Search

STRASSE (Silicon Tracker for RAdioactive nuclei Studies at SAMURAI Experiments) is a new detection system under construction for quasi-free scattering (QFS) measurements at 200-250 MeV/nucleon at the RIBF facility of the RIKEN Nishina Center. It consists of a charged-particle silicon tracker coupled with a dedicated thick liquid hydrogen target (up to 150-mm long) in a compact geometry to fit inside large scintillator or germanium arrays. Its design was optimized for two types of studies using QFS: missing-mass measurements and in-flight prompt $\gamma$-ray spectroscopy. This article describes (i) the resolution requirements needed to go beyond the sensitivity of existing systems for these two types of measurements, (ii) the conceptual design of the system using detailed simulations of the setup and (iii) its complete technical implementation and challenges. The final tracker aims at a sub-mm reaction vertex resolution and is expected to reach a missing-mass resolution below 2 MeV in $\sigma$ for $(p,2p)$ reactions when combined with the CsI(Na) CATANA array., Comment: 25 pages, 29 figures

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. Detailed $\gamma$-ray spectroscopy of the exotic neon isotope $^{28}$Ne has been performed for the first time using the one-neutron removal reaction from $^{29}$Ne 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 $^{28}$Ne 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.

13 pags., 8 figs., 1 tab., We measured 135 cross sections of residual nuclei produced in fragmentation reactions of C12, N14, and O13−16,20,22 projectiles impinging on a carbon target at kinetic energies of near 400A MeV, most of them for the first time, with the RB3/LAND setup at the GSI facility in Darmstadt (Germany). The use of this state-of-the-art experimental setup in combination with the inverse kinematics technique gave the full identification in atomic and mass numbers of fragmentation residues with a high precision. The cross sections of these residues were determined with uncertainties below 20% for most of the cases. These data are compared to other previous measurements with stable isotopes and are also used to benchmark different model calculations., This work has been partially supported by the Spanish Ministry for Science and Innovation under Grants No. PGC2018-099746-B-C21, No. PGC2018-099746-B-C22, and No. PID2019-104390GB-100; Xunta de Galicia under program “Grupos de referencia competitiva” (Project No. ED431C 2017/54); the German Bundesministerium für Bildung und Forschung (BMBF) (Grants No. 05P12RDFN8, No. 05P15RDFN1, and No. 05P19RDFN); the Swedish Research Council; and U.S. Department of Energy Grant No. DE-FG02-08ER41533. J.J.R.S. acknowledges the support of Xunta de Galicia under Grant No. ED481B-2017/002.

The one-neutron knockout from 52Ca in inverse kinematics onto a proton target was performed at ∼230 MeV/nucleon combined with prompt γ spectroscopy. Exclusive quasifree scattering cross sections to bound states in 51Ca and the momentum distributions corresponding to the removal of 1f7/2 and 2p3/2 neutrons were measured. The cross sections, interpreted within the distorted-wave impulse approximation reaction framework, are consistent with a shell closure at the neutron number N=32, found as strong as at N=28 and N=34 in Ca isotopes from the same observables. The analysis of the momentum distributions leads to a difference of the root-mean-square radii of the neutron 1f7/2 and 2p3/2 orbitals of 0.61(23) fm, in agreement with the modified-shell-model prediction of 0.7 fm suggesting that the large root-mean-square radius of the 2p3/2 orbital in neutron-rich Ca isotopes is responsible for the unexpected linear increase of the charge radius with the neutron number.

