration, resolves the "puzzle" that results from a two-levelmodel interpretation of the same data. In our analysis, the Mg32 ground state emerges naturally as dominated by intruder (2p2h and 4p4h) configurations, at the 95% level.

The electric quadrupole moment of the $_{13}^{33}\mathrm{Al}_{20}$ ground state, located at the border of the island of inversion, was obtained using continuous-beam $\ensuremath{\beta}$-detected nuclear quadrupole resonance ($\ensuremath{\beta}$-NQR). From the measured quadrupole coupling constant ${\ensuremath{\nu}}_{Q}=2.31(4)$ MHz in an $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ crystal, a precise value for the electric quadrupole moment is extracted: $\left|{Q}_{s}\left(^{33}\mathrm{Al}\right)\right|=141(3)$ mb. A comparison with large-scale shell model calculations shows that $^{33}\mathrm{Al}$ has at least 50% intruder configurations in the ground state wave function, favoring the excitation of two neutrons across the $N=20$ shell gap. $^{33}\mathrm{Al}$ therefore clearly marks the gradual transition north of the deformed Na and Mg nuclei towards the normal $Z\ensuremath{\ge}14$ isotopes.

With new-generation facilities able to provide beams of very unstable nuclei, one can explore how the traditional magic numbers and shell closures become fragile and break down in neutron-rich regions of the chart of nuclides. One important such region is near ${}^{32}$Mg, the so-called ''island of inversion'', suggesting a rearrangement of the nuclear shell structure. This study reports on $B(E2)$ transition probabilities for the most neutron-rich nuclei yet measured in that region, ${}^{30}$Ne and ${}^{36}$Mg. The results help map the evolution and extent of anomalous shell structure toward the neutron drip line.

There is significant evidence supporting the existence of deformed ground states within the neutron-rich N≈20 neon, sodium, and magnesium isotopes that make up what is commonly called the "island of inversion." However, the rotational band structures, which are a characteristic fingerprint of a rigid nonspherical shape, have yet to be observed. In this work, we report on a measurement and analysis of the yrast (lowest lying) rotational band in Mg32 up to spin I=6+ produced in a two-step projectile fragmentation reaction and observed using the state-of-the-art γ-ray tracking detector array, GRETINA (γ-ray energy tracking in-beam nuclear array). Large-scale shell-model calculations using the SDPF-U-MIX effective interaction show excellent agreement with the new data. Moreover, a theoretical analysis of the spectrum of rotational states as a function of the pairing gap, together with cranked-shell-model calculations, provides intriguing evidence for a reduction in pairing correlations with increased angular momentum, also in line with the shell-model results, This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Contracts No. DE-AC02-05CH11231 (LBNL) and No. DE-AC02-06CH11357 (ANL), by the Department of Energy National Nuclear Security Administration under Award No. DE-NA0000979 and the National Science Foundation (NSF) under PHY-1102511. GRETINA was funded by the U.S. DOE Office of Science. Operation of the array at NSCL is supported by NSF under Cooperative Agreement PHY11-02511 (NSCL) and DOE under Grant No. DE-AC02- 05CH11231 (LBNL). A.P. is partly supported by MINECO (Spain) Grant FPA2014-57196 and Programme “Centros de Excelencia Severo Ochoa” SEV-2012-0249

Mass measurements have been performed of the neutron-rich Al isotopes near $N\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}\phantom{\rule{4.pt}{0ex}}20$, along the island of inversion. In general, the precision of the mass values was improved by an order of magnitude. Based on these new and more accurate data, we find the two-neutron separation energy of $_{13}^{34}\mathrm{Al}$ to be 24.8(10.8) keV less than that of $_{12}^{33}\mathrm{Mg}$. This represents the first known crossover on the ${S}_{2n}$ surface of the chart of nuclides. Large-scale shell-model calculations show that the crossover can be attributed to exceptionally large energy gains of the Mg isotopes in the island of inversion.

5 pages, 1 table, 3 figures.-- PACS nrs.: 23.40.Hc; 27.20.+n; 21.10.Hw.-- Printed version published Jun 29, 2009.-- ArXiv available at: http://arxiv.org/abs/0904.4212v1, Coincidences between charged particles emitted in the β-decay of 11Li were observed using highly segmented detectors. The breakup channels involving three particles were studied in full kinematics allowing for the reconstruction of the excitation energy of the 11Be states participating in the decay. In particular, the contribution of a previously unobserved state at 16.3 MeV in 11Be has been identified selecting the α + 7He → α + 6He + n channel. The angular correlations between the α particle and the center of mass of the 6He + n system favors spin and parity assignment of 3/2− for this state as well as for the previously known state at 18 MeV., This work has been supported by the Spanish Ministerio de Ciencia e Innovación (MICINN), under the projects FPA2007-62170 and CSD2007-00042, the European Union Sixth Framework through RII3-EURONS (contract No. 506065) and the Swedish Knut and Alice Wallenberg Foundation.

