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The heaviest bound isotope of boron ¹⁹B has been investigated using exclusive measurements of its Coulomb dissociation, into ¹⁷B and two neutrons, in collisions with Pb at 220 MeV/nucleon. Enhanced electric dipole (E1) strength is observed just above the two-neutron decay threshold with an integrated E1 strength of B(E1)=1.64±0.06(stat)±0.12(sys) e² fm² for relative energies below 6 MeV. This feature, known as a soft E1 excitation, provides the first firm evidence that ¹⁹B has a prominent two-neutron halo. Three-body calculations that reproduce the energy spectrum indicate that the valence neutrons have a significant s-wave configuration and exhibit a dineutronlike correlation., コンパクトな新奇中性子対の新たな証拠を発見 --不安定核ビームを用いた実験と少数系理論により実現--. 京都大学プレスリリース. 2020-06-22.

Background: Neutron-rich nuclei with mass number between 100 and 110 attract much attention, since several kinds of shapes, such as spherical, prolate, oblate, and triaxial shapes, are predicted. In particular, for neutron-rich Mo isotopes, different models predict different magnitudes and rigidity of triaxial deformation. Previous interpretations of experimental results based solely on low-lying 2+2 states are insufficient to distinguish between the rigid triaxial shape, γ vibration, or γ -soft rotor. Purpose: The shape evolution of 106Mo, 108Mo, and 110Mo is investigated through their 2+1-state lifetimes, decay-branching ratios of the 2+2 state, and energies of the low-lying collective excited states with Kπ = 0+, 2+, and 4+. Method: β -delayed γ -ray spectroscopy was employed for neutron-rich Nb and Zr isotopes produced at the RIKEN RI Beam Factory to populate excited states in 106Mo , 108Mo , and 110Mo . The EUroball-RIKEN Cluster Array was used for high-resolution γ -ray detection and lifetimes of the 2+1 states were determined using the UK fast-timing array of LaBr3(Ce) detectors. Results: New γ -ray transitions and levels are reported, including newly assigned 0+2 states in 108,110Mo . Quadrupole deformations were obtained for 106,108,110Mo from their 2+1 energies and lifetimes. The β -delayed neutron-emission probabilities of 108 Nb and 110 Nb were determined by examining the γ rays of their respective daughter decays. Conclusions: The even-odd energy staggering in the 2+2 band was compared with typical patterns of the γ -vibrational band, rigid triaxial rotor, and γ -soft rotor. The very small even-odd staggering of 106 Mo, 108Mo, and 110Mo favors a γ -vibrational band assignment. The kinematic moment of inertia for the 2+2 band showed a trend similar to the ground-state band, which is as expected for the γ -vibrational band. Beyond-mean-field calculations employing the constrained Hartree-Fock-Bogoliubov and local quasiparticle-random-phase approximation method using the SLy 5 + T interaction reproduced the ground and 2+2 bands in 106Mo and 108Mo . The collective wave functions are consistent with the interpretation of the 2+2 band as the γ -vibrational band of the prolate shape. However, the staggering pattern observed in 110Mo differs from the one suggested in the calculations which predict a γ -soft rotor. There was no experimental indication of the oblate shape or the γ -soft rotor predicted in these Mo isotopes.