16 pags., 9 figs., 5 tabs., A first -ray study of spectroscopy was performed at the Radioactive Isotope Beam Factory with projectiles at 217 MeV/nucleon, impinging on the liquid hydrogen target of the MINOS device. Prompt deexcitation rays were measured with the NaI(Tl) array DALI2. Through the one-proton knockout reaction , a spin assignment could be determined for the low-lying states of from the momentum distribution obtained with the SAMURAI spectrometer. A spin-parity is deduced for the ground state of , similar to the recently studied isotope . The evolution of the energy difference is compared to state-of-the-art theoretical predictions., We thank the RIKEN Nishina Center accelerator staff for their work in the primary beam delivery and the BigRIPS team for preparing the secondary beams. The development of MINOS has been supported by the European Research Council through the ERC Grant No. MINOS258567. B.D.L., L.X.C., and N.D.T. acknowledge support from the Vietnam Ministry of Science and Technology under Grant No. ĐTCB.01/21/VKHKTHN. M.G.R. and A.M.M. are supported by the Spanish Ministerio de Ciencia, Innovación y Universidades (including FEDER funds) under project FIS2017-88410-P. F.B. was supported by the RIKEN Special Postdoctoral Researcher Program. Y.L.S. acknowledges the support of Marie Skłodowska-Curie Individual Fellowship (H2020-MSCAIF-2015-705023) from the European Union. I.G. has been supported by HIC for FAIR and Croatian Science Foundation. R.-B.G. is supported by the Deutsche Forschungsgemeinschaft (DFG) under Grant No. BL 1513/1-1. K.I.H., D.K., and S.Y.P. acknowledge the support from the IBS grant funded by the Korea government (No. IBS-R031-D1). P.K. was supported in part by the BMBF Grant No. 05P19RDFN1 and HGS-HIRe. D.So. has been supported by the European Regional Development Fund Contract No. GINOP-2.3.3-15-2016-00034 and the National Research, Development and Innovation Fund of Hungary via Project No. K128947. This work was supported in part by JSPS KAKENHI Grants No. JP16H02179, No. JP18H05404, and No. JP20K03981. J.D.H. and R.S. acknowledge the support from NSERC and the National Research Council Canada. This work was supported by the Office of Nuclear Physics, U.S. Department of Energy, under Grants No. de-sc0018223 (NUCLEI SciDAC-4 collaboration) and the FieldWork Proposal ERKBP72 at Oak Ridge National Laboratory (ORNL). Computer time was provided by the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. This research used resources of the Oak Ridge Leadership Computing Facility located at ORNL, which is supported by the Office of Science of the Department of Energy under Contract No. DE-AC05-00OR22725. GGF calculations were performed by using HPC resources from GENCI-TGCC (Contracts No. A007057392 and No. A009057392) and at the DiRAC Complexity system at the University of Leicester (BIS National E-infrastructure capital Grant No. ST/K000373/1 and STFC Grant No. ST/K0003259/1). This work was supported by the United Kingdom Science and Technology Facilities Council (STFC) under Grant No. ST/L005816/1 and in part by the NSERC Grants No. SAPIN-2016-00033, No. SAPIN-2018-00027, and No. RGPAS-2018-522453. TRIUMF receives federal funding via a contribution agreement with the National Research Council of Canada. J.D.H. thanks S. R. Stroberg for the IMSRG++ code used to perform the VSIMSRG calculations [86]. N.T.T.P. was funded by Vingroup Joint Stock Company and supported by the Domestic Ph.D. Scholarship Programme of Vingroup Innovation Foundation (VINIF), Vingroup Big Data Institute (VINBIGDATA), code VINIF.2020.TS.52.

