Search

A machine-learning algorithm, Light Gradient Boosting Machine, was applied for the first time to investigate the fundamental experimental observables in even-even nuclei over the Segrè chart. Specifically, we focused on the excitation energies of the 21+ and 41+ states, and the reduced electric quadrupole transition probability B(E2;01+→21+). Present obtained results well reproduced experimental data within an accuracy of 1.07, 1.05, and 1.14 times for the 21+ and 41+ states as well as B(E2;01+→21+), respectively, being significantly precise than the results from any state-of-the-art nuclear models and from any machine-learning-based approaches. The predictive capability of our machine learning methodology was further validated using 17 newly measured data points which were not used in the training set. Taking O, Ca, Sn, and Pb isotopes as examples, it has been found that our methodology precisely captures both the isotopic trend and absolute values, surpassing all theoretical models hitherto. Our findings reveal the double-magic nature of 100Sn and the disappearance of the N=20 shell in 28O.

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 53Ca and 55Ca, populated from 54,56Ca(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 53Ca and 6000(250)keV in 55Ca indicate a significant l =3 component, providing the first experimental evidence for the ν0f7/2 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 ν0f7/2 orbital in 55Ca. These results are in good agreement with predictions from shell-model calculations using the effective GXPF1Bs interaction and ab initio calculations and diverge markedly from the experimental distributions in the nickel isotones at Z=28.

We performed ab initio valence-space in-medium similarity renormalization group calculations based on chiral nucleon-nucleon and three-nucleon interactions to study the neutron-rich nuclei around N=28, where the experimental and theoretical evidence point to a rapid transition from spherical to rotational regimes. First, the rotational spectra of 40Mg, 42Si, and 44S are established theoretically, anchored by the calculated E2 transition strength. The results suggest that 40Mg and 42Si exhibit large deformations. After that, the erosion of the N=28 shell closure, especially the evolutions of shape deformation and the emergence of shape coexistence, are discussed in N=28 even-even isotones ranging from 40Mg to 48Ca. Finally, the shell evolution of N=28 in Mg and Si chains is discussed based on the experimental and predicted excitation energies of low-lying states. The results from our ab initio calculations align well with available experimental data, furnishing essential insights into the structure of neutron-rich nuclei, particularly deformations in the vicinity of 40Mg and 42Si.

New magic numbers have been discovered in the neutron-rich region of the nuclear chart. However, there has been a lack of research on proton-rich nuclei. 22O, the mirror nucleus of 22Si, is a double-magic nucleus bearing a high E(21+). Whether 22Si exhibits double-magic characters is an intriguing topic. To investigate this matter, we utilized ab initio valence space in-medium similarity renormalization group for 22Si/22O, and their nearby nuclei. Our ab initio calculations provide good descriptions for the double magicity of 22O, as well as the shell evolution of N=14 and Z=14 through E(21+). The computed E(21+) indicate that the closure of Z=14 sub-shell in proton-rich nuclei is weaker than the N=14 sub-shell closure in their mirror nuclei. Particularly, the calculated E(21+) of 22Si is 800 keV lower than the one of 22O. To further explore the magicity of 22Si, the mirror energy difference (MED) of 26Si/26Mg, 24Si/24Ne, as well as 22Si/22O are calculated. The results demonstrate that the calculated MEDs agree well with available experimental data, and the E(21+) values of 22,24,26Si are all lower than their respective mirror nuclei due to the Thomas-Ehrman shift with large s1/2 occupation. Moreover, our calculation provides that the many-body configurations of the low-lying state of 22Si/22O are nearly identical despite the fact that the states bearing large MED. In conclusion, our ab initio results suggest that 22Si is a double magic nucleus, similar to its mirror nucleus 22O, albeit with a lower E(21+).