24 pags., 17 figs., 4 tabs., Background: Neutron-rich nuclei with mass number between 100 and 110 attract much attention, since several kinds of shapes, such as spherical, prolate, oblate, and triaxial shapes, are predicted. In particular, for neutron-rich Mo isotopes, different models predict different magnitudes and rigidity of triaxial deformation. Previous interpretations of experimental results based solely on low-lying 22+ states are insufficient to distinguish between the rigid triaxial shape, γ vibration, or γ-soft rotor. Purpose: The shape evolution of Mo106, Mo108, and Mo110 is investigated through their 21+-state lifetimes, decay-branching ratios of the 22+ state, and energies of the low-lying collective excited states with Kπ=0+,2+, and 4+. Method: β-delayed γ-ray spectroscopy was employed for neutron-rich Nb and Zr isotopes produced at the RIKEN RI Beam Factory to populate excited states in Mo106, Mo108, and Mo110. The EUroball-RIKEN Cluster Array was used for high-resolution γ-ray detection and lifetimes of the 21+ states were determined using the UK fast-timing array of LaBr3(Ce) detectors. Results: New γ-ray transitions and levels are reported, including newly assigned 02+ states in Mo108,110. Quadrupole deformations were obtained for Mo106,108,110 from their 21+ energies and lifetimes. The β-delayed neutron-emission probabilities of Nb108 and Nb110 were determined by examining the γ rays of their respective daughter decays. Conclusions: The even-odd energy staggering in the 22+ band was compared with typical patterns of the γ-vibrational band, rigid triaxial rotor, and γ-soft rotor. The very small even-odd staggering of Mo106, Mo108, and Mo110 favors a γ-vibrational band assignment. The kinematic moment of inertia for the 22+ band showed a trend similar to the ground-state band, which is as expected for the γ-vibrational band. Beyond-mean-field calculations employing the constrained Hartree-Fock-Bogoliubov and local quasiparticle-random-phase approximation method using the SLy5+T interaction reproduced the ground and 22+ bands in Mo106 and Mo108. The collective wave functions are consistent with the interpretation of the 22+ band as the γ-vibrational band of the prolate shape. However, the staggering pattern observed in Mo110 differs from the one suggested in the calculations which predict a γ-soft rotor. There was no experimental indication of the oblate shape or the γ-soft rotor predicted in these Mo isotopes., We would like to express our gratitude to the RIKEN Nishina Center accelerator staff for providing a stable and high intensity 238U primary beam. This work was supported by JSPS KAKENHI Grants No. 24740188, No. 25247045, No. 26800117, and No. 16K17680; NRF Grant No. 2016K1A3A7A09005575; STFC Grants No. ST/J000132/1, No. ST/J000051/1, and No. ST/K502431/1; DOE Grant No. DE-FG02-91ER-40609; BMBF Grant No. 05P19RDFN1; and Spanish Ministerio de Ciencia e Innovación under Contracts No. FPA2009-13377-C02 and No. FPA2011- 29854-C04. P.H.R. acknowledges support from the UK National Measurement Office (NMO). P.-A.S. was financed by JSPS Grant No. 2301752 and the RIKEN Foreign Postdoctoral Researcher Program. We acknowledge the EUROBALL Owners Committee for the loan of germanium detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors. N.H. acknowledges the JSPS-NSFC Bilateral Program for the Joint Research Project on “Nuclear mass and life for unravelling mysteries of r-process.” Numerical calculations were performed in part using the COMA (PACSIX) and Oakforest-PACS provided by Multidisciplinary Cooperative Research Program in Center for Computational Sciences, University of Tsukuba.

We report on new γ-ray spectroscopy results from β decays of In99 and Sn101. 30 new γ rays were observed following the β decay of In99, and inconsistencies in the literature with respect to the γ rays following the β decay of Sn101 were addressed with two confirmed cases and two new transitions. The experimental γ-ray energies, intensities, and coincidence relationships are discussed with shell model calculations, where theoretical β-decay branching ratios from the parent nuclei and γ-ray cascades of excited states from the daughter nuclei were combined to generate hypothetical βγ spectra and βγγ coincidence matrices. The most intense β-delayed γ-ray branches in both Cd99 and In101 were well reproduced with this approach, and several γ rays were assigned to new excited states based on their good agreement with shell model predictions.

