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H. Sakurai

Publications and source records attributed to H. Sakurai.

At least 19 recordsLinked to original sources
arXiv2026

Evolution of Spectroscopic Factors in Neutron-deficient $p$-shell Nuclei in connection with Short-Range Correlations

We report an uncertainty-controlled determination of neutron-removal spectroscopic factors in the neutron-deficient $p$-shell nuclei $^{12}\mathrm{C}$, $^{10}\mathrm{C}$, $^{9}\mathrm{C}$, and $^{8}\mathrm{B}$, all measured under uniform conditions through exclusive ground-state-to-ground-state $(p,d)$ reactions at about 50 MeV/nucleon in a liquid hydrogen target at the GANIL/LISE facility. The deuterons were detected in the highly-segmented MUST2 detector placed at forward angles and the excitation energy was reconstructed by the missing mass method. Several transitions, including a new state in $^7\mathrm{B}$, were observed. Differential cross sections, which display an $L=1$ pattern for all nuclei, were fitted with 416 sets of uncertainty-quantified optical-model potential parameters and considering various single-particle bound-state wavefunctions to derive $C^2S$ values. The ratio between experimental and shell-model calculated values, $R_S$, is found to be slightly decreasing as a function of increasing proton to neutron separation energy asymmetry $\Delta S$, with a slope of $-0.0049(36)_{\rm stat}(13)_{\rm omp}(8)_{\rm s.p.}$. The observed trend is compatible with the phenomenological expectation that the short-range correlation effect on $C^2S$ increases with the degree of nucleon minority. The present data set delivers a high-precision benchmark that will tightly constrain future microscopic descriptions of short-range correlations in asymmetric nuclei.

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arXiv2026

Orbital-Selective Coexistence of Interlayer Spin-Singlet Formation and SDW Order with Anomalous Spin Reconfiguration in Bilayer Nickelate La$_{3}$Ni$_{2}$O$_{7}$ Revealed by $^{17}$O-NMR

The spin structure of the spin density wave (SDW) order in the bilayer nickelate La$_3$Ni$_2$O$_7$ has been investigated using site-selective $^{17}$O-NMR measurements on the inner apical O(1), outer apical O(2), and planar O(3,4) sites. Below $T_{\rm SDW}$ (= 150 K), the peak of all planar O(3,4) sites significantly broadens due to the emergence of a finite internal magnetic field, whereas O(2) sites remain with no (or a negligibly small) internal field. These results are consistent with commensurate SDW order with a single spin-spinless (or large-tiny spin) stripe. As for the O(1) sites that bridge the NiO$_2$ planes, the internal field is nearly canceled below $T_{\rm SDW}$, indicating an antiparallel spin configuration between adjacent planes. However, below $T_\text{A}$ ($\sim$ 115 K), the spectrum of the O(1) site disappears even though the in-plane SDW order remains robust, implying that the antiparallel spin configuration through the Ni--O(1)--Ni bond is not particularly stable below $T_{\rm A}$, despite the expected strong interlayer spin coupling between the NiO$_2$ planes. Above all, we emphasize that the local spin susceptibility is extremely small at the O(2) site that has a strong covalency with the $d_{3z^2-r^2}$ orbital, indicating a well-developed interlayer spin-singlet formation in the Ni-$d_{3z^2-r^2}$ orbitals bridging the NiO$_2$ planes. These findings shed new light on the interlayer spin-singlet formation and the anomalous spin reconfiguration through the $\text{Ni--O(1)--Ni}$ bonding orbitals connecting the NiO$_2$ planes, which characterize the orbital-selective nature of the bilayer nickelate La$_3$Ni$_2$O$_7$.

cond-mat.str-el↗
arXiv2026

Probing the Size of Neutron and Proton Single-Particle Orbitals from Nucleon Knockout Reactions

The size of neutron and proton single-particle orbitals of $^{52}$Ca, $^{53}$Ca, $^{54}$Ca, and $^{55}$Sc were investigated via nucleon knockout reactions at $\sim$ 230 MeV/nucleon. The determination method is based on the measured fragment momentum distributions in $(p,pn)$ and $(p,2p)$ reactions, which are shown to be sensitive to the spatial extension of the wave function of the knocked-out nucleon, interpreted within the distorted wave impulse approximation (DWIA) framework. A systematic sensitivity study is carried out for the $(p,pn)$ recoil-momentum distribution method and is presented in this work. The experimental momentum distributions are compared to state-of-the-art mean field and $ab$ $initio$ in-medium similarity renormalization group and self-consistent Green's function calculations in combination with DWIA reaction theory calculations. Based on this work, the 1$p$ neutron orbitals are consistently found $0.48-0.78$ fm larger than the $0f_{7/2}$ neutron orbitals in $^{52-54}$Ca, while the size evolution of the valence proton orbitals remains inconclusive due to the large associated statistical uncertainties.