9 pags., 6 figs., 4 tabs., Low-lying excited states in the N=32 isotope Ar50 were investigated by in-beam γ-ray spectroscopy following proton- and neutron-knockout, multinucleon removal, and proton inelastic scattering at the RIKEN Radioactive Isotope Beam Factory. The energies of the two previously reported transitions have been confirmed, and five additional states are presented for the first time, including a candidate for a 3- state. The level scheme built using γγ coincidences was compared to shell-model calculations in the sd-pf model space and to ab initio predictions based on chiral two- and three-nucleon interactions. Theoretical proton- and neutron-knockout cross sections suggest that two of the new transitions correspond to 2+ states, while the previously proposed 41+ state could also correspond to a 2+ state., We thank the RIKEN Nishina Center accelerator staff and the BigRIPS team for the stable operation of the high-intensity Zn beam and for the preparation of the secondary beam setting. This work has been supported by the JSPS Grant-in-Aid for Scientific Research JP16K05352, JP18K03639, JP16H02179, and JP18H05404, the RIKEN Special Postdoctoral Researcher Program, Colciencias–Convocatoria 617 Becas Doctorados Nacionales, the Ministry of Science and Technology of Vietnam through the Physics Development Program Grant No. ĐTĐLCN.25/18, HIC for FAIR, the Croatian Science Foundation under Projects No. 1257 and No. 7194, the European Regional Development Fund GINOP-2.3.3-15- 2016-00034 and the National Research, Development and Innovation Fund K128947 projects, the NKFIH (128072), the Spanish Ministerio de Economía y Competitividad under Contract No. FPA2017-84756-C4-2-P, the NRF Grants No. 2018R1A5A1025563 and No. 2019M7A1A1033186 funded by the Korean government, the MEXT as “Priority issue on post-K computer” (Elucidation of the fundamental laws and evolution of the universe), the Joint Institute for Computational Fundamental Science (JICFuS), the Ramón y Cajal program RYC-2017-22781 of the Spanish Ministry of Science, Innovation and Universities, the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Project-ID 279384907–SFB 1245 and Grant No. BL 1513/1-1, the PRISMA Cluster of Excellence, and the BMBF under Contracts No. 05P15RDFN1, No. 05P18RDFN1, and No. 05P19RDFN1. TRIUMF receives funding via a contribution through the National Research Council Canada. Computations were performed with an allocation of computing resources on Cedar at WestGrid and Compute Canada, and on the Oak Cluster at TRIUMF managed by the University of British Columbia, Department of Advanced Research Computing (ARC). The development of MINOS was supported by the European Research Council (ERC) through Grant No. MINOS-258567.

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.

Detailed spectroscopy of the neutron-unbound nucleus ^{28}F has been performed for the first time following proton/neutron removal from ^{29}Ne/^{29}F beams at energies around 230 MeV/nucleon. The invariant-mass spectra were reconstructed for both the ^{27}F^{(*)}+n and ^{26}F^{(*)}+2n coincidences and revealed a series of well-defined resonances. A near-threshold state was observed in both reactions and is identified as the ^{28}F ground state, with S_{n}(^{28}F)=-199(6) keV, while analysis of the 2n decay channel allowed a considerably improved S_{n}(^{27}F)=1620(60) keV to be deduced. Comparison with shell-model predictions and eikonal-model reaction calculations have allowed spin-parity assignments to be proposed for some of the lower-lying levels of ^{28}F. Importantly, in the case of the ground state, the reconstructed ^{27}F+n momentum distribution following neutron removal from ^{29}F indicates that it arises mainly from the 1p_{3/2} neutron intruder configuration. This demonstrates that the island of inversion around N=20 includes ^{28}F, and most probably ^{29}F, and suggests that ^{28}O is not doubly magic.

The gyromagnetic factor of the low-lying E=251.96(9) keV isomeric state of the nucleus ^{99}Zr was measured using the time-dependent perturbed angular distribution technique. This level is assigned a spin and parity of J^{π}=7/2^{+}, with a half-life of T_{1/2}=336(5) ns. The isomer was produced and spin aligned via the abrasion-fission of a ^{238}U primary beam at RIKEN RIBF. A magnetic moment |μ|=2.31(14)μ_{N} was deduced showing that this isomer is not single particle in nature. A comparison of the experimental values with interacting boson-fermion model IBFM-1 results shows that this state is strongly mixed with a main νd_{5/2} composition. Furthermore, it was found that monopole single-particle evolution changes significantly with the appearance of collective modes, likely due to type-II shell evolution. ispartof: Physical Review Letters vol:124 issue:11 ispartof: location:United States status: Published online

