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Nishimura, S.

Publications and source records attributed to Nishimura, S..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Shape evolution of neutron-rich 106,108,110 Mo isotopes in the triaxial degree of freedom

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, $\gamma$ vibration, or $\gamma$-soft rotor. Purpose: The shape evolution of 106 Mo, 108 Mo, and 110 Mo 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 $K^π = 0^+, 2^+$, and $4^+$. Method: $\beta$-delayed $\gamma$-ray spectroscopy was employed for neutron-rich Nb and Zr isotopes produced at the RIKEN RI Beam Factory to populate excited states in 106 Mo, 108 Mo, and 110 Mo. The EUroball-RIKEN Cluster Array was used for high-resolution $\gamma$-ray detection and lifetimes of the $2^+_1$ states were determined using the UK fast-timing array of LaBr 3 (Ce) detectors. Results: New $\gamma$-ray transitions and levels are reported, including newly assigned $0^+_2$ states in 108,110 Mo. Quadrupole deformations were obtained for 106,108,110 Mo from their $2^+_1$ energies and lifetimes. The $\beta$-delayed neutron-emission probabilities of 108 Nb and 110 Nb were determined by examining the $\gamma$ rays of their respective daughter decays. Conclusions: In this work, the even-odd energy staggering in the $2^+_2$ band was compared with typical patterns of the $\gamma$-vibrational band, rigid triaxial rotor, and $\gamma$-soft rotor. The very small even-odd staggering of 106 Mo, 108 Mo, and 110 Mo favors a $\gamma$-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 $\gamma$-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 $2^+_2$ bands in 106 Mo and 108 Mo. The collective wave functions are consistent with the interpretation of the $2^+_2$ band as the $\gamma$-vibrational band of the prolate shape. However, the staggering pattern observed in 110 Mo differs from the one suggested in the calculations which predict a $\gamma$-soft rotor. There was no experimental indication of the oblate shape or the $\gamma$-soft rotor predicted in these Mo isotopes.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Strong one-neutron emission from two-neutron unbound states in β decays of the r -process nuclei Ga 86 , 87

β-delayed one-neutron and two-neutron branching ratios (P$_{1n}$ and P$_{2n}$) have been measured in the decay of A = 84 to 87 Ga isotopes at the Radioactive-Isotope Beam Factory (RIBF) at the RIKEN Nishina Center using a high-efficiency array of 3 He neutron counters (BRIKEN). Two-neutron emission was observed in the decay of 84, 85, 87 Ga for the first time and the branching ratios were measured to be P$_{2n}$ = 1.6 (2)%, 1.3 (2)%, and 10.2 ( 28 ) stat ( 5 ) sys %, respectively. One-neutron branching ratio of 87 Ga (P$_{1n}$= 81 (9) stat (8) sys %) and half-life of 29(4) ms were measured for the first time. The branching ratios of 86 Ga were also measured to be P$_{1n}$ = 74 (2) stat (8) sys % and 16.2 (9) stat ( 6 ) sys % with better precision than a previous study. The observation that P$_{1n}$ > P$_{2n}$ for both 86,87 Ga was unexpected and is interpreted as a signature of dominating one-neutron emission from the two-neutron unbound excited states in 86, 87 Ge. In order to interpret the experimental results, shell-model and Hauser-Feshbach statistical model calculations of delayed particle and γ-ray emission probabilities were performed. This model framework reproduces the experimental results. The shell model alone predicts P$_{2n}$ significantly larger than P$_{1n}$ for the 87 Ga decay, and it is necessary to invoke a statistical description to successfully explain the observation that P$_{1n}$ > P$_{2n}$. Our new results demonstrate the relevance and importance of a statistical description of neutron emission for the prediction of the decay properties of multineutron emitters and that it must be included in the r -process modeling.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Properties of γ-decaying isomers in the 100 Sn region populated in fragmentation of a 124 Xe beam

