arXiv subjects

T. Otsuka

Publications and source records attributed to T. Otsuka.

At least 19 recordsLinked to original sources

arXiv2026

Quenching of the proton $\pi0p_{3/2}$-$\pi0p_{1/2}$ spin-orbit splitting in $^{20}$O and the effect of the tensor force

We present the first direct measurement of the Z=6 shell gap in the neutron-rich 20O nucleus. The one-proton removal transfer reaction 2H(20O,3He)19N has been studied using the ACTAR TPC setup at GANIL. The use of ACTAR TPC enabled the measurement of low-cross section proton-removal reactions while preserving resolution. Eight p-hole states with l=1 were identified in 19N accounting for total strengths of 86% and 72% of the 0p3/2 and 0p1/2 single-particle orbitals, respectively. The energies and spectroscopic factors of the measured states allowed to determine the proton spin-orbit splitting 0p3/2 - 0p1/2 in 20O. The Z=6 shell gap has been established to be 5.30(14) MeV. These findings indicate a reduction of the Z=6 shell gap while adding neutrons to the sd-valence orbitals, consistent with the effects of the tensor force predicted by state-of-the-art shell model interaction SFO-tls while at variance with the emergence of a large Z=6 gap observed in other studies.

nucl-ex↗

arXiv2025

Lifetime of the $4^+_1$ state of $^{132}$Te

The evolution of the collectivity of tellurium isotopes from mid-shell towards $N=82$ is currently based mainly on properties of the first excited $2^+$ states. To extend structural information in this isotopic chain, in particular with respect to the balance of microscopic, seniority-type and collective excitations, electric quadrupole transition strengths from $4^+$ states need to be considered. An experiment was performed to determine the $4_1^+$ lifetime of $^{132}$Te via the recoil-distance Doppler-shift method at the University of Cologne tandem accelerator. The isotope of interest was populated in the two neutron-transfer reaction $^{130}$Te($^{18}$O,$^{16}$O)$^{132}$Te$^*$. The $E2$ decay transition strength has been determined to be $B(E2; 4^+_1\rightarrow 2^+_1) = 9.3(10)\, \text{W.u.}$ and compares favourably to shell model calculations.

nucl-ex↗

arXiv2025

Isolated one-phonon mixed-symmetry 2+ state of the radioactive neutron-rich nuclide 132Te

The $M1$ transition strengths between excited $2^+$ states of the neutron-rich, radioactive nuclide $^{132}$Te have been studied through direct lifetime measurements using the Doppler-shift attenuation method in a two-neutron transfer reaction on a $^{130}$Te target. An unambiguous identification of the lowest-lying mixed-symmetry $2^+$ state has been achieved on the basis of the large $B(M1;2^+_2\rightarrow2^+_1$)=0.18(2) $μ_\mathrm{N}^2$ transition strength, in agreement with shell-model calculations. Results are compared to the shell model, and the analysis of both, data and calculations, unambiguously identifies the second-excited $2^+$ state of $^{132}$Te as the one-quadrupole phonon mixed-symmetry state of this isotope. A lowering of the energy and $B(M1;2^+_\mathrm{ms}\rightarrow 2^+_1)$ strength within the $N$=80 isotones toward the $Z$=50 shell closure is observed, which goes alongside with the lowering of the $E2$ collectivity approaching the magic proton shell.

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arXiv2025

Vibrational Modes in Strongly Deformed Nuclei

Low-energy vibrational excitations associated with the fluctuation of quadrupole deformed shapes are discussed within the frame of state-of-the-art Configuration Interaction calculations, actually performed via the Quasi-particle Vacua Shell Model version of the Monte Carlo Shell Model. Recently, low-lying $\gamma$ bands in heavy strongly deformed nuclei were shown to be rotational $K^P$ = 2$^+$ excitations of triaxially deformed states (see T. Otsuka \etal, Eur. Phys. J. A 61, 126 (2025)) rather than vibrational excitations as traditionally interpreted. In this context, it is important to identify possible low-lying vibrational excitations and to characterize the excitation energy at which they emerge. Focusing on two typical examples, $^{166}$Er and $^{162}$Dy, vibrational states are indeed identified above the $\gamma$ band using an extended version of the so-called T-plot. The phenomenon of shape coexistence is also shown to produce low-lying states below such vibrational band heads. These results suggest novel and rich structures in heavy deformed nuclei. While experimental counterparts are seen for some of such states, others are predictions opening doors to future dedicated experiments.