In this article we analyze the nuclear matrix elements (NME) of the neutrinoless double beta decays ( 0 ν β β ) of the nuclei 48 Ca, 76 Ge, 82 Se, 124 Sn, 128 Te, 130 Te and 136 Xe in the framework of the interacting shell model (ISM). We study the relative value of the different contributions to the NME's, such as higher order terms in the nuclear current, finite nuclear size effects and short range correlations, as well as their evolution with the maximum seniority permitted in the wave functions. We discuss also the build-up of the NME's as a function of the distance between the decaying neutrons. We calculate the decays to 0 1 + final states and find that these decays are at least 25 times more suppressed with respect to the ground state to ground state transition.

In this article, we analyze some nuclear-structure aspects of the 0ν double-beta decay nuclear matrix elements (NME). We give results for the decays of 48Ca , 76Ge , 82Se , 124Sn , 128Te , 130Te , and 136Xe , using improved effective interactions and valence spaces. We examine the dependence of the NMEs on the effective interaction and the valence space, and analyze the effects of the short-range correlations and the finite size of the nucleon. Finally, we study the influence of the deformation on the values of the NMEs.

The independent particle model was introduced in nuclear physics more than fifty years ago by Mayer and Jensen [1,2]. Their basic idea was to assume that, at zero order, the result of the complicated two body interactions among the nucleons is to produce an average self-binding potential. The proposed spherical mean field consists of an isotropic harmonic oscillator plus a strongly attractive spin-orbit potential and an orbit-orbit term that corrects the bad surface behavior of the harmonic oscillator.

The determination of accurate nuclear matrix elements for ββ decay processes is a challenge for nuclear theory and can have a strong impact in neutrino physics. Large Scale Shell Model (LSSM) calculations are among the best tools for such determination and recent developments have allowed to extend its application domains. In particular, systematic studies of nuclear matrix elements calculations have been now undertaken in this framework for most of the ββ emitters. These calculations are crucial in the determination of the most favorable emitters in the forthcoming generation of ββ experiments. The present paper focuses on the recent advances and remaining difficulties of shell model calculations for the neutrinoless mode. Stability and predictive power of the results will be discussed.

10 pags.; 11 figs.; 2 tabs.; PACS number(s): 23.40.Hc, 29.30.Kv, 27.50.+e, 21.10.Pc; Open Access funded by Creative Commons Atribution Licence 3.0

5 págs.; 3 figs.; PACS number(s): 21.60.−n, 75.10.Jm, 02.60.−x, 27.50.+e, We present an efficient implementation of the Density Matrix Renormalization Group (DMRG) algorithm that includes an optimal ordering of the proton and neutron orbitals and an efficient expansion of the active space utilizing various concepts of quantum information theory. We first show how this new DMRG methodology could solve a previous 400 keV discrepancy in the ground state energy of 56Ni. We then report the first DMRG results in the pf + g9/2 shell model space for the ground 0+ and first 2+ states of 64Ge which are benchmarked with reference data obtained from a Monte Carlo shell model. The corresponding correlation structure among the proton and neutron orbitals is determined in terms of two-orbital mutual information. Based on such correlation graphs we propose several further algorithmic improvement possibilities that can be utilized in a new generation of tensor network based algorithms. ©2015 American Physical Society, This work was supported in part by the Hungarian Research Fund (OTKA) through Grants No. K100908 and No. NN110360, and by the Spanish Ministry of Economy and Competitiveness through Grants No. FIS2012- 34479, No. FPA2014-57196, and No. SEV2012-0249.

Artículo escrito por un elevado número de autores, solo se referencian el que aparece en primer lugar, el nombre del grupo de colaboración, si le hubiere, y los autores pertenecientes a la UAM, Background: The neutron-rich calcium isotopes have gained particular interest as evidence of closed-shell structures has recently been found in two exotic nuclei, at N=32 and N=34. Additionally, the study of such neutron-rich systems has revealed new aspects of nuclear forces, in particular regarding the role of three-nucleon forces. Purpose: We study the electromagnetic properties of Ca isotopes around the neutron number N=32. Methods: High-resolution bunched-beam collinear laser spectroscopy was used to measure the optical hyperfine spectra of the Ca43-51 isotopes. Results: The ground-state magnetic moments of Ca49,51 and quadrupole moments of Ca47,49,51 were measured for the first time, and the Ca51 ground-state spin I=3/2 was determined in a model-independent way. Our experimental results are compared with state-of-the-art shell-model calculations using both phenomenological interactions and microscopic interactions derived from chiral effective field theory. Conclusions: The results for the ground-state moments of neutron-rich isotopes are in excellent agreement with predictions of interactions derived from chiral effective field theory including three-nucleon forces. Lighter isotopes illustrate the presence of particle-hole excitations of the Ca40 core in their ground state. Our results provide a critical test of modern nuclear theories, and give direct answer to the evolution of ground-state electromagnetic properties in the Ca isotopic chain across three doubly closed-shell configurations at N=20, 28, 32 of this unique system, This work was supported by the IAP-project P7/12, the GOA grant 15/010 KU Leuven, NSF Grant No. PHY-1068217, the BMBF Contracts No. 05P12RDCIC and No. 06DA70471, the Max-Planck Society, the EU FP7 via ENSAR No. 262010, the DFG through Grant No. SFB 634, ERC Grant No. 307986 STRONGINT, the Helmholtz Alliance HA216/EMMI, and MINECO (SPAIN) (FPA2011-29854). Computations were performed on JUROPA at the Julich Supercomputing Center. We would like to thank the ISOLDE technical group for their support and assistance. International Research Fellowship of the Japan Society for the Promotion of Science (JSPS) and JSPS KAKENHI Grant No. 26.04323