7 pags., 3 figs., 1 tab., We report on the first γ-ray spectroscopy of K produced via the Ca(p,2p) reactions at ∼250 MeV/nucleon. Unambiguous final-state angular-momentum assignments were achieved for beam intensities down to few particles per second by using a new technique based on reaction vertex tracking combined with a thick liquid-hydrogen target. Through γ-ray spectroscopy and exclusive parallel momentum distribution analysis, 3/2 ground states and 1/2 first excited states in K were established quantifying the natural ordering of the 1d and 2s proton-hole states that are restored at N = 32 and 34. State-of-the-art ab initio calculations and shell-model calculations with improved phenomenological effective interactions reproduce the present data and predict consistently the increase of the E(1/2 ) - E(3/2 ) energy differences towards N = 40., We are very grateful to the RIKEN Nishina Center accelerator staff for providing the stable and high-intensity zinc beam and to the BigRIPS team for the smooth operation of the secondary beams. The development of MINOS has been supported by the European Research Council through the ERC Grant No. MINOS-258567. Green's function calculations were performed using HPC resources from GENCI-TGCC, France (Projects A0030507392 and A0050507392) and from the DiRAC Data Intensive service at Leicester, UK (funded by the UK BEIS via STFC capital grants ST/K000373/1 and ST/R002363/1 and STFC DiRAC Operations grant ST/R001014/1). This work (C. B.) was also supported by the United Kingdom Science and Technology Facilities Council (STFC) under Grants No. ST/P005314/1 and No. ST/L005816/1. K. O. acknowledges the support by Grant-in-Aid for Scientific Research JP16K05352. Y. L. S. acknowledges the support of Marie Skłodowska-Curie Individual Fellowship (H2020-MSCA-IF-2015-705023) from the European Union and the support from the Helmholtz International Center for FAIR. The valuable discussions with C. Qi are gratefully acknowledged. H. N. L. acknowledges the support from the Enhanced Eurotalents program (PCOFUND-GA-2013-600382) co-funded by CEA and the European Union. H. N. L. and A. O. acknowledge the support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Project No. 279384907-SFB 1245. Y. L. S. and A. O. acknowledge the support from the Alexander von Humboldt Foundation. L. X. C. and B. D. L would like to thank MOST for its support through the Physics Development Program Grant No. ĐTĐLCN.25/18. I.G. has been supported by HIC for FAIR and HRZZ under project No. 1257 and 7194. K. I. H., D. K. and S. Y. P. acknowledge the support from the NRF grant funded by the Korea government (No. 2017R1A2B2012382 and 2019M7A1A1033186). F. B. acknowledge the support from the RIKEN Special Postdoctoral Researcher Program. D.S. was supported by projects No. GINOP-2.3.3-15-2016-00034 and No. K128947. V. V. acknowledges support from the Spanish Ministerio de Economía y Competitividad under Contract No. FPA2017-84756-C4-2-P. V. W. acknowledges support from BMBF grants 05P15RDFN1, 05P19RDFN1 and DFG grant SFB 1245. P. K. acknowledges support from HGS-HIRe and BMBF grant 05P19RDFN1. This work was also supported by NKFIH (128072).

Low-lying excited states in the $N=32$ isotope $^{50}$Ar were investigated by in-beam $\gamma$-ray spectroscopy following proton- and neutron-knockout, multi-nucleon removal, and proton inelastic scattering at the RIKEN Radioactive Isotope Beam Factory. The energies of the two previously reported transitions have been confirmed, and five additional states are presented for the first time, including a candidate for a 3$^-$ state. The level scheme built using $\gamma\gamma$ coincidences was compared to shell-model calculations in the $sd-pf$ model space, and to ab initio predictions based on chiral two- and three-nucleon interactions. Theoretical proton- and neutron-knockout cross sections suggest that two of the new transitions correspond to $2^+$ states, while the previously proposed $4^+$ state could also correspond to a $2^+$ state., Comment: Accepted for Phys. Rev. C