A systematic study was performed of microsecond γ-decaying isomers around 100 Sn produced in a fragmentation reaction of a 124 Xe beam at 345 MeV/u at the Radioactive Ion Beam Factory of the RIKEN Nishina Center in Saitama, Japan. Half-lives of isomeric states in that region were remeasured allowing us to improve the currently available experimental information. Reduced transition probabilities were deduced and compared to shell-model calculations in various model spaces. Furthermore, the recently reported low-energy transitions in 92 Rh and 96 Ag were remeasured with improved precision. Additionally, experimental information on isomeric ratios, including five new ones, were extracted and compared to a previous experimental study and the sharp cutoff model of fragmentation reaction.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Observation of a $μs$ isomer in ${134\atop{49}}$ In $_{85}$: Proton-neutron coupling "southeast" of ${132\atop{50}}$ Sn $_{82}$

In this paper, we report on the observation of a microsecond isomeric state in the single-proton-hole, three-neutron-particle nucleus 134 In . The nuclei of interest were produced by in-flight fission of a 238 U beam at the Radioactive Isotope Beam Factory at RIKEN. The isomer depopulates through a γ ray of energy 56.7(1) keV and with a half-life of $T_{1/2}$ = 3.5 (4) $μs$. Based on the comparison with shell-model calculations, we interpret the isomer as the $I^π$ = 5 - member of the $π0g{-1\atop{9/2}}Ⓧv1f{3\atop{7/2}}$ multiplet, decaying to the $I^π$ = 7 - ground state with a reduced-transition probability of $B (E$ 2; 5 - $\rightarrow$ 7 - = 0.53 (6) W. u. Observation of this isomer, and lack of evidence in the current work for a $I^π$ = 5 - isomer decay in 132 In , provides a benchmark of the proton-neutron interaction in the region of the nuclear chart “southeast” of 132 Sn , where experimental information on excited states is sparse.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Proton Shell Evolution below Sn 132 : First Measurement of Low-Lying β -Emitting Isomers in Ag 123 , 125

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.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Segmented YSO scintillation detectors as a new β -implant detection tool for decay spectroscopy in fragmentation facilities

A newly developed segmented YSO scintillator detector was implemented for the first time at the RI-beam Factory at RIKEN Nishina Center as an implantation-decay counter. The results from the experiment demonstrate that the detector is a viable alternative to conventional silicon-strip detectors with its good timing resolution and high detection efficiency for β particles. A Position-Sensitive Photo-Multiplier Tube (PSPMT) is coupled with a 48 × 48 segmented YSO crystal. To demonstrate its capabilities, a known short-lived isomer in 76 Ni and the β decay of 74 Co were measured by implanting those ions into the YSO detector. The half-lives and γ-rays observed in this work are consistent with the known values. The β -ray detection efficiency is more than 80 % for the decay of 74 Co.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Toward the limit of nuclear binding on the N = Z line: Spectroscopy of Cd 96

A γ-decaying isomeric state (τ 1/2 = $197$ $^{+19}_{–17}$ ns) has been identified in 96 Cd, which is one α particle away from the last known bound N = Z nucleus, 100 Sn. 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. Here, 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.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Commissioning of the BRIKEN detector for the measurement of very exotic β -delayed neutron emitters

A new detection system has been installed at the RIKEN Nishina Center (Japan) to investigate decay properties of very neutron-rich nuclei. The setup consists of three main parts: a moderated neutron counter, a detection system sensitive to the implantation and decay of radioactive ions, and -ray detectors. We describe here the setup, the commissioning experiment and some selected results demonstrating its performance for the measurement of half-lives and β-delayed neutron emission probabilities. Here, the methodology followed in the analysis of the data is described in detail. Particular emphasis is placed on the correction of the accidental neutron background.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

Properties of $\gamma $-decaying Isomers in the $^{100}$Sn Region Revisited

The study of nuclei in the region around the N = Z doubly-magic nucleus 100Sn has been of long standing interest for the nuclear structure and nuclear astrophysics. Recently, Park et al. have reported on properties of γ-decaying isomers and isomeric ratios in the vicinity of 100Sn. That experiment was performed at the Radioactive Ion Beam Factory (RIBF) of the RIKEN Nishina Center in Japan as a part of the EURICA campaign. Neutron-deficient nuclei were produced in a fragmentation reaction of a 124 Xe primary beam on a 9 Be target at an energy of 345 MeV/A. Secondary ions were separated and identified in the BigRIPS fragment separator and implanted in the silicon detector array WAS3ABi. The data presented here were obtained in another experiment performed at the RIBF using the same reaction but slightly different separator settings. New results of ratios of isomeric population and half-lives of γ-decaying isomers populated in the experiment are presented.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