Gamma decays were observed in 56Ca and 58Ca following quasi-free one-proton knockout reactions from 57,59Sc beams at ≈200 MeV/nucleon. For 56Ca, a γ ray transition was measured to be 1456(12) keV, while for 58Ca an indication for a transition was observed at 1115(34) keV. Both transitions were tentatively assigned as the 21+→0gs+ 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 21+ 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 0f5/2 and 0g9/2 orbitals, are almost degenerate. This degeneracy precludes the possibility for a doubly magic 60Ca and potentially drives the dripline of Ca isotopes to 70Ca or even beyond.

Detailed γ-ray spectroscopy of the exotic neon isotope 28Ne has been performed for the first time using the one-neutron removal reaction from 29Ne on a liquid hydrogen target at 240 MeV/nucleon. Based on an analysis of parallel momentum distributions, a level scheme with spin-parity assignments has been constructed for 28Ne and the negative-parity states are identified for the first time. The measured partial cross sections and momentum distributions reveal a significant intruder p-wave strength providing evidence of the breakdown of the N=20 and N=28 shell gaps. Only a weak, possible f-wave strength was observed to bound final states. Large-scale shell-model calculations with different effective interactions do not reproduce the large p-wave and small f-wave strength observed experimentally, indicating an ongoing challenge for a complete theoretical description of the transition into the island of inversion along the Ne isotopic chain.

Some emerging concepts of nuclear structure are overviewed. (i) Background: the many-body quantum structure of atomic nucleus, a complex system comprising protons and neutrons (called nucleons collectively), has been studied largely based on the idea of the quantum liquid (à la Landau), where nucleons are quasiparticles moving in a (mean) potential well, with weak “residual” interactions between nucleons. The potential is rigid in general, although it can be anisotropic. While this view was a good starting point, it is time to look into kaleidoscopic aspects of the nuclear structure brought in by underlying dynamics and nuclear forces. (ii) Methods: exotic features as well as classical issues are investigated from fresh viewpoints based on the shell model and nucleon–nucleon interactions. The 70-year progress of the shell–model approach, including effective nucleon–nucleon interactions, enables us to do this. (iii) Results: we go beyond the picture of the solid potential well by activating the monopole interactions of the nuclear forces. This produces notable consequences in key features such as the shell/magic structure, the shape deformation, the dripline, etc. These consequences are understood with emerging concepts such as shell evolution (including type-II), T-plot, self-organization (for collective bands), triaxial-shape dominance, new dripline mechanism, etc. The resulting predictions and analyses agree with experiment. (iv) Conclusion: atomic nuclei are surprisingly richer objects than initially thought.

In this paper, the validity of the shell-evolution picture is investigated on the basis of shell-model calculations for the atomic mass number 25≲A≲55 neutron-rich nuclei. For this purpose, the so-called SDPF-MU interaction is used. Its central, two-body spin–orbit, and tensor forces are taken from a simple Gaussian force, the M3Y (Michigan 3-range Yukawa) interaction, and a π+ρ meson exchange force, respectively. Carrying out almost a complete survey of the predicted effective single-particle energies, it is confirmed here that the present scheme is quite effective for describing shell evolution in exotic nuclei.

One ambitious goal of nuclear physics is a predictive model of all nuclei, including the ones at the fringes of the nuclear chart which may remain out of experimental reach. Certain regions of the chart are providing formidable testing grounds for nuclear models in this quest as they display rapid structural evolution from one nucleus to another or phenomena such as shape coexistence. Observables measured for such nuclei can confirm or refute our understanding of the driving forces of the evolution of nuclear structure away from stability where textbook nuclear physics has been proven to not apply anymore. This paper briefly reviews the emerging picture for the very neutron-rich Fe, Cr, and Ti isotopes within the so-called N=40 island of inversion as obtained with nucleon knockout reactions. These have provided some of the most detailed nuclear spectroscopy in very neutron-rich nuclei produced at rare-isotope facilities. The results indicate that our current understanding, as encoded in large-scale shell-model calculations, appears correct with exciting predictions for the N=40 island of inversion left to be proven in the experiment. A bright future emerges with predictions of continued shell evolution and shape coexistence out to neutron number N=50, below 78Ni on the chart of nuclei.