7 pags., 4 figs., 1 tab., Exclusive cross sections and momentum distributions have been measured for quasifree one-neutron knockout reactions from a Ca54 beam striking on a liquid hydrogen target at ∼200 MeV/u. A significantly larger cross section to the p3/2 state compared to the f5/2 state observed in the excitation of Ca53 provides direct evidence for the nature of the N=34 shell closure. This finding corroborates the arising of a new shell closure in neutron-rich calcium isotopes. The distorted-wave impulse approximation reaction formalism with shell model calculations using the effective GXPF1Bs interaction and ab initio calculations concur our experimental findings. Obtained transverse and parallel momentum distributions demonstrate the sensitivity of quasifree one-neutron knockout in inverse kinematics on a thick liquid hydrogen target with the reaction vertex reconstructed to final state spin-parity assignments., We would like to express our gratitude to the RIKEN Nishina Center accelerator staff for providing the stable and high-intensity beam andtotheBigRIPSteam for operatingthe secondary beams. S. C. acknowledges the support of the IPA program at RIKEN Nishina Center. J. L. acknowledges the support from Research Grants Council (RGC) of Hong Kong with grant of Early Career Scheme (ECS-27303915). K. O., K. Y., and Y. C. acknowledge the support from Grants-in-Aid of the Japan Society for the Promotion of Science under Grants No. JP16K05352. Y. L. S. acknowledges the support of the Marie Skłodowska-Curie Individual Fellowship (H2020-MSCA-IF-2015-705023). V. V. acknowledges support from the Spanish Ministerio de Economía y Competitividad under Contract No. FPA2017- 84756-C4-2-P. L. X. C. and B. D. L. would like to thank MOST for its support through the Physics Development Program Grant No. ĐTĐLCN.25/18. D. R. and V. W. acknowledge the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Grant No. SFB1245. V. W. and P. K. acknowledge the German BMBF Grant No. 05P19RDFN1. P. K. was also supported by HGSHIRe. D. S. was supported by Projects No. GINOP-2.3.3- 15-2016-00034 and No. NKFIH-NN114454. I. G. has been supported by HIC for FAIR and Croatian Science Foundation under Projects No. 1257 and No. 7194. K. I. H., D. K., and S. Y. P. acknowledge the support from the NRF grant funded bythe Korea government (No. 2016K1A3A7A09005580 and No. 2018R1A5A1025563). This work was also supported by the United Kingdom Science and Technology Facilities Council (STFC) under Grants No. ST/P005314/1 and No. ST/L005816/1, and by NKFIH (128072), and by JSPS KAKENHI Grant No. 16H02179, and by MEXT KAKENHI Grant No. 18H05404. The development of MINOS were supported by the European Research Council through the ERC Grant No. MINOS-258567. Green’s function calculations were performed using HPC resources from the DiRAC Data Intensive service at Leicester, UK (funded by the UK BEIS via STFC capital Grants No. ST/K000373/1 and No. ST/R002363/1 and STFC DiRAC Operations Grant No. ST/R001014/1) and from GENCI-TGCC, France (Project No. A0050507392).

7 pags., 3 figs., 1 tab. -- Open Access funded by Creative Commons Atribution Licence 4.0, The first γ-ray spectroscopy of Ar52, with the neutron number N=34, was measured using the K53(p,2p) one-proton removal reaction at ∼210 MeV/u at the RIBF facility. The 21+ excitation energy is found at 1656(18) keV, the highest among the Ar isotopes with N>20. This result is the first experimental signature of the persistence of the N=34 subshell closure beyond Ca54, i.e., below the magic proton number Z=20. Shell-model calculations with phenomenological and chiral-effective-field-theory interactions both reproduce the measured 21+ systematics of neutron-rich Ar isotopes, and support a N=34 subshell closure in Ar52., We thank the RIKEN Nishina Center accelerator staff for their work in the primary beam delivery and the Big RIP Steam for preparing the secondary beams. The development of MINOS has been supported by the European Research Council through the ERC Grant No. MINOS 258567. Acknowledges the support from the Enhanced Eurotalents program (PCOFUND-GA-2013-600382) co-funded by CEA and the European Union. H.N.L., A.O. and A.S. acknowledge the support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Project No. 279384907- SFB 1245. C.A.B. acknowledges support from the U.S. NSF Grant No. 1415656 and the U.S. DOE Grant No.DE-FG02-08ER41533.J.D.H.and R.S.acknowledge the support from NSERC and the National Research Council Canada. Y.L.S. acknowledges the support of Marie Skłodowska-Curie Individual Fellowship (H2020-MSCAIF-2015-705023) from the European Union. I.G. has been supported by HIC for FAIR andCroatianScienceFoundation. L.X.C. and B.D.L. have been supported by the Vietnam MOST through the Physics Development Program Grant No. ĐTĐLCN.25/18. K.I.H., D.K. and S.Y.P. have been supported by the NRF grant funded by the Korea government (No. 2017R1A2B2012382 and 2018R1A5A1025563). This work was supported in part by JSPS KAKENHI Grant No. 16H02179, MEXT KAKENHI Grants No. 24105005 and No. 18H05404. This work was also supported by the Office of Nuclear Physics,U.S.Department of Energy,under Grants No.de-sc 0018223 (NUCLEISciDAC-4collaboration) and the Field Work Proposal ERKBP72 at Oak Ridge National Laboratory (ORNL). Computer time was provided by the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. Thisresearch used resources of the Oak Ridge Leadership Computing Facility located at ORNL, which is supported by the Office of Science of the Department of Energy under Contract No. DE-AC0500OR22725.