nucl-th↗

arXiv2025

Unbound neutron $\nu0d_{3/2}$ strength in $^{17}$C and the N=16 shell gap

Significant continuum strength has been observed to be populated in $^{17}$C produced in the d($^{16}$C,p) reaction at a beam energy of 17.2~MeV/nucleon. The strength appears at greater than $\sim$2~MeV above the single-neutron decay threshold and has been identified as arising from transfer into the neutron $0d_{3/2}$ orbital. Guided by shell model predictions the greater majority of the strength is associated with a 3/2$^+$ state at an excitation energy of 4.40$_{-0.14}^{+0.33}$ MeV and a much weaker 3/2$^+$ level at 5.60$_{-0.45}^{+1.35}$ MeV. The corresponding total widths were determined to be 3.45$_{-0.78}^{+1.82}$ and 1.6$_{-1.4}^{+4.6}$ MeV, respectively. From the backward angle proton differential cross sections and the branching ratios for neutron decay to the $^{16}$C(2$_{1}^{+}$) level, the corresponding spectroscopic factors to the ground state were deduced to be 0.47$\pm{10}$ and $<$0.09. Shell-model calculations employing the phenomenological SFO-tls interaction as well as Gamow Shell-Model calculations including continuum effects are in reasonable agreement with experiment, although the predicted strength lies at somewhat lower energy. The size of the N=16 shell gap ($\varepsilon_{ \nu0d_{3/2}}-\varepsilon _{ν1s_{1/2}}$) was estimated to be 5.08$_{-0.33}^{+0.43}$~MeV - some 1.3~MeV larger than found in the SFO-tls shell model calculation.

nucl-ex↗

arXiv2025

Charge Radii Measurements of Exotic Tin Isotopes in the Proximity of $N=50$ and $N=82$

We report nuclear charge radii for the isotopes $^{104-134}$Sn, measured using two different collinear laser spectroscopy techniques at ISOLDE-CERN. These measurements clarify the arch-like trend in charge radii along the isotopic chain and reveal an odd-even staggering that is more pronounced near the $N=50$ and $N=82$ shell closures. The observed local trends are well described by both nuclear density functional theory and valence space in-medium similarity renormalization group calculations. Both theories predict appreciable contributions from beyond-mean-field correlations to the charge radii of the neutron-deficient tin isotopes. The models, however, fall short of reproducing the magnitude of the known $B(E2)$ transition probabilities, highlighting the remaining challenges in achieving a unified description of both ground-state properties and collective phenomena.

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arXiv2025

Prevailing Triaxial Shapes in Atomic Nuclei and a Quantum Theory of Rotation of Composite Objects

In the traditional view, heavy deformed nuclei are like axially-symmetric prolate ellipsoids, rotating about one of the short axes. In the present picture, their shapes may be triaxial. The triaxial shape yields complex rotations, which actually well reproduce experimental data, as confirmed by state-of-the-art Configuration Interaction calculations. Two origins are suggested for the triaxiality: (i) binding-energy gain by the symmetry restoration for triaxial shapes, and (ii) another gain by specific components of the nuclear force, like tensor force and high-multipole (e.g. hexadecupole) central force. While the origin (i) produces basic smaller triaxiality for virtually all deformed nuclei, the origin (ii) produces medium triaxiality for a certain class of nuclei. An example of the former is 154Sm, a typical showcase of axial symmetry but is now suggested to depict a modest yet finite triaxiality. The latter, medium triaxiality, is discussed from various viewpoints for some exemplified nuclei including 166Er, and experimental findings. Many-body structures of the gamma band and the double-gamma band are clarified. Regarding the general features of rotational states of deformed many-body systems including triaxial ones, the well-known J(J+1) rule of rotational excitation energies is discussed, within the quantum mechanical many-body theory, without resorting to the quantization of a rotating classical rigid body. The picture of prevailing triaxial shapes thus emerges, where the empirically known rotational-band pattern appears with good K quantum number, but the internal structure is dfferent from conventional picture a la A. Bohr. The possible relations to Davydov's rigid-triaxial-rotor model are mentioned.

nucl-th↗

arXiv2025

Refined topology of the N = 20 island of inversion with high precision mass measurements of $^{31-33}$Na and $^{31-35}$Mg

Mass measurements of $^{31-33}$Na and $^{31-35}$Mg using the TITAN MR-TOF-MS at TRIUMF's ISAC facility are presented, with the uncertainty of the $^{33}$Na mass reduced by over two orders of magnitude. The excellent performance of the MR-TOF-MS has also allowed the discovery of a millisecond isomer in $^{32}$Na. The precision obtained shows that the binding energy of the normally closed N = 20 neutron shell reaches a minimum for $^{32}$Mg but increases significantly for $^{31}$Na, hinting at the possibility of enhanced shell strength toward the unbound $^{28}$O. We compare the results with new ab initio predictions that raise intriguing questions of nuclear structure beyond the dripline.