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arXiv2026

Femtoscopy Measurement with S$\pi$RIT TPC in Radioactive BeamHeavy-ion Collisions

Femtoscopy is a powerful tool for exploring the dynamic emitting structure in heavy-ion collisions, while radioactive beam heavy-ion collisions enable the investigation of nuclear matter under extreme isospin conditions. Here, we successfully perform femtoscopy measurements using the S$\pi$RIT Time Projection Chamber (TPC). A dedicated correction scheme for track merging and splitting is proposed, which is well applicable to rectangular TPCs housed inside dipole magnets and effectively improves the reconstructed correlation functions at small relative momenta. Focusing on the proton-proton (p-p) correlation function in the 270 MeV/u $^{132}\text{Sn}+^{124}\text{Sn}$ system, we successfully apply the track merging and splitting correction; additionally, the TPC angular acceptance exhibits a negligible impact on the correlation function. A systematic uncertainty quantification framework is established. The experimental results of the p-p correlation function confirm the feasibility of the S$\pi$RIT TPC for femtoscopy measurements and provide technical support for high-precision femtoscopy studies using rectangular TPCs in radioactive beam heavy-ion collisions.

physics.ins-det↗
arXiv2026

Large amplification of the isospin-dependence of proton emitting source size in radioactive heavy-ion collisions: a signal of n-p correlation

We report proton-proton correlation function measurements in central $^{132}$Sn+$^{124}$Sn and $^{108}$Sn+$^{112}$Sn collisions at 270 MeV/nucleon. The proton emitting source sizes are extracted for the systems by using femtoscopic imaging technique. The fast dynamic core radius for the neutron-rich system is found to be $2.22 \pm 0.13\ \text{(stat.)} \pm 0.07\ \text{(syst.)}$ fm, which is approximately 24\% larger than that for the neutron-deficient system, $1.74 \pm 0.08\ \text{(stat.)} \pm 0.05\ \text{(syst.)}$ fm. This difference is an order of magnitude larger than the $\sim$3\% difference in the ground-state charge radii of the projectile nuclei. Transport model simulations based on mean-field dynamics cannot reproduce this amplification. The observation reveals a beyond-mean-field mechanism associated to short-range neutron-proton correlations, which dynamically enhance the proton emitting source in the neutron-rich environment. Our results demonstrate that heavy-ion collisions induced by radioactive beam, combined with femtoscopic precision, provide a new hadronic probe of short-range correlation, and that careful treatment of the beyond-mean-field interactions are required in modeling such processes.

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arXiv2026

Single-particle strength toward N = 32: Spectroscopy of 51 Ca via the 50 Ca(d, p) reaction

States in the neutron-rich isotope 51 Ca were populated via the 50 Ca(d, p) transfer reaction in inverse kinematics at a beam energy of about 14 AMeV. The experiment was performed using a decelerated radioactive 50 Ca beam from the OEDO facility and the TiNA2 silicon array in combination with the SHARAQ magnetic spectrometer at RIBF/RIKEN. The energies of excited states in 51 Ca were reconstructed via missing mass spectroscopy, and angular distributions of protons were measured to extract differential cross sections. From a comparison with adiabatic distorted wave approximation (ADWA) calculations, spectroscopic factors were deduced for several states, including the ground state and excited states up to 4.2 MeV. These results are compared with shell-model calculations, as well as ab initio valence-space in-medium similarity renormalization group (VS-IMSRG) predictions. The data support the assignment of the 1/2- and 5/2- single-particle states and provide evidence for a candidate 9/2+ state with a structure consistent with neutron excitation into the 0g9/2 orbital. These findings contribute new constraints on the single-particle structure and shell evolution in neutron-rich calcium isotopes.