First results are reported on the ground state configurations of the neutron-rich $^{29,30}$Na isotopes, obtained via Coulomb dissociation (CD) measurements as a method of the direct probe. The invariant mass spectra of those nuclei have been obtained through measurement of the four-momentum of all decay products after Coulomb excitation on a $^{208}Pb$ target at energies of 400-430 MeV/nucleon using FRS-ALADIN-LAND setup at GSI, Darmstadt. Integrated Coulomb-dissociation cross-sections (CD) of 89 $(7)$ mb and 167 $(13)$ mb up to excitation energy of 10 MeV for one neutron removal from $^{29}$Na and $^{30}$Na respectively, have been extracted. The major part of one neutron removal, CD cross-sections of those nuclei populate core, in its' ground state. A comparison with the direct breakup model, suggests the predominant occupation of the valence neutron in the ground state of $^{29}$Na${(3/2^+)}$ and $^{30}$Na${(2^+)}$ is the $d$ orbital with small contribution in the $s$-orbital which are coupled with ground state of the core. The ground state configurations of these nuclei are as $^{28}$Na$_{gs (1^+)\otimes\nu_{s,d}$ and $^{29}$Na$_{gs}(3/2^+)\otimes\nu_{ s,d}$, respectively. The ground state spin and parity of these nuclei, obtained from this experiment are in agreement with earlier reported values. The spectroscopic factors for the valence neutron occupying the $s$ and $d$ orbitals for these nuclei in the ground state have been extracted and reported for the first time. A comparison of the experimental findings with the shell model calculation using MCSM suggests a lower limit of around 4.3 MeV of the sd-pf shell gap in $^{30}$Na.

The transition strengths of the first-excited 2+ states and deformation lengths of the nuclei Ne30 and Mg36 were determined via Coulomb- and nuclear-force-dominated inelastic scattering at intermediate energies. Beams of these exotic nuclei were produced at the RIKEN Radioactive Isotope Beam Factory and were incident on lead and carbon targets at energies above 200 MeV/u. Absolute excitation cross sections on the lead target yielded reduced transition probabilities of 0.0277(79) and 0.0528(121) e2b2, while the measurements with the carbon target revealed nuclear deformation lengths of δN=1.98(11) and 1.93(11) fm for Ne30 and Mg36, respectively. Corresponding quadrupole deformation parameters of β2∼0.5 from the two probes were found comparable in magnitude, showing no indication for a reduction in deformation along isotopic and isotonic chains from Mg32 towards the neutron drip-line. Comparisons to shell-model calculations illustrate the importance of neutron excitations across the N=20 shell for Ne30 and suggest that shallow maximums of collectivity may occur around N=22 and 24 along the neon and magnesium isotopic chains, respectively, A.P. is partly supported by MINECO (Spain) Grant No. FPA2014-57196 and Programme “Centros de Excelencia Severo Ochoa” SEV-2012-0249

The one-neutron knock-out reaction H-1(C-20, C-19 gamma) was studied at RIKEN using the DALI2 array. A gamma-ray transition was observed at 198(10) keV. Based on the comparison between the experimental production cross section and theoretical predictions, the transition was assigned to the de-excitation of the 3/2(1)(+) state to the ground state.