International audience; The heaviest bound isotope of boron B19 has been investigated using exclusive measurements of its Coulomb dissociation, into B17 and two neutrons, in collisions with Pb at 220 MeV/nucleon. Enhanced electric dipole (E1) strength is observed just above the two-neutron decay threshold with an integrated E1 strength of B(E1)=1.64±0.06(stat)±0.12(sys) e2 fm2 for relative energies below 6 MeV. This feature, known as a soft E1 excitation, provides the first firm evidence that B19 has a prominent two-neutron halo. Three-body calculations that reproduce the energy spectrum indicate that the valence neutrons have a significant s-wave configuration and exhibit a dineutronlike correlation.

International audience; Background: Neutron-rich nuclei with mass number between 100 and 110 attract much attention, since several kinds of shapes, such as spherical, prolate, oblate, and triaxial shapes, are predicted. In particular, for neutron-rich Mo isotopes, different models predict different magnitudes and rigidity of triaxial deformation. Previous interpretations of experimental results based solely on low-lying 22+ states are insufficient to distinguish between the rigid triaxial shape, γ vibration, or γ-soft rotor. Purpose: The shape evolution of Mo106, Mo108, and Mo110 is investigated through their 21+-state lifetimes, decay-branching ratios of the 22+ state, and energies of the low-lying collective excited states with Kπ=0+,2+, and 4+. Method: β-delayed γ-ray spectroscopy was employed for neutron-rich Nb and Zr isotopes produced at the RIKEN RI Beam Factory to populate excited states in Mo106, Mo108, and Mo110. The EUroball-RIKEN Cluster Array was used for high-resolution γ-ray detection and lifetimes of the 21+ states were determined using the UK fast-timing array of LaBr3(Ce) detectors. Results: New γ-ray transitions and levels are reported, including newly assigned 02+ states in Mo108,110. Quadrupole deformations were obtained for Mo106,108,110 from their 21+ energies and lifetimes. The β-delayed neutron-emission probabilities of Nb108 and Nb110 were determined by examining the γ rays of their respective daughter decays. Conclusions: The even-odd energy staggering in the 22+ band was compared with typical patterns of the γ-vibrational band, rigid triaxial rotor, and γ-soft rotor. The very small even-odd staggering of Mo106, Mo108, and Mo110 favors a γ-vibrational band assignment. The kinematic moment of inertia for the 22+ band showed a trend similar to the ground-state band, which is as expected for the γ-vibrational band. Beyond-mean-field calculations employing the constrained Hartree-Fock-Bogoliubov and local quasiparticle-random-phase approximation method using the SLy5+T interaction reproduced the ground and 22+ bands in Mo106 and Mo108. The collective wave functions are consistent with the interpretation of the 22+ band as the γ-vibrational band of the prolate shape. However, the staggering pattern observed in Mo110 differs from the one suggested in the calculations which predict a γ-soft rotor. There was no experimental indication of the oblate shape or the γ-soft rotor predicted in these Mo isotopes.

International audience; We report on new γ-ray spectroscopy results from β decays of In99 and Sn101. 30 new γ rays were observed following the β decay of In99, and inconsistencies in the literature with respect to the γ rays following the β decay of Sn101 were addressed with two confirmed cases and two new transitions. The experimental γ-ray energies, intensities, and coincidence relationships are discussed with shell model calculations, where theoretical β-decay branching ratios from the parent nuclei and γ-ray cascades of excited states from the daughter nuclei were combined to generate hypothetical βγ spectra and βγγ coincidence matrices. The most intense β-delayed γ-ray branches in both Cd99 and In101 were well reproduced with this approach, and several γ rays were assigned to new excited states based on their good agreement with shell model predictions. (APS)