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arXiv2025

Neutron emission following nuclear muon capture on palladium isotopes

The energy spectra of the neutrons emitted following nuclear muon capture on palladium isotopes ($A=104$, 105, 106, 108, and 110) were measured using isotopically enriched target. \item[Method] The experiment was performed at the MuSIC-M1 beamline at the Research Center for Nuclear Physics (RCNP), Osaka University. The neutrons and $\gamma$ rays were detected with twenty-one liquid scintillators and BaF$_2$ detectors. The time-of-flight method was used to determine the neutron energy. \item[Results] Neutron energy spectra from 1\,MeV up to 20\,MeV were measured for five palladium isotopes, providing the first systematic data in the $A\sim100$ region. The spectral shapes were compared with the previous measurement for heavy nuclei and theoretical calculations. The neutron-neutron opening angle distribution was also measured and an indication of small angle correlation was found. \item[Conclusions] The spectral shape below 4\,MeV was well explained consistently with the previous measurement by the evaporation model introducing a mass number scaling. The neutron energy spectrum around 10\,MeV plays a key role in understanding the dynamics of the nuclear muon capture reaction because it is the result of the transition from the direct and pre-equilibrium neutron emission onto the evaporation process.

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arXiv2025

Impact of newly measured $\beta$\nobreakdash-delayed neutron emitters around \myisoSimp{78}{Ni} on light element nucleosynthesis in the neutrino-wind following a neutron star merger

Neutron emission probabilities and half-lives of 37 beta-delayed neutron emitters from 75Ni to 92Br were measured at the RIKEN Nishina Center in Japan, including 11 one-neutron and 13 two-neutron emission probabilities and 6 half-lives measured for the first time, which supersede theoretical estimates. These nuclei lie in the path of the weak r-process occurring in neutrino-driven winds from the accretion disk formed after the merger of two neutron stars, synthesizing elements in the A~80 abundance peak. The presence of such elements dominates the accompanying kilonova emission over the first few days and has been identified in the AT2017gfo event, associated with the gravitational wave detection GW170817. Abundance calculations based on over 17000 simulated trajectories describing the evolution of matter properties in the merger outflows show that the new data lead to an increase of 50-70 percent in the abundance of Y, Zr, Nb, and Mo. This enhancement is large compared to the scatter of relative abundances observed in old very metal-poor stars and is therefore significant in the comparison with other possible astrophysical processes contributing to light-element production. These results underline the importance of including experimental decay data for very neutron-rich beta-delayed neutron emitters into r-process models.

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arXiv2024

Spectroscopy of $^{52}$K

The first spectroscopy of $^{52}$K was investigated via in-beam $\gamma$-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory after one-proton and one-neutron knockout from $^{53}$Ca and $^{53}$K beams impinging on a 15-cm liquid hydrogen target at $\approx$ 230~MeV/nucleon. The energy level scheme of $^{52}$K was built using single $\gamma$ and $\gamma$-$\gamma$ coincidence spectra. The spins and parities of the excited states were established based on momentum distributions of the fragment after the knockout reaction and based on exclusive cross sections. The results were compared to state-of-the-art shell model calculations with the SDPF-Umod interaction and ab initio IMSRG calculations with chiral effective field theory nucleon-nucleon and three-nucleon forces.

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arXiv2024

Isospin breaking in the $^{71}$Kr and $^{71}$Br mirror system

Isospin symmetry is a fundamental concept in nuclear physics. Even though isospin symmetry is partially broken, it holds approximately for most nuclear systems, which makes exceptions very interesting from the nuclear structure perspective. In this framework, it is expected that the spins and parities of the ground states of mirror nuclei should be the same, in particular for the simplest systems where a proton is exchanged with a neutron or vice versa. In this work, we present evidence that this assumption is broken in the mirror pair $^{71}$Br and $^{71}$Kr system. Our conclusions are based on a high-statistics $\beta$ decay study of $^{71}$Kr and on state-of-the-art shell model calculations. In our work, we also found evidence of a new state in $^{70}$Se, populated in the $\beta$-delayed proton emission process which can be interpreted as the long sought coexisting 0$^+$ state.