The one neutron knock-out reaction $^1$H($^{20}$C,$^{19}$C$��$) was studied at RIKEN using the DALI2 array. A $��$ ray transition was observed at 198(10) keV. Based on the comparison between the experimental production cross section and theoretical predictions, the transition was assigned to the decay of the 3/2$_1^+$ state to the ground state., 5 pages, 3 figures, submitted in Physical Review C

A NaI(Tl) detector array called DALI2 (Detector Array for Low Intensity radiation 2) has been constructed for in-beam $��$-ray spectroscopy experiments with fast radioactive isotope (RI) beams. It consists typically of 186 NaI(Tl) scintillators covering polar angles from $\sim$15$^{\circ}$ to $\sim$160$^{\circ}$ with an average angular resolution of 6$^{\circ}$ in full width at half maximum. Its high granularity (good angular resolution) enables Doppler-shift corrections that result in, for example, 10% energy resolution and 20% full-energy photopeak efficiency for 1-MeV $��$ rays emitted from fast-moving nuclei (velocities of $v/c \simeq 0.6$). DALI2 has been employed successfully in numerous experiments using fast RI beams with velocities of $v/c = 0.3 - 0.6$ provided by the RIKEN RI Beam Factory., 10 pages, 8 figures

One-neutron knockout from 24O leading to the first excited state in 23O has been measured for a proton target at a beam energy of 62 MeV/nucleon. The decay energy spectrum of the neutron unbound state of 23O was reconstructed from the measured four momenta of the 22O fragment and emitted neutron. A sharp peak was found at Edecay=50$\pm$3 keV, corresponding to an excited state in 23O at 2.78$\pm$0.11 MeV, as observed in previous measurements. The longitudinal momentum distribution for this state was consistent with d -wave neutron knockout, providing support for a J{\pi} assignment of 5/2+. The associated spectroscopic factor was deduced to be C2S(0d5/2)=4.1$\pm$0.4 by comparing the measured cross section (View the MathML source) with a distorted wave impulse approximation calculation. Such a large occupancy for the neutron 0d5/2 orbital is in line with the N=16 shell closure in 24O., Comment: Published in Physics Letters B (18 October 2014)

The $^{18}$Ne($��$,$p$)$^{21}$Na reaction is thought to be one of the key breakout reactions from the hot CNO cycles to the rp-process in type I x-ray bursts. In this work, the resonant properties of the compound nucleus $^{22}$Mg have been investigated by measuring the resonant elastic scattering of $^{21}$Na+$p$. An 89 MeV $^{21}$Na radioactive beam delivered from the CNS Radioactive Ion Beam Separator bombarded an 8.8 mg/cm$^2$ thick polyethylene (CH$_{2}$)$_{n}$ target. The $^{21}$Na beam intensity was about 2$\times$10$^{5}$ pps, with a purity of about 70% on target. The recoiled protons were measured at the center-of-mass scattering angles of $��_{c.m.}$$\approx$175.2${^\circ}$, 152.2${^\circ}$, and 150.5${^\circ}$ by three sets of $��E$-$E$ telescopes, respectively. The excitation function was obtained with the thick-target method over energies $E_x$($^{22}$Mg)=5.5--9.2 MeV. In total, 23 states above the proton-threshold in $^{22}$Mg were observed, and their resonant parameters were determined via an $R$-matrix analysis of the excitation functions. We have made several new $J^��$ assignments and confirmed some tentative assignments made in previous work. The thermonuclear $^{18}$Ne($��$,$p$)$^{21}$Na rate has been recalculated based on our recommended spin-parity assignments. The astrophysical impact of our new rate has been investigated through one-zone postprocessing x-ray burst calculations. We find that the $^{18}$Ne($��$,$p$)$^{21}$Na rate significantly affects the peak nuclear energy generation rate, reaction fluxes, as well as the onset temperature of this breakout reaction in these astrophysical phenomena.

