Mass, Spectroscopy, and Two-Neutron Decay of

The structure and decay of the most neutron-rich beryllium isotope, ^{16}Be, has been investigated following proton knockout from a high-energy ^{17}B beam. Two relatively narrow resonances were observed for the first time, with energies of 0.84(3) and 2.15(5) MeV above the two-neutron decay threshold and widths of 0.32(8) and 0.95(15) MeV, respectively. These were assigned to be the ground (J^{π}=0^{+}) and first excited (2^{+}) state, with E_{x}=1.31(6) MeV. The mass excess of ^{16}Be was thus deduced to be 56.93(13) MeV, some 0.5 MeV more bound than the only previous measurement. Both states were observed to decay by direct two-neutron emission. Calculations incorporating the evolution of the wave function during the decay as a genuine three-body process reproduced the principal characteristics of the neutron-neutron energy spectra for both levels, indicating that the ground state exhibits a strong spatially compact dineutron component, while the 2^{+} level presents a far more diffuse neutron-neutron distribution.

Quasifree Neutron Knockout Reaction Reveals a Small s-Orbital Component in the Borromean Nucleus

A kinematically complete quasifree (p,pn) experiment in inverse kinematics was performed to study the structure of the Borromean nucleus ^{17}B, which had long been considered to have a neutron halo. By analyzing the momentum distributions and exclusive cross sections, we obtained the spectroscopic factors for 1s_{1/2} and 0d_{5/2} orbitals, and a surprisingly small percentage of 9(2)% was determined for 1s_{1/2}. Our finding of such a small 1s_{1/2} component and the halo features reported in prior experiments can be explained by the deformed relativistic Hartree-Bogoliubov theory in continuum, revealing a definite but not dominant neutron halo in ^{17}B. The present work gives the smallest s- or p-orbital component among known nuclei exhibiting halo features and implies that the dominant occupation of s or p orbitals is not a prerequisite for the occurrence of a neutron halo.

Sequential Nature of (p,3p) Two-Proton Knockout from Neutron-Rich Nuclei.

Twenty-one two-proton knockout (p,3p) cross sections were measured from neutron-rich nuclei at ∼250 MeV/nucleon in inverse kinematics. The angular distribution of the three emitted protons was determined for the first time, demonstrating that the (p,3p) kinematics are consistent with two sequential proton-proton collisions within the projectile nucleus. Ratios of (p,3p) over (p,2p) inclusive cross sections follow the trend of other many-nucleon removal reactions, further reinforcing the sequential nature of (p,3p) in neutron-rich nuclei.

Metastable States of

Here we present new information on the shape evolution of the very neutron-rich ^{92,94}Se nuclei from an isomer-decay spectroscopy experiment at the Radioactive Isotope Beam Factory at RIKEN. High-resolution germanium detectors were used to identify delayed γ rays emitted following the decay of their isomers. New transitions are reported extending the previously known level schemes. The isomeric levels are interpreted as originating from high-K quasineutron states with an oblate deformation of ß∼0.25, with the high-K state in ^{94}Se being metastable and K hindered. Following this, ^{94}Se is the lowest-mass neutron-rich nucleus known to date with such a substantial K hindrance. Furthermore, it is the first observation of an oblate K isomer in a deformed nucleus. This opens up the possibility for a new region of K isomers at low Z and at oblate deformation, involving the same neutron orbitals as the prolate orbitals within the classic Z∼72 deformed hafnium region. From an interpretation of the level scheme guided by theoretical calculations, an oblate deformation is also suggested for the ^{94}Se_{60} ground-state band.

Surface Localization of the Dineutron in

The formation of a dineutron in the ^{11}Li nucleus is found to be localized to the surface region. The experiment measured the intrinsic momentum of the struck neutron in ^{11}Li via the (p,pn) knockout reaction at 246 MeV/nucleon. The correlation angle between the two neutrons is, for the first time, measured as a function of the intrinsic neutron momentum. A comparison with reaction calculations reveals the localization of the dineutron at r∼3.6 fm. The results also support the density dependence of dineutron formation as deduced from Hartree-Fock-Bogoliubov calculations for nuclear matter.

78Ni revealed as a doubly magic stronghold against nuclear deformation.

Nuclear magic numbers 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. Although the sequence of magic numbers is well established for stable nuclei, experimental evidence has revealed modifications for nuclei with a large asymmetry between proton and neutron numbers. Here we provide a spectroscopic study of the doubly magic nucleus 78Ni, which contains fourteen neutrons more than the heaviest stable nickel isotope. We provide direct evidence of 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. Our results also indicate 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 a rapid transition from spherical to deformed ground states, with 78Ni as the turning point.

