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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 $^{132}$Sn using the newly obtained experimental data on a long-lived isomer in $^{127}$Ag. 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 $^{127}$Cd. The underlying mechanism of a strong GT transition from the $^{127}$Ag isomer is discussed in terms of configuration-dependent optimization of the effective single-particle energies in the framework of a shell-model approach.

Reduced transition probabilities have been measured for the neutron-rich Ni73,74,75 nuclei with relativistic Coulomb excitation performed at the RIKEN Nishina Center. For the even-even Ni isotope the determined B(E2;0+→2+) value is compatible within errors with the result of a previous intermediate energy Coulomb excitation experiment, although being somewhat larger. For the odd Ni isotopes B(E2) values have been determined for the first time. In the middle νg9/2 shell nucleus Ni73 three peak candidates have been observed and tentatively assigned to the (5/2+), (13/2+), and (11/2+) excitations. Two peak candidates have been observed in Ni75 and tentatively assigned to the (13/2+) and (11/2+) levels. The measured B(E2) values are in line with the large B(E2) reported in Ni70 and at variance with the small strength measured in Ni72. Excitation energies compare well with the result of large-scale shell-model calculations, while B(E2) values are larger than predicted. This could indicate an increasing contribution of proton excitations across the Z=28 shell closure when approaching Ni78.

International audience; Neutron-rich Pm (Z=61) isotopes were studied by delayed γ-ray spectroscopy at RIBF, RIKEN Nishina Center using the in-flight fission of a 345 MeV/nucleon U238 beam. A cluster-type Ge detector array, EURICA, was used to measure the delayed γ rays from stopped ions. Isomeric γ decays were observed in Pm159 and Pm161 with half-lives of 4.97(12) μs and 0.79(4) μs, respectively. Level schemes for Pm159 and Pm161 were constructed in this study. The isomeric states of Pm159 and Pm161 could be interpreted as two quasiparticle excitations of neutrons with the configurations of ν(7/2[633]⊗5/2[523]) and ν(7/2[633]⊗1/2[521]), respectively. They are analogous to the isomers that have been observed systematically in other even-mass N=98 and N=100 isotones in this region. A projected shell model calculation was performed and it reproduced the order of three-quasiparticle states only if new Nilsson parameters with an N-dependent spin-orbit interaction were used. This work demonstrates that the strength of spin-orbit interactions in standard Nilsson parameters needs to be modified to study the properties of neutron-rich rare-earth nuclei around A=165, and provides new evidence supporting the existence of the deformed N=98 subshell gap in odd-mass nuclei for the first time.

7 pags., 5 figs., 1 tab., Neutron-rich Pm (Z=61) isotopes were studied by delayed γ-ray spectroscopy at RIBF, RIKEN Nishina Center using the in-flight fission of a 345 MeV/nucleon U238 beam. A cluster-type Ge detector array, EURICA, was used to measure the delayed γ rays from stopped ions. Isomeric γ decays were observed in Pm159 and Pm161 with half-lives of 4.97(12) μs and 0.79(4) μs, respectively. Level schemes for Pm159 and Pm161 were constructed in this study. The isomeric states of Pm159 and Pm161 could be interpreted as two quasiparticle excitations of neutrons with the configurations of ν(7/2[633] - 5/2[523]) and ν(7/2[633] - 1/2[521]), respectively. They are analogous to the isomers that have been observed systematically in other even-mass N=98 and N=100 isotones in this region. A projected shell model calculation was performed and it reproduced the order of three-quasiparticle states only if new Nilsson parameters with an N-dependent spin-orbit interaction were used. This work demonstrates that the strength of spin-orbit interactions in standard Nilsson parameters needs to be modified to study the properties of neutron-rich rare-earth nuclei around A=165, and provides new evidence supporting the existence of the deformed N=98 subshell gap in odd-mass nuclei for the first time., We acknowledge financial support from JSPS KAKENHI (Grants No. 17H06090, No. 25247045). This experiment was carried out at the RI Beam Factory operated by RIKEN, Nishina Center and CNS, the University of Tokyo. The authors are grateful to the RIBF accelerator crew for providing the U beam. We acknowledge the EUROBALL Owners Committee for the loan of germanium detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors. Part of the WAS3ABi was supported by the Rare Isotope Science Project, which is funded by the Ministry of Education, Science and Technology (MEST) and the National Research Foundation (NRF) of Korea. This work was supported by the Spanish Ministerio de Ciencia e Innovación under Contracts No. FPA2009-13377-C02 and No. FPA2011-29854-C04 and the Hungarian Research Fund (OTKA), Contract No. K100835. The author R.Y. was supported by the ALPS program of the University of Tokyo and by a JSPS fellowship, No. JP15J10788. The theoretical work was partially supported by the National Natural Science Foundation of China (No. U1932206, No. U1832139, No. 11947410, No. 11905175, No. 11875225), and by the National Key Program for S&T Research and Development of China (No. 2016YFA0400501)