A machine-learning algorithm, Light Gradient Boosting Machine, was applied for the first time to investigate the fundamental experimental observables in even-even nuclei over the Segrè chart. Specifically, we focused on the excitation energies of the 2 1 + and 4 1 + states, and the reduced electric quadrupole transition probability B (E 2 ; 0 1 + → 2 1 +). Present obtained results well reproduced experimental data within an accuracy of 1.07, 1.05, and 1.14 times for the 2 1 + and 4 1 + states as well as B (E 2 ; 0 1 + → 2 1 +) , respectively, being significantly precise than the results from any state-of-the-art nuclear models and from any machine-learning-based approaches. The predictive capability of our machine learning methodology was further validated using 17 newly measured data points which were not used in the training set. Taking O, Ca, Sn, and Pb isotopes as examples, it has been found that our methodology precisely captures both the isotopic trend and absolute values, surpassing all theoretical models hitherto. Our findings reveal the double-magic nature of 100Sn and the disappearance of the N = 20 shell in 28O. [ABSTRACT FROM AUTHOR]

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 53Ca and 55Ca, populated from 54,56Ca(p , p n) 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 53Ca and 6000(250) keV in 55Ca indicate a significant l = 3 component, providing the first experimental evidence for the ν 0 f 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 ν 0 f 7 / 2 orbital in 55Ca. These results are in good agreement with predictions from shell-model calculations using the effective GXPF1Bs interaction and ab initio calculations and diverge markedly from the experimental distributions in the nickel isotones at Z = 28. [ABSTRACT FROM AUTHOR]

Sexual dimorphism in the shell size and shape of land snails has been less explored compared to that of other marine and freshwater snail taxa. This study examined the differences in shell size and shape across both sexes of Leptopoma perlucidum land snails. We collected 84 land snails of both sexes from two isolated populations on two islands off Borneo. A total of five shell size variables were measured: (1) shell height, (2) shell width, (3) shell spire height, (4) aperture height, and (5) aperture width. We performed frequentist and Bayesian t-tests to determine if there was a significant difference between the two sexes of L. perlucidum on each of the five shell measurements. Additionally, the shell shape was quantified based on nine landmark points using the geometric morphometric approach. We used generalised Procrustes and principal component analyses to test the effects of sex and location on shell shape. The results showed that female shells were larger than male shells across all five measurements (all with p-values < 0.05), but particularly in regards to shell height and shell width. Future taxonomic studies looking to resolve the Leptopoma species' status should consider the variability of shell size caused by sexual dimorphism. [ABSTRACT FROM AUTHOR]

The systematic search of axial ground state and the emergence of new gaps in neutron-rich 4 4 − 5 2 Ar isotopes are the challenging of current calculations of relativistic and nonrelativistic mean field models. For this purpose, we have employed the model based on Relativistic Hartree–Bogoliubov (RHB) with the density-dependent effective interaction of point coupling DD-PCX type. However, the nonrelativistic mean field calculations based on Hartree-Fock model were carried out with Skyrme SV-min parametrization. All the present calculations were performed within axial symmetries. The results provide that most of isotopes show an oblate-deformed ground state. In particular, 5 0 Ar and 5 2 Ar display a spherical shape in their ground states with SV-Min calculations. There is a notable signature of the sub-shell closure at neutron number N = 3 2 , whereas N = 3 4 is expected to be nearly quenched in the spherical ground. Furthermore, coexistence features are predicted in the neutron-rich 4 4 Ar isotope. We have also studied the neutron axial densities and its central depletion in the well-deformed grounds for the first time. In addition to these, the proton single-particle evolution and bubble-like structure at 4 6 Ar are very well reproduced with DD-PCX calculations. [ABSTRACT FROM AUTHOR]