Neutron-rich N=22, 24, 26 magnesium isotopes were studied via in-beam γ-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory following secondary fragmentation reactions on a carbon target at ≈200 MeV/nucleon. In the one- and two-proton removal channels from 39Al and 40Si beams, two distinct γ-ray transitions were observed in 38Mg, while in the one-proton removal reaction from 37Al a new transition was observed in addition to the known 2(1)(+)→0(g.s.)(+) decay. From the experimental systematics and comparison to theoretical predictions it is concluded that the transitions belong to the 2(1)(+)→0(g.s.)(+) and 4(1)(+)→2(1)(+) decays in 36Mg and 38Mg, respectively. For 34Mg, previously reported 2(1)(+) and 4(1)(+) level energies were remeasured. The deduced E(4(1)(+))/E(2(1)(+)) ratios for 34,36,38Mg of 3.14(5), 3.07(5), and 3.07(5) are almost identical and suggest the emergence of a large area of deformation extending from the N=20 to the N=28 shell quenching.

Excited states in (38,40,42) Si nuclei have been studied via in-beam γ-ray spectroscopy with multinucleon removal reactions. Intense radioactive beams of ^{40}S and (44)S provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled γ-γ coincidence measurements. A prominent γ line observed with an energy of 742(8) keV in (42) Si confirms the 2(+) state reported in an earlier study. Among the γ lines observed in coincidence with the 2^{+} → 0+ transition, the most probable candidate for the transition from the yrast 4(+) state was identified, leading to a 4(1)+) energy of 2173(14) keV. The energy ratio of 2.93(5) between the 2(1)+ and 4(1)(+) states indicates well-developed deformation in (42) Si at N = 28 and Z = 14. Also for 38,40)Si energy ratios with values of 2.09(5) and 2.56(5) were obtained. Together with the ratio for (42)Si, the results show a rapid deformation development of Si isotopes from N = 24 to N = 28.

A Gamow-Teller (GT) transition from the drip-line nucleus 14Be to 14B was studied via the (p,n) reaction in inverse kinematics using a secondary 14Be beam at 69 MeV/nucleon. The invariant mass method is employed to reconstruct the energy spectrum. A peak is observed at an excitation energy of 1.27(2) MeV in 14B, together with bumps at 2.08 and 4.06(5) MeV. The observed forward peaking of the state at 1.27 MeV and a good description for the differential cross section, obtained with a DWBA calculation provide support for the 1+ assignment to this state. By extrapolating the cross section to zero momentum transfer the GT-transition strength is deduced. The value is found to compare well with that reported in a beta-delayed neutron emission study., Comment: 5 pages, 2 figures

The structure of the neutron rich sodium isotopes 31,32,33Na was investigated by means of in-beam gammaray spectroscopy following one-neutron knockout and inelastic scattering of radioactive beams provided by the RIKEN Radioactive Ion Beam Factory. The secondary beams were selected and separated by the fragment separator BigRIPS and incident at ~240 meV/u on a natural carbon (secondary) target, which was surrounded by the DALI2 array to detect coincident de-excitation gammarays. Scattered particles were identified by the spectrometer ZeroDegree. In 31Na, a new decay gammaray was observed in coincidence with the known (5/2+)-> 3/2(+) transition, while for 32,33Na excited states are reported for the first time. From a comparison to state-of-the-art shell model calculations it is concluded that the newly observed excited state in 31Na belongs to a rotational band formed by a 2p2h intruder configuration within the "Island of Inversion"., Accepted for Publication in Phys. Rev. C, 11 pages, 6 figures

The structure of neutron rich nitrogen nuclei has been studied by use of neutron removal reaction and inelastic scattering. Mass and charge deformations have been deduced for the first excited state of 21N, which indicates the partial persitence of the N=14 subshell closure in nitrogen isotopes. The spectroscopic information obtained on the structure of 19,20,21N confirms the results from a previous experiment.

We report on the first spectroscopic study of the N=22 nucleus 32Ne at the newly completed RIKEN Radioactive Ion Beam Factory. A single gamma-ray line with an energy of 722(9) keV was observed in both inelastic scattering of a 226 MeV/u 32Ne beam on a Carbon target and proton removal from 33Na at 245 MeV/u. This transition is assigned to the de-excitation of the first J^pi = 2+ state in 32Ne to the 0+ ground state. Interpreted through comparison with state-of-the-art shell model calculations, the low excitation energy demonstrates that the Island of Inversion extends to at least N=22 for the Ne isotopes., Accepted for publication in Phys. Rev. Lett. 11 pages, 3 figures