12 pags., 6 figs., 3 tabs., Background: The evolution of shell structure around doubly magic exotic nuclei is of great interest in nuclear physics and astrophysics. In the 'southwest' region of Ni78, 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 β decay of Co75, with one-proton and two-neutron holes relative to Ni78, to access new nuclear structure information in Co75 and its β-decay daughters Ni75 and Ni74. Methods: The nucleus Co75 is produced in relativistic in-flight fission reactions of U238 at the Radioactive Ion Beam Factory in the RIKEN Nishina Center. Its isomeric and β decay are studied exploiting the BigRIPS and EURICA setups. Results: We obtain partial β-decay spectra for Ni75 and Ni74, and report a new isomeric transition in Co75. The energy [Eγ=1914(2)keV] and half-life [t1/2=13(6)μs] of the delayed γ ray lend support for the existence of a Jπ=(1/2-) isomeric state at 1914(2) keV. A comparison with PFSDG-U shell-model calculations provides a good account for the observed states in Ni75, but the first calculated 1/2- level in Co75, 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 Ni75 is proved. In the case of Co75, the results suggest that the dominance of the spherical configurations over the deformed ones might be stronger than expected below Ni78. Further experimental efforts to discern the nature of the Jπ=(1/2-) isomer are necessary., The excellent work of the RIKEN accelerator staff for providing a stable and high-intensity beam of 238U is acknowledged. This work was partially supported by KAKENHI (Grants No. 25247045, No. 23.01752, and No. 25800130); US DOE Grant No. DE-FG02-91ER-40609; Spanish Ministerio de Ciencia e Innovación Contracts No. FPA2009-13377-C02, No. FPA2011-29854-C04, No. PID2019-104714GB-C21 (AEI), and No. FPA2017-83946-C2-1-P (MCIU/AEI/FEDER), the Severo Ochoa Programme SEV-2016-0597 and Grant No. PGC-2018-94583; Subvenciones grupos de excelencia de la Generalitat Valenciana: PROMETEO/2019/007; Spanish Comunidad de Madrid via the “Atracción de Talento Investigador” Program No. 2019- T1/TIC-13194; European Regional Development Fund Contract No. GINOP-2.3.3-15-2016-00034; National Research, Development and Innovation Fund of Hungary via Project No. K128947; and the German BMBF Grant No. 05P19RDFN1. The authors acknowledge the EUROBALL Owners Committee for the loan of germanium detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors. Part of the WAS3ABi was supported by the Rare Isotope Science Project which is funded by the Ministry of Education, Science and Technology (MEST) and National Research Foundation (NRF) of Korea

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.

9 pags., 7 figs., 1 tab., Several experiments were conducted at the 10 MV Van-de-Graaff tandem accelerator at the Institute of Nuclear Physics, Cologne, to detect proton emission from the isomeric 6457-keV 10 state in Ni. Excitation functions for two fusion–evaporation reactions were measured to maximise the population of the rare two-neutron evaporation channel from a Ni compound nucleus. The search for delayed proton emission was based on the Si (Si , 2 n)Ni reaction at a beam energy of 70 MeV. For this reaction, a cross-section limit for the population of the 10 state in Ni and its proton-decay branch was determined to be σ< 22 nb., Open Access funding provided by Projekt DEAL. We would like to thank the accelerator staff at the University of Cologne for the efforts to deliver heavy-ion beams of excellent quality, as well as the Swedish Research Council (contract VR 2008-4240 and VR 2016- 3969) for financial support.

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.

8 pags., 2 figs., 2 tabs., Reduced transition probabilities have been measured for the neutron-rich Ni73,74,75 nuclei with relativistic Coulomb excitation performed at the RIKEN Nishina Center. For the even-even Ni isotope the determined B(E2;0+→2+) value is compatible within errors with the result of a previous intermediate energy Coulomb excitation experiment, although being somewhat larger. For the odd Ni isotopes B(E2) values have been determined for the first time. In the middle νg9/2 shell nucleus Ni73 three peak candidates have been observed and tentatively assigned to the (5/2+), (13/2+), and (11/2+) excitations. Two peak candidates have been observed in Ni75 and tentatively assigned to the (13/2+) and (11/2+) levels. The measured B(E2) values are in line with the large B(E2) reported in Ni70 and at variance with the small strength measured in Ni72. Excitation energies compare well with the result of large-scale shell-model calculations, while B(E2) values are larger than predicted. This could indicate an increasing contribution of proton excitations across the Z=28 shell closure when approaching Ni78., The excellent work of the Riken-Nishina Center accelerator staff is acknowledged. We acknowledge the support of Istituto Nazionale di Fisica Nucleare, Italy. We also acknowledge the National Research, Development, and Innovation Fund of Hungary (Project No. K128947); the European Regional Development Fund (Contract No. GINOP-2.3.3-15- 2016-00034); and the National Research, Development, and Innovation Office (NKFIH Contract No. PD124717).