Some emerging concepts of nuclear structure are overviewed. (i) Background: the many-body quantum structure of atomic nucleus, a complex system comprising protons and neutrons (called nucleons collectively), has been studied largely based on the idea of the quantum liquid (à la Landau), where nucleons are quasiparticles moving in a (mean) potential well, with weak "residual" interactions between nucleons. The potential is rigid in general, although it can be anisotropic. While this view was a good starting point, it is time to look into kaleidoscopic aspects of the nuclear structure brought in by underlying dynamics and nuclear forces. (ii) Methods: exotic features as well as classical issues are investigated from fresh viewpoints based on the shell model and nucleon–nucleon interactions. The 70-year progress of the shell–model approach, including effective nucleon–nucleon interactions, enables us to do this. (iii) Results: we go beyond the picture of the solid potential well by activating the monopole interactions of the nuclear forces. This produces notable consequences in key features such as the shell/magic structure, the shape deformation, the dripline, etc. These consequences are understood with emerging concepts such as shell evolution (including type-II), T-plot, self-organization (for collective bands), triaxial-shape dominance, new dripline mechanism, etc. The resulting predictions and analyses agree with experiment. (iv) Conclusion: atomic nuclei are surprisingly richer objects than initially thought. [ABSTRACT FROM AUTHOR]

In this paper, the validity of the shell-evolution picture is investigated on the basis of shell-model calculations for the atomic mass number 25 ≲ A ≲ 55 neutron-rich nuclei. For this purpose, the so-called SDPF-MU interaction is used. Its central, two-body spin–orbit, and tensor forces are taken from a simple Gaussian force, the M3Y (Michigan 3-range Yukawa) interaction, and a π + ρ meson exchange force, respectively. Carrying out almost a complete survey of the predicted effective single-particle energies, it is confirmed here that the present scheme is quite effective for describing shell evolution in exotic nuclei. [ABSTRACT FROM AUTHOR]

States in the N=35 and 37 isotopes 55,57Ca have been populated by direct proton-induced nucleon removal reactions from 56,58Sc and 56Ca beams at the RIBF. In addition, the (p,2p) quasi-free single-proton removal reaction from 56Ca was studied. Excited states in 55K, 55Ca, and 57Ca were established for the first time via in-beam γ-ray spectroscopy. Results for the proton and neutron removal reactions from 56Ca to states in 55K and 55Ca 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 57Ca suggests a transition in the calcium isotopic chain from single-particle dominated states at N=35 to collective excitations at N=37.

One ambitious goal of nuclear physics is a predictive model of all nuclei, including the ones at the fringes of the nuclear chart which may remain out of experimental reach. Certain regions of the chart are providing formidable testing grounds for nuclear models in this quest as they display rapid structural evolution from one nucleus to another or phenomena such as shape coexistence. Observables measured for such nuclei can confirm or refute our understanding of the driving forces of the evolution of nuclear structure away from stability where textbook nuclear physics has been proven to not apply anymore. This paper briefly reviews the emerging picture for the very neutron-rich Fe, Cr, and Ti isotopes within the so-called N = 40 island of inversion as obtained with nucleon knockout reactions. These have provided some of the most detailed nuclear spectroscopy in very neutron-rich nuclei produced at rare-isotope facilities. The results indicate that our current understanding, as encoded in large-scale shell-model calculations, appears correct with exciting predictions for the N = 40 island of inversion left to be proven in the experiment. A bright future emerges with predictions of continued shell evolution and shape coexistence out to neutron number N = 50 , below 78 Ni on the chart of nuclei. [ABSTRACT FROM AUTHOR]

The shell evolution of even–even drip line argon isotopes 4 6 – 5 4 Ar has been investigated via the shell model calculations using SDPF-U and SDPF-NR two-body effective interactions in two different shell model spaces π (s d) − ν (f p) and π (s d) − ν (f 7 / 2 p). In this work, the energy of first 2 + , reduced transition probability B (E 2) , excitation energy levels as well as how the proton shells evolve with N have been studied. Excellent agreements were obtained for the first 2 + level along the investigated isotopes within π (s d) − ν (f p) and π (s d) − ν (f 7 / 2 p) model spaces. [ABSTRACT FROM AUTHOR]