We present Large Scale Shell Model calculations using the effective interaction LNPS, which reproduce nicely the experimental data available in the N=40 island of deformation. We deal as well with the triple (shape?) coexistence in the low energy spectrum of 68Ni, showing that the highly deformed band based upon the excited 0+ state at 2627 keV is in fact the doorway to the island. The onset of prolate quadrupole collectivity and shape coexistence in the heavy N=Z nuclei, is explained by means of the realizations of the pseudo plus quasi SU3 symmetry, with special emphasis on the shape mixing in the ground state of 72Kr, This work is partly supported by a grant of the MICINN (Spain) (FPA2011-29854), by the Nupnet network SARFEN (PRI-PIMMNUP-2011-1361) and by MINECO’s (Spain) Programme ”Centros de Excelencia Severo Ochoa” grant SEV-2012-0249

NESTER; The fast timing $\beta\gamma\gamma(t)$ method was applied to investigate the level lifetimes in $^{30,31,32}$Mg. Levels in Mg have been populated in $\beta$ and $\beta$-delayed neutron emission of Na at the ISOLDE facility. From the $\gamma\gamma$ coincidences a number of new states have been identified and new level schemes were constructed for $^{30,31,32}$Mg. The following preliminary half lives have been determined: T$_{1/2}$ = 3.9(4) ns for the 1789 keV state in $^{30}Mg, T$_{1/2}$ = 133(8) ps and 10.5(8) ns for the 221 keV and 461 keV states in $^{31}$Mg, respectively, and T$_{1/2}$ = 16(4) ps for the 885 keV level in $^{32}$Mg. The 1789 keV level was established as a candidate for the intruder 0$^+$ configuration in $^{30}$Mg with a possible strong E0 branch to the ground state.

The evolution of the N = 28 neutron shell closure is studied in the N = 28 isotones from neutron drip line's 40Mg to N = Z doubly magic 56Ni. It is found that the N = 28 closure vanishes at Z = 12 in favor of a deformed ground state. For Z = 14 and Z = 16 closed shell and intruder configurations are almost degenerate, leading to highly mixed ground states and to very low lying excited 0 + states. For Z > 20 the intruder states are always above the closed shell ground states. We examine their structure and their possible existence as collective yrare bands of well-defined particle–hole character.

Present large scale shell model calculations give a very accurate and comprehensive description of light and medium light nuclei. Full pf -shell calculations have made it possible to describe many collective features in an spherical shell model context. Calculations including two major oscillator shells can cope with superdeformed bands. The underlying symmetries are seen to be variants of Elliott's SU3.

The neutron-rich nuclei 23 60-63V have been produced at GANIL via interactions of a 61.8A·MeV 76Ge beam with a 58Ni target. Beta-decay to 24 60-63Cr has been investigated using combined β- and γ-ray spectroscopy. Half-lives of the 60-63V nuclei have been determined, and the existence of a beta-decay isomer in the 60V nucleus is strongly supported. The observation of low-energy 2+ states in 60Cr (646keV) and 62Cr (446keV) suggests that these isotopes are strongly deformed with β2 ∼ 0.3. This is confirmed by shell model calculations which show the dominant influence of the intruder g and d orbitals to obtain low 2+ energies in the neutron-rich Cr isotopes.

We study in this Letter the double beta decay of 136Xe with emission of two neutrinos which has been recently measured by the EXO-200 Collaboration. We use the same shell model framework, valence space, and effective interaction that we have already employed in our calculation of the nuclear matrix element (NME) of its neutrinoless double beta decay. Using the quenching factor of the Gamow–Teller operator which is needed to reproduce the very recent high resolution 136Xe (3He, t) 136Cs data, we obtain a nuclear matrix element M 2 ν = 0.025 MeV − 1 compared with the experimental value M 2 ν = 0.019 ( 2 ) MeV − 1 .