11 pags., 10 figs., 3 tabs., We report on new γ-ray spectroscopy results from β decays of In99 and Sn101. 30 new γ rays were observed following the β decay of In99, and inconsistencies in the literature with respect to the γ rays following the β decay of Sn101 were addressed with two confirmed cases and two new transitions. The experimental γ-ray energies, intensities, and coincidence relationships are discussed with shell model calculations, where theoretical β-decay branching ratios from the parent nuclei and γ-ray cascades of excited states from the daughter nuclei were combined to generate hypothetical βγ spectra and βγγ coincidence matrices. The most intense β-delayed γ-ray branches in both Cd99 and In101 were well reproduced with this approach, and several γ rays were assigned to new excited states based on their good agreement with shell model predictions., The authors would like to thank the personnel at the RIKEN Nishina Center for providing the exotic radioactive isotope beam with record intensities. This experiment was performed at RI Beam Factory operated by RIKEN Nishina Center and CNS, University of Tokyo. We acknowledge the EUROBALL Owners Committee for loaning the germanium detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors of EURICA. Support for the WAS3ABi setup was provided by the Rare Isotope Science Project, funded by the Ministry of Education, Science and Technology (MEST) and National Research Foundation (NRF) of Korea, as well as KAKENHI (Grant No. 25247045) of Japan Society for the Promotion of Science (JSPS). This work was supported by the Institute for Basic Science (IBS-R031-D1). The authors acknowledge also the support of the DFG cluster of excellence “Origin and Structure of the Universe,” German BMBF under Contracts No. 05P15PKFNA and No. 05P19PKFNA, and the Spanish Ministerio de Economía y Competitividad via Project No. FPA2017-84756-C4-2-P. Part of the research was funded by the Natural Sciences and Engineering Research Council (NSERC) of Canada and also supported by FJNSP LIA (French-Japanese International Associated Laboratory for Nuclear Structure Problems)

6 pags., 4 figs., A record number of Sn100 nuclei was detected and new isotopic species toward the proton dripline were discovered at the RIKEN Nishina Center. Decay spectroscopy was performed with the high-efficiency detector arrays WAS3ABi and EURICA. Both the half-life and the β-decay end point energy of Sn100 were measured more precisely than the literature values. The value and the uncertainty of the resulting strength for the pure 0+→1+ Gamow-Teller decay was improved to BGT=4.4-0.7+0.9. A discrimination between different model calculations was possible for the first time, and the level scheme of In100 is investigated further., This work was carried out at the RIBF operated by RIKEN Nishina Center, RIKEN and CNS, University of Tokyo. It was supported and financed by the DFG cluster of excellence “Origin and Structure of the Universe,” the German BMBF under Contracts No. 05P12WOFNF and No. 05P15PKFNA, the Hanns-Seidel-Stiftung, and the RIKEN IPA Programme. We wish to acknowledge the EURICA RIBF09 collaboration as well as the support on the WAS3ABi setup from the Rare Isotope Science Project, funded by the Ministry of Education, Science and Technology and National Research Foundation of Korea KAKENHI (Grant No. 25247045). Part of the research was also funded by the Natural Sciences and Engineering Research Council of Canada, supported by FJNSP LIA (French-Japanese International Associated Laboratory for Nuclear Structure Problems) and by the Spanish Ministerio de Economía y Competitividad under Contract No. FPA2017-84756-C4-2-P.

The β-delayed γ-ray spectroscopy of neutron-rich ^{123,125}Ag isotopes is investigated at the Radioactive Isotope Beam Factory of RIKEN, and the long-predicted 1/2^{-} β-emitting isomers in ^{123,125}Ag are identified for the first time. With the new experimental results, the systematic trend of energy spacing between the lowest 9/2^{+} and 1/2^{-} levels is extended in Ag isotopes up to N=78, providing a clear signal for the reduction of the Z=40 subshell gap in Ag towards N=82. Shell-model calculations with the state-of-the-art V_{MU} plus M3Y spin-orbit interaction give a satisfactory description of the low-lying states in ^{123,125}Ag. The tensor force is found to play a crucial role in the evolution of the size of the Z=40 subshell gap. The observed inversion of the single-particle levels around ^{123}Ag can be well interpreted in terms of the monopole shift of the π1g_{9/2} orbitals mainly caused by the increasing occupation of ν1h_{11/2} orbitals. ispartof: PHYSICAL REVIEW LETTERS vol:122 issue:21 ispartof: location:United States status: published