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arXiv2024

Spectroscopy of deeply bound orbitals in neutron-rich Ca isotopes

The calcium isotopes are an ideal system to investigate the evolution of shell structure and magic numbers. Although the properties of surface nucleons in calcium have been well studied, probing the structure of deeply bound nucleons remains a challenge. Here, we report on the first measurement of unbound states in $^{53}$Ca and $^{55}$Ca, populated from \ts{54,56}Ca($p,pn$) reactions at a beam energy of around 216 MeV/nucleon at the RIKEN Radioactive Isotopes Beam Factory. The resonance properties, partial cross sections, and momentum distributions of these unbound states were analyzed. Orbital angular momentum $l$ assignments were extracted from momentum distributions based on calculations using the distorted wave impulse approximation (DWIA) reaction model. The resonances at excitation energies of 5516(41)\,keV in $^{53}$Ca and 6000(250)\,keV in $^{55}$Ca indicate a significant $l$\, =\,3 component, providing the first experimental evidence for the $\nu 0f_{7/2}$ single-particle strength of unbound hole states in the neutron-rich Ca isotopes. The observed excitation energies and cross-sections point towards extremely localized and well separated strength distributions, with some fragmentation for the $\nu 0f_{7/2}$ orbital in $^{55}$Ca. These results are in good agreement with predictions from shell-model calculations using the effective GXPF1Bs interaction and \textit{ab initio} calculations and diverge markedly from the experimental distributions in the nickel isotones at $Z=28$.

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arXiv2024

First Exploration of Monopole-Driven Shell Evolution above the N = 126 shell closure: new Millisecond Isomers in 213Tl and 215Tl

Isomer spectroscopy of heavy neutron-rich nuclei beyond the N=126 closed shell has been performed for the first time at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. New millisecond isomers have been identified at low excitation energies, 985.3(19) keV in 213Tl and 874(5) keV in 215Tl. The measured half-lives of 1.34(5) ms in 213Tl and 3.0(3) ms in 215Tl suggest spins and parities 11/2- with the single proton-hole configuration h11/2 as leading component. They are populated via E1 transitions by the decay of higher-lying isomeric states with proposed spin and parity 17/2+, interpreted as arising from a single s1/2 proton hole coupled to the 8+ seniority isomer in the (A+1)Pb cores. The lowering of the 11/2- states is ascribed to an increase of the h11/2 proton effective single-particle energy as the second g9/2 orbital is filled by neutrons, owing to a significant reduction of the proton-neutron monopole interaction between the h11/2 and g9/2 orbitals. The new ms-isomers provide the first experimental observation of shell evolution in the almost unexplored N>126 nuclear region below doubly-magic 208Pb.

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arXiv2023

Development of wide range photon detection system for muonic X-ray spectroscopy

We have developed a photon detection system for muonic X-ray spectroscopy. The detector system consists of high-purity germanium detectors with BGO Compton suppressors. The signals from the detectors are readout with a digital acquisition system. The absolute energy accuracy, energy and timing resolutions, photo-peak efficiency, the performance of the Compton suppressor, and high count rate durability are studied with standard $γ$-ray sources and in-beam experiment using $^{27}\mathrm{Al}(p, γ){}^{28}\mathrm{Si}$ resonance reaction. The detection system was demonstrated at Paul Scherrer Institute. A calibration method for a photon detector at a muon facility using muonic X-rays of $^{197}$Au and $^{209}$Bi is proposed.

physics.ins-det↗
arXiv2023

Constraining nucleon effective masses with flow and stopping observables from the S$\pi$RIT experiment

Properties of the nuclear equation of state (EoS) can be probed by measuring the dynamical properties of nucleus-nucleus collisions. In this study, we present the directed flow ($v_1$), elliptic flow ($v_2$) and stopping (VarXZ) measured in fixed target Sn + Sn collisions at 270 AMeV with the S$\pi$RIT Time Projection Chamber. We perform Bayesian analyses in which EoS parameters are varied simultaneously within the Improved Quantum Molecular Dynamics-Skyrme (ImQMD-Sky) transport code to obtain a multivariate correlated constraint. The varied parameters include symmetry energy, $S_0$, and slope of the symmetry energy, $L$, at saturation density, isoscalar effective mass, $m_{s}^*/m_{N}$, isovector effective mass, $m_{v}^{*}/m_{N}$ and the in-medium cross-section enhancement factor $\eta$. We find that the flow and VarXZ observables are sensitive to the splitting of proton and neutron effective masses and the in-medium cross-section. Comparisons of ImQMD-Sky predictions to the S$\pi$RIT data suggest a narrow range of preferred values for $m_{s}^*/m_{N}$, $m_{v}^{*}/m_{N}$ and $\eta$.