Prominence of Pairing in Inclusive (p,2p) and (p,pn) Cross Sections from Neutron-Rich Nuclei.

Fifty-five inclusive single nucleon-removal cross sections from medium mass neutron-rich nuclei impinging on a hydrogen target at ∼250 MeV/nucleon are measured at the RIKEN Radioactive Isotope Beam Factory. Systematically higher cross sections are found for proton removal from nuclei with an even number of protons as compared to odd-proton number projectiles for a given neutron separation energy. Neutron removal cross sections display no even-odd splitting, contrary to nuclear cascade model predictions. Both effects are understood through simple considerations of neutron separation energies and bound state level densities originating in pairing correlations in the daughter nuclei. These conclusions are supported by comparison with semimicroscopic model predictions, highlighting the enhanced role of low-lying level densities in nucleon-removal cross sections from loosely bound nuclei.

Erratum: Persistence of the Z=28 Shell Gap Around

This corrects the article DOI: 10.1103/PhysRevLett.119.192501.

Persistence of the Z=28 Shell Gap Around

In-beam γ-ray spectroscopy of ^{79}Cu is performed at the Radioactive Isotope Beam Factory of RIKEN. The nucleus of interest is produced through proton knockout from a ^{80}Zn beam at 270 MeV/nucleon. The level scheme up to 4.6 MeV is established for the first time and the results are compared to Monte Carlo shell-model calculations. We do not observe significant knockout feeding to the excited states below 2.2 MeV, which indicates that the Z=28 gap at N=50 remains large. The results show that the ^{79}Cu nucleus can be described in terms of a valence proton outside a ^{78}Ni core, implying the magic character of the latter.

Shape Evolution in Neutron-Rich Krypton Isotopes Beyond N=60: First Spectroscopy of

We report on the first γ-ray spectroscopy of low-lying states in neutron-rich ^{98,100}Kr isotopes obtained from ^{99,101}Rb(p,2p) reactions at ∼220 MeV/nucleon. A reduction of the 2_{1}^{+} state energies beyond N=60 demonstrates a significant increase of deformation, shifted in neutron number compared to the sharper transition observed in strontium and zirconium isotopes. State-of-the-art beyond-mean-field calculations using the Gogny D1S interaction predict level energies in good agreement with experimental results. The identification of a low-lying (0_{2}^{+}, 2_{2}^{+}) state in ^{98}Kr provides the first experimental evidence of a competing configuration at low energy in neutron-rich krypton isotopes consistent with the oblate-prolate shape coexistence picture suggested by theory.

Are There Signatures of Harmonic Oscillator Shells Far from Stability? First Spectroscopy of

The first measurement of the low-lying states of the neutron-rich ^{110}Zr and ^{112}Mo was performed via in-beam γ-ray spectroscopy after one proton removal on hydrogen at ∼200 MeV/nucleon. The 2_{1}^{+} excitation energies were found at 185(11) keV in ^{110}Zr, and 235(7) keV in ^{112}Mo, while the R_{42}=E(4_{1}^{+})/E(2_{1}^{+}) ratios are 3.1(2), close to the rigid rotor value, and 2.7(1), respectively. These results are compared to modern energy density functional based configuration mixing models using Gogny and Skyrme effective interactions. We conclude that first levels of ^{110}Zr exhibit a rotational behavior, in agreement with previous observations of lighter zirconium isotopes as well as with the most advanced Monte Carlo shell model predictions. The data, therefore, do not support a harmonic oscillator shell stabilization scenario at Z=40 and N=70. The present data also invalidate predictions for a tetrahedral ground state symmetry in ^{110}Zr.

Extension of the N=40 Island of Inversion towards N=50: Spectroscopy of (66)Cr, (70,72)Fe.

We report on the measurement of the first 2(+) and 4(+) states of (66)Cr and (70,72)Fe via in-beam γ-ray spectroscopy. The nuclei of interest were produced by (p,2p) reactions at incident energies of 260 MeV/nucleon. The experiment was performed at the Radioactive Isotope Beam Factory, RIKEN, using the DALI 2γ-ray detector array and the novel MINOS device, a thick liquid hydrogen target combined with a vertex tracker. A low-energy plateau of 2(1)(+) and 4(1)(+) energies as a function of the neutron number was observed for N≥38 and N≥40 for even-even Cr and Fe isotopes, respectively. State-of-the-art shell model calculations with a modified Lenzi-Nowacki-Poves-Sieja (LNPS) interaction in the pfg(9/2)d(5/2) valence space reproduce the observations. Interpretation within the shell model shows an extension of the island of inversion at N=40 for more neutron-rich isotopes towards N=50.