International audience; Reduced transition probabilities have been measured for the neutron-rich Ni73,74,75 nuclei with relativistic Coulomb excitation performed at the RIKEN Nishina Center. For the even-even Ni isotope the determined B(E2;0+→2+) value is compatible within errors with the result of a previous intermediate energy Coulomb excitation experiment, although being somewhat larger. For the odd Ni isotopes B(E2) values have been determined for the first time. In the middle νg9/2 shell nucleus Ni73 three peak candidates have been observed and tentatively assigned to the (5/2+), (13/2+), and (11/2+) excitations. Two peak candidates have been observed in Ni75 and tentatively assigned to the (13/2+) and (11/2+) levels. The measured B(E2) values are in line with the large B(E2) reported in Ni70 and at variance with the small strength measured in Ni72. Excitation energies compare well with the result of large-scale shell-model calculations, while B(E2) values are larger than predicted. This could indicate an increasing contribution of proton excitations across the Z=28 shell closure when approaching Ni78.

13 pags., 11 figs., 3 tabs., Background: Detailed spectroscopy of neutron-rich odd-A Cu isotopes is of great importance for studying the shell evolution in the region of Ni78. While there is experimental information on excited states in Cu69-73,77,79 isotopes, the information concerning Cu75 is very limited. Purpose: Experimentally observed single-particle, core-coupling, and proton-hole intruder states in Cu75, will complete the systematics of these states in the chain of isotopes. Method: Excited states in Cu75 were populated in the β decay of Ni75 isotopes. The Ni nuclei were produced by the in-flight fission of U238 projectiles, and were separated, identified, and implanted in a highly segmented Si detector array for the detection of the β-decay electrons. The β-delayed γ rays were detected in a HPGe cluster array. Monte Carlo shell model calculations were performed using the A3DA interaction built on the pfg9/2d5/2 model space for both neutrons and protons. Results: A level scheme of Cu75 was built up to ≈4 MeV by performing a γ-γ coincidence analysis. The excited states below 2 MeV were interpreted based on the systematics of neutron-rich odd-A Cu isotopes and the results of the shell model calculations. Conclusions: The evolution of the single-particle, core-coupling, and proton-hole intruder states in the chain of neutron-rich odd-A Cu isotopes is discussed in the present work, in connection with the newly observed level structure of Cu75., This work was carried out at the RIBF operated by RIKEN Nishina Center, RIKEN and CNS, University of Tokyo. The research leading to the results has received funding from the Research Council of Norway under project Grants No. 240104 and No. 213442 and from KAKENHI (under Grants No. 25247045, No. 23.01752, and No. 25800130); U.S. DOE Grant No. DE-FG02-91ER-40609; German BMBF Grant No. 05P19RDFN1 and DFG Grant No. SFB 1245; Spanish Ministerio de Ciencia e Innovación Contracts No. FPA2009-13377C02 and No. FPA2011-29854- C04; and GINOP-2.3.3-15-2016-00034 and the Hungarian Scientific Research Fund OTKA Contract No. K128947. The Monte Carlo shell-model calculations were performed on K computer at RIKEN AICS (hp140210, hp150224, hp160211). This work was supported in part by the HPCI Strategic Program (The Origin of Matter and the Universe), by “Priority Issue on Post-K computer” (Elucidation of the Fundamental Laws and Evolution of the Universe) (hp160211), and by CNS-RIKEN joint project for large-scale nuclear structure calculations. The authors acknowledge the EUROBALL Owners Committee for the loan of germanium detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors. Part of the WAS3ABi was supported by the Rare Isotope Science Project which is funded by the Ministry of Education, Science and Technology (MEST) and National Research Foundation (NRF) of Korea.

Background: Detailed spectroscopy of neutron-rich odd-A Cu isotopes is of great importance for studying the shell evolution in the region of 78Ni. While there is experimental information on excited states in 69−73,77,79Cu isotopes, the information concerning 75Cu is very limited. Purpose: Experimentally observed single-particle, core-coupling, and proton-hole intruder states in 75Cu, will complete the systematics of these states in the chain of isotopes. Method: Excited states in 75Cu were populated in the β decay of 75Ni isotopes. The Ni nuclei were produced by the in-flight fission of 238U projectiles, and were separated, identified, and implanted in a highly segmented Si detector array for the detection of the β-decay electrons. The β-delayed γ rays were detected in a HPGe cluster array. Monte Carlo shell model calculations were performed using the A3DA interaction built on the pfg9/2d5/2 model space for both neutrons and protons. Results: A level scheme of 75Cu was built up to ≈4 MeV by performing a γ−γ coincidence analysis. The excited states below 2 MeV were interpreted based on the systematics of neutron-rich odd-A Cu isotopes and the results of the shell model calculations. Conclusions: The evolution of the single-particle, core-coupling, and proton-hole intruder states in the chain of neutron-rich odd-A Cu isotopes is discussed in the present work, in connection with the newly observed level structure of 75Cu.