nucl-ex↗

arXiv2024

Giant Dipole Resonance photofission and photoneutron reactions in $^{238}$U and $^{232}$Th

New measurements of photofission and photoneutron reactions on $^{238}$U and $^{232}$Th in the Giant Dipole Resonance (GDR) energy region have been performed at the laser Compton-scattering $γ$-ray source of the NewSUBARU synchrotron radiation facility using a high-and-flat efficiency moderated $^3$He detection array. The neutron-multiplicity sorting of high-multiplicity fission neutron coincidence events has been performed using a dedicated energy dependent, multiple firing statistical treatment. The photoneutron $(γ,\,in)$ with $i$~=~1~--~3 and photofission $(γ,\,F)$ reactions have been discriminated by considering a Gaussian distribution of prompt-fission-neutron (PFN) multiplicities predicted by the theory of evaporation in sequential neutron emission from excited fission fragments. We report experimental $(γ,\,n)$, $(γ,\,2n)$, $(γ,\,3n)$ and $(γ,\,F)$ cross sections, average energies of PFNs and of $(γ,\,in)$ photoneutrons, as well as the mean number of PFNs per fission act and the width of the PFNs multiplicity distribution. Based on these primary experimental results and combined with reasonable assumptions, we extract also the first- and second-chance fission contributions. The new experimental results are compared with statistical-model calculations performed with the EMPIRE-3.2 Malta and TALYS-1.964 codes on the present data and with prompt fission emission calculations obtained with the Los Alamos model in the frame of the most probable fragmentation approach with and without sequential emission.

nucl-ex↗

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.

nucl-ex↗

arXiv2023

Intruder configurations in $^{29}$Ne at the transition into the island of inversion: Detailed structure study of $^{28}$Ne

Detailed $γ$-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.

nucl-ex↗

arXiv2023

Cross-shell states in $^{15}$C: a test for p-sd interactions

The low-lying structure of $^{15}$C has been investigated via the neutron-removal $^{16}$C$(d,t)$ reaction. Along with bound neutron sd-shell hole states, unbound p-shell hole states have been firmly confirmed. The excitation energies and the deduced spectroscopic factors of the cross-shell states are an important measure of the $[(p)^{-1}(sd)^{2}]$ neutron configurations in $^{15}$C. Our results show a very good agreement with shell-model calculations using the SFO-tls interaction for $^{15}$C. However, a modification of the $p$-$sd$ and $sd$-$sd$ monopole terms was applied in order to reproduce the $N=9$ isotone $^{17}$O. In addition, the excitation energies and spectroscopic factors have been compared to the first calculations of $^{15}$C with the $ab~ initio$ self-consistent Green's function method employing the NNLO$_{sat}$ interaction. The results show the sensitivity to the size of the $N=8$ shell gap and highlight the need of going beyond the current truncation scheme in the theory.

nucl-ex↗

arXiv2022

A new study of the $N=32$ and $N=34$ shell gap for Ti and V by the first high-precision MRTOF mass measurements at BigRIPS-SLOWRI

The atomic masses of $^{55}$Sc, $^{56,58}$Ti, and $^{56-59}$V have been determined using the high-precision multi-reflection time-of-flight technique. The radioisotopes have been produced at RIKEN's RIBF facility and delivered to the novel designed gas cell and multi-reflection system (ZD MRTOF), which has been recently commissioned downstream of the ZeroDegree spectrometer following the BigRIPS separator. For $^{56,58}$Ti and $^{56-59}$V the mass uncertainties have been reduced down to the order of $10\,\mathrm{keV}$, shedding new light on the $N=34$ shell effect in Ti and V isotopes by the first high-precision mass measurements of the critical species $^{58}$Ti and $^{59}$V. With the new precision achieved, we reveal the non-existence of the $N=34$ empirical two-neutron shell gaps for Ti and V, and the enhanced energy gap above the occupied $νp_{3/2}$ orbit is identified as a feature unique to Ca. We perform new Monte Carlo shell model calculations including the $νd_{5/2}$ and $νg_{9/2}$ orbits and compare the results with conventional shell model calculations, which exclude the $νg_{9/2}$ and the $νd_{5/2}$ orbits. The comparison indicates that the shell gap reduction in Ti is related to a partial occupation of the higher orbitals for the outer two valence neutrons at $N=34$.

nucl-ex↗

arXiv2022

Proton distribution radii of $^{16-24}$O: signatures of new shell closures and neutron skin