The ORNL analysis technique for extracting β -delayed multi-neutron branching ratios with BRIKEN

Here, many choices are available in order to evaluate large radioactive decay networks. There are many parameters that influence the calculated -decay delayed single and multi-neutron emission branching fractions. We describe assumptions about the decay model, background, and other parameters and their influence on β-decay delayed multi-neutron emission analysis. An analysis technique, the ORNL BRIKEN analysis procedure, for determining -delayed multi-neutron branching ratios in β-neutron precursors produced by means of heavy-ion fragmentation is presented. The technique is based on estimating the initial activities of zero, one, and two neutrons occurring in coincidence with an ion-implant and β trigger. The technique allows one to extract β-delayed multi-neutron decay branching ratios measured with the 3 He BRIKEN neutron counter. As an example, two analyses of the -neutron emitter 77 Cu based on different a priori assumptions are presented along with comparisons to literature values.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

β - γ and isomeric decay spectroscopy of 168 Dy

Here, this contribution will report on the experimental work on the level structure of 168 Dy. The experimental data have been taken as part of the EURICA decay spectroscopy campaign at RIBF, RIKEN in November 2014. In the experiment, a 238 U primary beam is accelerated up to 345 MeV/u with an average intensity of 12 pnA. The nuclei of interest are produced by in-flight fission of 238 U impinging on Be target with a thickness of 5 mm. The excited states of 168 Dy have been populated through the decay from a newly identified isomeric state and via the beta decay from 168 Tb. In this contribution, scientific motivations, experimental procedure and some preliminary results for this study are presented.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Isomer spectroscopy of neutron-rich 168 Tb 103

In-flight fission of a 345 MeV per nucleon 238U primary beam on a 2 mm thick 9Be target has been used to produce and study the decays of a range of neutron-rich nuclei centred around the doubly mid-shell nucleus 170Dy at the RIBF Facility, RIKEN, Japan. The produced secondary fragments of interest were identified event-by-event using the BigRIPS separator. The fragments were implanted into the WAS3ABI position sensitive silicon active stopper which allowed pixelated correlations between implants and their subsequent β-decay. Discrete γ-ray transitions emitted following decays from either metastable states or excited states populated following beta decay were identified using the 84 coaxial high-purity germanium (HPGe) detectors of the EURICA spectrometer, which was complemented by 18 additional cerium-doped lanthanum bromide (LaBr3) fast-timing scintillation detectors from the FATIMA collaboration. This paper presents the internal decay of a metastable isomeric excited state in the odd-odd nucleus 168Tb, which corresponds to a single proton-neutron hole configuration in the valence maximum nucleus 170Dy. These data represent the first information on excited states in this nucleus, which is the most neutron-rich odd-odd isotope of terbium (Z=65) studied to date. Nilsson configurations associated with an axially symmetric, prolate-deformed nucleus are proposed for the 168Tb ground state the observed isomeric state by comparison with Blocked BCS-Nilsson calculations.

Gamma-ray spectroscopy and nuclear isomerism↗
DOE OSTI2017

Isomer-delayed gamma-ray spectroscopy of neutron-rich 166 Tb

Here, this short paper presents the identification of a metastable, isomeric-state decay in the neutron-rich odd-odd, prolate-deformed nucleus 166 Tb. The nucleus of interest was formed using the in-flight fission of a 345 MeV per nucleon 238 U primary beam at the RIBF facility, RIKEN, Japan. Gamma-ray transitions decaying from the observed isomeric states in 166 Tb were identified using the EURICA gamma-ray spectrometer, positioned at the final focus of the BigRIPS fragments separator. The current work identifies a single discrete gamma-ray transition of energy 119 keV which de-excites an isomeric state in 166 Tb with a measured half-life of 3.5(4) μs. The multipolarity assignment for this transition is an electric dipole and is made on the basis internal conversion and decay lifetime arguments. Possible two quasi-particle Nilsson configurations for the initial and final states which are linked by this transition in 166 Tb are made on the basis of comparison with Blocked BCS Nilsson calculations, with the predicted ground state configuration for this nucleus arising from the coupling of the v(1-/2)[521] and π(3+/2) Nilsson orbitals.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