The change of the shell structure in atomic nuclei, so-called “nuclear shell evolution”, occurs due to changes of major configurations through particle-hole excitations inside one nucleus, as well as due to variation of the number of constituent protons or neutrons. We have investigated how the shell evolution affects Gamow-Teller (GT) transitions that dominate the β decay in the region below 132Sn using the newly obtained experimental data on a long-lived isomer in 127Ag. The T1/2=67.5(9) ms isomer has been identified with a spin and parity of (27/2+) at an excitation energy of 1942−20+14 keV, and found to decay via an internal transition of an E3 character, which competes with the dominant β-decay branches towards the high-spin states in 127Cd. The underlying mechanism of a strong GT transition from the 127Ag isomer is discussed in terms of configuration-dependent optimization of the effective single-particle energies in the framework of a shell-model approach.

The evolution of the traditional nuclear magic numbers away from the valley of stability is an active field of research. Experimental efforts focus on providing key spectroscopic information that will shed light into the structure of exotic nuclei and understanding the driving mechanism behind the shell evolution. In this work, we investigate the Z=6 spin-orbit shell gap towards the neutron dripline. To do so, we employed NA(p,2p)CA−1 quasi-free scattering reactions to measure the proton component of the 21+ state of 16,18,20C. The experimental findings support the notion of a moderate reduction of the proton 1p1/2−1p3/2 spin-orbit splitting, at variance to recent claims for a prevalent Z=6 magic number towards the neutron dripline.

Parallel evolution in phenotype may result when closely related taxa are adapting in the face of similar ecological pressures. Here, we discuss possible parallelism in shell morphology in the context of the microgeographic phylogeography of two conchologically distinct sympatric hydrocenid snails inhabiting a limestone outcrop and its cave system, Georissa pyrrhoderma and Georissa silaburensis, respectively, at Mount Silabur in Sarawak, Malaysian Borneo. Our results show a certain degree of morphological parallelism of a third, possibly new, cryptic Georissa species within the same cave that strongly diverged from its above‐ground sister species, G. pyrrhoderma. We found that both sympatric cave species have shifted from a more sculptured, conical shell toward a broader, less sculptured form. [ABSTRACT FROM AUTHOR]

The Muensterelloidea is a superfamily of teudopseid octobrachians with a posteriorly patella‐shaped gladius. A morphometric comparison based on 148 muensterelloid gladii has yielded five new species accommodated in three new genera: Engeseriteuthis arcuatus gen. et sp. nov., Muensterella jillae sp. nov., Muensterella spinosa sp. nov., Tyrionella fauseri gen. et sp. nov. and Muensterellina johnjagti gen. et sp. nov. Cretaceous taxa 'Tusoteuthis' cobbani and 'Muensterella' tonii are re‐combined and placed within the genus Enchoteuthis. We introduce categories for gladius proportions applicable for both muensterelloid and non‐muensterelloid octobrachian gladii. A 2D‐landmark analysis including 64 muensterelloid and non‐muensterelloid gladii statistically confirms that the Muensterelloidea possess the smallest median field sizes of all Mesozoic gladii. We consider the lateral field‐dominated 'gladius' of the family Patelloctopodidae (Patelloctopus, Pearceiteuthis) to be vestigial (i.e. shorter than the mantle length) suggesting that the Patelloctopodidae are the last shared ancestors of incirrate and cirrate octopods. According to a phylogenetic analysis based on 31 gladius characters, the Muensterelloidea mainly consists of the 'Muensterella‐Enchoteuthidae' and the 'patelloctopodid' clade. Ancestral character state reconstructions suggest that an increasing posterior growth front is accompanied by a continuous decrease of the median field length. This milestone in the evolution of the octopod gladius vestige occurred between the Early and Middle Jurassic. The benthic life style of incirrate octopods (including Cretaceous palaeoctopodids) was adopted by Jurassic Patelloctopodidae, which arose from nectonic to nectobenthic teudopseid ancestors. There is currently no evidence to suggest a pelagic origin for benthic octopods. [ABSTRACT FROM AUTHOR]