International audience; A record number of Sn100 nuclei was detected and new isotopic species toward the proton dripline were discovered at the RIKEN Nishina Center. Decay spectroscopy was performed with the high-efficiency detector arrays WAS3ABi and EURICA. Both the half-life and the β-decay end point energy of Sn100 were measured more precisely than the literature values. The value and the uncertainty of the resulting strength for the pure 0+→1+ Gamow-Teller decay was improved to BGT=4.4-0.7+0.9. A discrimination between different model calculations was possible for the first time, and the level scheme of In100 is investigated further.

The β-delayed γ-ray spectroscopy of neutron-rich 123;125Ag isotopes is investigated at the Radioactive Isotope Beam Factory of RIKEN and the long-predicted 1/2¯ β-emitting isomers in 123;125Ag are identified for the first time. With the new experimental results, the systematic trend of energy spacing between the lowest 9/2+ and 1/2¯ levels is extended in Ag isotopes up to N = 78, providing a clear signal for the reduction of the Z = 40 subshell gap in Ag towards N = 82. Shellmodel calculations with the state-of-the-art VMU plus M3Y spin-orbit interaction give a satisfactory description of the low-lying states in 123;125Ag. The tensor force is found to play a crucial role in the evolution of the size of the Z = 40 subshell gap. The observed inversion of the single particle levels around 123Ag can be well interpreted in terms of the monopole shift of the π1g9/2 orbitals mainly caused by the increasing occupation of ʋ1h11/2 orbitals.

The neutron-rich isotopes of palladium have attracted considerable interest in terms of the evolution of the N=82 neutron shell closure and its influence on the r-process nucleosynthesis. In this Letter, we present the first spectroscopic information on the excited states in $^{125}$Pd$_{79}$ and $^{127}$Pd$_{81}$ studied using the EURICA γ-ray spectrometer, following production via in-flight fission of a high-intensity $^{238}$U beam at the RIBF facility. New isomeric states with half-lives of 144(4) ns and 39(6) μs have been assigned spins and parities of (23/2+) and (19/2+) in $^{125}$Pd and $^{127}$Pd, respectively. The observed level properties are compared to a shell-model calculation, suggesting the competition between proton excitations and neutron excitations in the proton-hole and neutron-hole systems in the vicinity of the doubly magic nucleus $^{132}$Sn.

A γ-decaying isomeric state (τ1/2=197-17+19 ns) has been identified in Cd96, which is one α particle away from the last known bound N=Z nucleus, Sn100. Comparison of the results with shell-model calculations has allowed a tentative experimental level scheme to be deduced and the isomer to be interpreted as a medium-spin negative-parity spin trap based on the coupling of isoscalar (T=0) and isovector (T=1) neutron-proton pairs. The data also suggest evidence for the population of a 9+ T=1 state, which is predicted by shell-model calculations to be yrast. Such a low-lying T=1 state, which is unknown in lighter mass even-even self-conjugate nuclei, can also be interpreted in terms of the coupling of T=0 and T=1 neutron-proton pairs.

6 pags., 5 figs., A γ-decaying isomeric state (τ1/2=197-17+19 ns) has been identified in Cd96, which is one α particle away from the last known bound N=Z nucleus, Sn100. Comparison of the results with shell-model calculations has allowed a tentative experimental level scheme to be deduced and the isomer to be interpreted as a medium-spin negative-parity spin trap based on the coupling of isoscalar (T=0) and isovector (T=1) neutron-proton pairs. The data also suggest evidence for the population of a 9+ T=1 state, which is predicted by shell-model calculations to be yrast. Such a low-lying T=1 state, which is unknown in lighter mass even-even self-conjugate nuclei, can also be interpreted in terms of the coupling of T=0 and T=1 neutron-proton pairs., This work was partially supported by the German BMBF Grants No. 05P12PKFNE and No. 05P15RDFN1, the UK STFC under Grants No. ST/J000124/1 and No. ST/L005727/1, the Natural Sciences and Engineering Research Council (NSERC) of Canada, the U.S. DOE Grant No. DE-FG02-91ER40609, KAKENHI under Grant No. 25247045, and the Spanish Ministerio de Economía y Competitividad under Contract No. FPA2014-57196-C5-4-P.