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arXiv2023

Level Structures of $^{56,58}$Ca Cast Doubt on a doubly magic $^{60}$Ca

Gamma decays were observed in $^{56}$Ca and $^{58}$Ca following quasi-free one-proton knockout reactions from $^{57,59}$Sc beams at $\approx 200$ MeV/nucleon. For $^{56}$Ca, a $γ$ ray transition was measured to be 1456(12) keV, while for $^{58}$Ca an indication for a transition was observed at 1115(34) keV. Both transitions were tentatively assigned as the $2^+_1 \rightarrow 0^+_{gs}$ decays, and were compared to results from ab initio and conventional shell-model approaches. A shell-model calculation in a wide model space with a marginally modified effective nucleon-nucleon interaction depicts excellent agreement with experiment for $2^+_1$ level energies, two-neutron separation energies, and reaction cross sections, corroborating the formation of a new nuclear shell above the $N$ = 34 shell. Its constituents, the $0f_{5/2}$ and $0g_{9/2}$ orbitals, are almost degenerate. This degeneracy precludes the possibility for a doubly magic $^{60}$Ca and potentially drives the dripline of Ca isotopes to $^{70}$Ca or even beyond.

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arXiv2023

Multiple Mechanisms in Proton-Induced Nucleon Removal at $\sim$100 MeV/Nucleon

We report on the first proton-induced single proton- and neutron-removal reactions from the neutron-deficient $^{14}$O nucleus with large Fermi-surface asymmetry $S_n-S_p$ = 18.6 MeV at $\sim$100 MeV/nucleon, a widely used energy regime for rare-isotope studies. The measured inclusive cross sections and parallel momentum distributions of the $^{13}$N and $^{13}$O residues are compared to the state-of-the-art reaction models, with nuclear structure inputs from many-body shell-model calculations. Our results provide the first quantitative contributions of multiple reaction mechanisms including the quasifree knockout, inelastic scattering and nucleon transfer processes. It is shown that the inelastic scattering and nucleon transfer, usually neglected at such energy regime, contribute about 50% and 30% to the loosely bound proton and deeply bound neutron removal, respectively. These multiple reaction mechanisms should be considered in analyses of inclusive one-nucleon removal cross sections measured at intermediate energies for quantitative investigation of single-particle strengths and correlations in atomic nuclei.

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arXiv2023

Unusually strong electronic correlation and field-induced ordered phase in YbCo$_2$

We report the first study of electrical resistivity, magnetization, and specific heat on YbCo$_2$. The measurements on a single-phased sample of YbCo$_2$ bring no evidence of magnetic ordering down to 0.3 K in a zero magnetic field. The manifestations of low Kondo temperature are observed. The specific heat value divided by temperature, C/T, keeps increasing logarithmically beyond 7 J/mol.K2 with decreasing temperature down to 0.3 K without no sign of magnetic ordering, suggesting a very large electronic specific heat. Analysis of the magnetic specific heat indicates that the large portion of the low-temperature specific heat is not explained simply by the low Kondo temperature but is due to the strong intersite magnetic correlation in both the 3d and 4f electrons. Temperature-dependent measurements under static magnetic fields up to 7 T are carried out, which show the evolution of field-induced transition above 2 T. The transition temperature increases with increasing field, pointing to a ferromagnetic character. The extrapolation of the transition temperature to zero field suggests that YbCo$_2$ is in the very proximity of the quantum critical point. These results indicate that in the unique case of YbCo$_2$, the itinerant electron magnetism of Co 3d-electrons and the Kondo effect within the vicinity of quantum criticality of Yb 4f-local moments can both play a role.

cond-mat.str-el↗
arXiv2022

Isoscaling in central Sn+Sn collisions at 270 MeV/u

Experimental information on fragment emissions is important in understanding the dynamics of nuclear collisions and in the development of transport model simulating heavy-ion collisions. The composition of complex fragments emitted in the heavy-ion collisions can be explained by statistical models, which assume that thermal equilibrium is achieved at collision energies below 100 MeV/u. Our new experimental data together with theoretical analyses for light particles from Sn+Sn collisions at 270 MeV/u, suggest that the hypothesis of thermal equilibrium breaks down for particles emitted with high transfer momentum. To inspect the system's properties in such limit, the scaling features of the yield ratios of particles from two systems, a neutron-rich system of ${}^{132}\mathrm{Sn}+{}^{124}\mathrm{Sn}$ and a nearly symmetric system of ${}^{108}\mathrm{Sn}+{}^{112}\mathrm{Sn}$, are examined in the framework of the statistical multifragmentation model and the antisymmetrized molecular dynamics model. The isoscaling from low energy particles agree with both models. However the observed breakdown of isoscaling for particles with high transverse momentum cannot be explained by the antisymmetrized molecular dynamics model.

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