The low-lying unbound level structure of the halo nucleus 19C has been investigated using single-neutron knockout from 20C on a carbon target at 280 MeV/nucleon. The invariant mass spectrum, derived from the momenta of the forward going beam velocity 18C fragment and neutrons, was found to be dominated by a very narrow near threshold (Erel=0.036(1) MeV) peak. Two less strongly populated resonance-like features were also observed at Erel=0.84(4) and 2.31(3) MeV, both of which exhibit characteristics consistent with neutron p-shell hole states. Comparisons of the energies, measured cross sections and parallel momentum distributions to the results of shell-model and eikonal reaction calculations lead to spin-parity assignments of 5/21+ and 1/21− for the levels at Ex=0.62(9) and 2.89(10) MeV with Sn=0.58(9) MeV. Spectroscopic factors were also deduced and found to be in reasonable accord with shell-model calculations. The valence neutron configuration of the 20C ground state is thus seen to include, in addition to the known 1s1/22 component, a significant 0d5/22 contribution. The level scheme of 19C, including significantly the 1/21− cross-shell state, is well accounted for by the YSOX shell-model interaction developed from the monopole-based universal interaction.

Isomeric states in 59,61Ti have been populated in the projectile fragmentation of a 345 A MeV 238U beam at the Radioactive Isotope Beam Factory. The decay lifetimes and delayed γ -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. [ABSTRACT FROM AUTHOR]

We performed ab initio valence-space in-medium similarity renormalization group calculations based on chiral nucleon-nucleon and three-nucleon interactions to study the neutron-rich nuclei around N = 28 , where the experimental and theoretical evidence point to a rapid transition from spherical to rotational regimes. First, the rotational spectra of 40Mg, 42Si, and 44S are established theoretically, anchored by the calculated E 2 transition strength. The results suggest that 40Mg and 42Si exhibit large deformations. After that, the erosion of the N = 28 shell closure, especially the evolutions of shape deformation and the emergence of shape coexistence, are discussed in N = 28 even-even isotones ranging from 40Mg to 48Ca. Finally, the shell evolution of N = 28 in Mg and Si chains is discussed based on the experimental and predicted excitation energies of low-lying states. The results from our ab initio calculations align well with available experimental data, furnishing essential insights into the structure of neutron-rich nuclei, particularly deformations in the vicinity of 40Mg and 42Si. [ABSTRACT FROM AUTHOR]

New magic numbers have been discovered in the neutron-rich region of the nuclear chart. However, there has been a lack of research on proton-rich nuclei. 22O, the mirror nucleus of 22Si, is a double-magic nucleus bearing a high E (2 1 +). Whether 22Si exhibits double-magic characters is an intriguing topic. To investigate this matter, we utilized ab initio valence space in-medium similarity renormalization group for 22Si/22O, and their nearby nuclei. Our ab initio calculations provide good descriptions for the double magicity of 22O, as well as the shell evolution of N = 14 and Z = 14 through E (2 1 +). The computed E (2 1 +) indicate that the closure of Z = 14 sub-shell in proton-rich nuclei is weaker than the N = 14 sub-shell closure in their mirror nuclei. Particularly, the calculated E (2 1 +) of 22Si is 800 keV lower than the one of 22O. To further explore the magicity of 22Si, the mirror energy difference (MED) of 26Si/26Mg, 24Si/24Ne, as well as 22Si/22O are calculated. The results demonstrate that the calculated MEDs agree well with available experimental data, and the E (2 1 +) values of 22,24,26Si are all lower than their respective mirror nuclei due to the Thomas-Ehrman shift with large s 1 / 2 occupation. Moreover, our calculation provides that the many-body configurations of the low-lying state of 22Si/22O are nearly identical despite the fact that the states bearing large MED. In conclusion, our ab initio results suggest that 22Si is a double magic nucleus, similar to its mirror nucleus 22O, albeit with a lower E (2 1 +). [ABSTRACT FROM AUTHOR]