International audience; A γ -decaying isomeric state (τ1/2 = 197$^{+19}_{−17}$ ns) has been identified in 96Cd, which is one α particle away from the last known bound N = Z nucleus, 100Sn. Comparison of the results with shell-model calculations has allowed a tentative experimental level scheme to be deduced and the isomer to be interpreted as a medium-spin negativeparity spin trap based on the coupling of isoscalar (T = 0) and isovector (T = 1) neutron-proton pairs. The data also suggest evidence for the population of a 9+ T = 1 state, which is predicted by shell-model calculations to be yrast. Such a low-lying T = 1 state, which is unknown in lighter mass even-even self-conjugate nuclei, can also be interpreted in terms of the coupling of T = 0 and T = 1 neutron-proton pairs.

International audience; The β-delayed γ-ray spectroscopy of neutron-rich Ag123,125 isotopes is investigated at the Radioactive Isotope Beam Factory of RIKEN, and the long-predicted 1/2− β-emitting isomers in Ag123,125 are identified for the first time. With the new experimental results, the systematic trend of energy spacing between the lowest 9/2+ and 1/2− levels is extended in Ag isotopes up to N=78, providing a clear signal for the reduction of the Z=40 subshell gap in Ag towards N=82. Shell-model calculations with the state-of-the-art VMU plus M3Y spin-orbit interaction give a satisfactory description of the low-lying states in Ag123,125. The tensor force is found to play a crucial role in the evolution of the size of the Z=40 subshell gap. The observed inversion of the single-particle levels around Ag123 can be well interpreted in terms of the monopole shift of the π1g9/2 orbitals mainly caused by the increasing occupation of ν1h11/2 orbitals.

A record number of Sn100 nuclei was detected and new isotopic species toward the proton dripline were discovered at the RIKEN Nishina Center. Decay spectroscopy was performed with the high-efficiency detector arrays WAS3ABi and EURICA. Both the half-life and the β-decay end point energy of Sn100 were measured more precisely than the literature values. The value and the uncertainty of the resulting strength for the pure 0+→1+ Gamow-Teller decay was improved to BGT=4.4-0.7+0.9. A discrimination between different model calculations was possible for the first time, and the level scheme of In100 is investigated further. © 2019 American Physical Society.

International audience; A decay spectroscopy experiment on proton-rich nuclei in the vicinity of the doubly magic Sn100 was carried out at RIKEN Nishina Center. More than 20 nuclei with 43≤Z≤50 and N≤51, produced by fragmentation reactions were investigated via analyses of β-decay, βp-decay, and subsequent γ-ray data. Owing to higher statistics, the precision on the half-lives of many of the ground states and isomers was improved. β-decay endpoint energies of 11 states in 8 nuclei were measured for the first time, and the corresponding QEC and excitation energies were generally consistent with various mass models. Many β-delayed proton emission branching ratios were measured either for the first time or with higher precision compared to literature values, and some of them differed by more than 2σ. Many of the large discrepancies were associated with nuclei with long-lived isomeric states, highlighting large systematic uncertainties involved in these measurements. Twenty-five new γ rays were observed, and ten new states are proposed with unambiguous excitation energies, spins, and parities. Most of the energies of the excited states were consistent within 300 keV or 20%, whichever was greater, compared to shell model predictions in the proton/neutron (p1/2,g9/2) model space assuming a Sr76 core. A signature of a new (1/2−) isomer in Cd97 with T1/2=0.73(7) s was found, in good agreement with shell model predictions.