Detailed γ -ray spectroscopy of the exotic neon isotope 28Ne has been performed for the first time using the one-neutron removal reaction from 29Ne on a liquid hydrogen target at 240 MeV/nucleon. Based on an analysis of parallel momentum distributions, a level scheme with spin-parity assignments has been constructed for 28Ne and the negative-parity states are identified for the first time. The measured partial cross sections and momentum distributions reveal a significant intruder p -wave strength providing evidence of the breakdown of the N = 20 and N = 28 shell gaps. Only a weak, possible f -wave strength was observed to bound final states. Large-scale shell-model calculations with different effective interactions do not reproduce the large p -wave and small f -wave strength observed experimentally, indicating an ongoing challenge for a complete theoretical description of the transition into the island of inversion along the Ne isotopic chain. [ABSTRACT FROM AUTHOR]

Gamma decays were observed in 56Ca and 58Ca following quasi-free one-proton knockout reactions from 57,59Sc beams at ≈200 MeV/nucleon. For 56Ca, a γ ray transition was measured to be 1456(12) keV, while for 58Ca an indication for a transition was observed at 1115(34) keV. Both transitions were tentatively assigned as the 2 1 + → 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 0 f 5 / 2 and 0 g 9 / 2 orbitals, are almost degenerate. This degeneracy precludes the possibility for a doubly magic 60Ca and potentially drives the dripline of Ca isotopes to 70Ca or even beyond. [ABSTRACT FROM AUTHOR]

The low-lying unbound level structure of the halo nucleus 19 C has been investigated using single-neutron knockout from 20 C on a carbon target at 280 MeV/nucleon. The invariant mass spectrum, derived from the momenta of the forward going beam velocity 18 C fragment and neutrons, was found to be dominated by a very narrow near threshold ( E rel = 0.036 ( 1 ) MeV) peak. Two less strongly populated resonance-like features were also observed at E rel = 0.84 ( 4 ) and 2.31 ( 3 ) MeV, both of which exhibit characteristics consistent with neutron p -shell hole states. Comparisons of the energies, measured cross sections and parallel momentum distributions to the results of shell-model and eikonal reaction calculations lead to spin-parity assignments of 5 / 2 1 + and 1 / 2 1 − for the levels at E x = 0.62 ( 9 ) and 2.89 ( 10 ) MeV with S n = 0.58 ( 9 ) MeV. Spectroscopic factors were also deduced and found to be in reasonable accord with shell-model calculations. The valence neutron configuration of the 20 C ground state is thus seen to include, in addition to the known 1 s 1 / 2 2 component, a significant 0 d 5 / 2 2 contribution. The level scheme of 19 C, including significantly the 1 / 2 1 − cross-shell state, is well accounted for by the YSOX shell-model interaction developed from the monopole-based universal interaction. [ABSTRACT FROM AUTHOR]

Understanding the properties of nuclei inside 'island of inversion' is still an interesting issue. Based on a simple Nilsson model with a new set of isospin-dependent parameters, and with non-axial deformations considered, we have performed three-dimensional potential-energy-surface calculations for Ne, Na, Mg and Al isotopes that are claimed to be in or nearby the A ~ 30 island of inversion. It is found that shape coexistence and triaxial deformation (or softness) exist in these nuclei. Large deformations are obtained by the improved Nilsson parameters, which explains the observed large electric quadrupole transition probabilities. The large deformations happening in Ne, Na, Mg and Al indicate the quenching of the spherical N = 20 neutron shell gap. The calculations of nuclear binding and two-neutron separation energies have been also improved with the isospin-dependent parameters and the inclusion of the non-axial deformation degree of freedom. [ABSTRACT FROM AUTHOR]