The root mean square radii of the proton density distribution in $^{16-24}$O derived from measurements of charge changing cross sections with a carbon target at $\sim$900$A$ MeV together with the matter radii portray thick neutron skin for $^{22 - 24}$O despite $^{22,24}$O being doubly magic. Imprints of the shell closures at $N$ = 14 and 16 are reflected in local minima of their proton radii that provide evidence for the tensor interaction causing them. The radii agree with it ab initio calculations employing the chiral NNLO$_{\mathrm{sat}}$ interaction, though skin thickness predictions are challenged. Shell model predictions agree well with the data.

nucl-ex↗

arXiv2022

Enhanced superconducting pairing strength near a nonmagnetic nematic quantum critical point

The quest for high-temperature superconductivity at ambient pressure is a central issue in physics. In this regard, the relationship between unconventional superconductivity and the quantum critical point (QCP) associated with the suppression of some form of symmetry-breaking order to zero temperature has received particular attention. The key question is how the strength of the electron pairs changes near the QCP, and this can be verified by high-field experiments. However, such studies are limited mainly to superconductors with magnetic QCPs, and the possibility of unconventional mechanisms by which nonmagnetic QCP promotes strong pairing remains a nontrivial issue. Here, we report systematic measurements of the upper critical field $H_{\rm c2}$ in nonmagnetic FeSe$_{1-x}$Te$_{x}$ superconductors, which exhibit a QCP of electronic nematicity characterized by spontaneous rotational-symmetry breaking. As the magnetic field increases, the superconducting phase of FeSe$_{1-x}$Te$_{x}$ shrinks to a narrower dome surrounding the nematic QCP. The analysis of $H_{\rm c2}$ reveals that the Pauli-limiting field is enhanced toward the QCP, implying that the pairing interaction is significantly strengthened via nematic fluctuations emanated from the QCP. Remarkably, this nonmagnetic nematic QCP is not accompanied by a divergent effective mass, distinct from the magnetically mediated pairing. Our observation opens up a nonmagnetic route to high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2021

Ground-state properties of light $4n$ self-conjugate nuclei in $ab$ $initio$ no-core Monte Carlo shell model calculations with nonlocal $NN$ interactions

We report $J^π= 0^+$ ground-state energies and point-proton radii of $^4$He, $^8$Be, $^{12}$C, $^{16}$O and $^{20}$Ne nuclei calculated by the $ab$ $initio$ no-core Monte Carlo shell model with the JISP16 and Daejeon16 nonlocal $NN$ interactions. Ground-state energies are obtained in the basis spaces up to seven oscillator shells ($N_{\rm shell} = 7$) with several oscillator energies ($\hbar ω$) around the optimal oscillator energy for the convergence of ground-state energies. These energy eigenvalues are extrapolated to obtain estimates of converged ground state energies in each basis space using energy variances of computed energy eigenvalues. We further extrapolate these energy-variance-extrapolated energies obtained in the finite basis spaces to infinite basis-space results with an empirical exponential form. This form features a dependence on the basis-space size but is independent of the value of $\hbarω$ used for the harmonic-oscillator basis functions. Point-proton radii for these states of atomic nuclei are also calculated following techniques employed for the energies. From these results, it is found that the Daejeon16 $NN$ interaction provides good agreement with experimental data up to approximately $^{16}$O, while the JISP16 $NN$ interaction provides good agreement with experimental data up to approximately $^{12}$C. Beyond these nuclei, the interactions produce overbinding accompanied by radii that are too small. These findings suggest and encourage further revisions of nonlocal $NN$ interactions towards the investigation of nuclear structure in heavier-mass regions.

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arXiv2021

Material survey for millimeter-wave absorber using 3-D printed mold

Radio absorptive materials (RAMs) are key elements for receivers in the millimeter-wave range. For astronomical applications, cryogenic receivers are widely used to achieve a high-sensitivity. These cryogenic receivers, in particular the receivers for the cosmic microwave background, require that the RAM has low surface reflectance ($\lesssim 1\%$) in a wide frequency range (20--300 GHz) to minimize the undesired stray light to detectors. We develop a RAM that satisfies this requirement based on a production technology using a 3D-printed mold (named as RAM-3pm). This method allows us to shape periodic surface structures to achieve a low reflectance. A wide range of choices for the absorptive materials is an advantage. We survey the best material for the RAM-3pm. We measure the index of refraction ($n$) and the extinction coefficient ($κ$) at liquid nitrogen temperature as well as at room temperature of 17 materials. We also measure the reflectance at the room temperature for the selected materials. The mixture of an epoxy adhesive (STYCAST-2850FT) and a carbon fiber (K223HE) achieves the best performance. We estimate the optical performance at the liquid nitrogen temperature by a simulation based on the measured $n$ and $κ$. The RAM-3pm made with this material satisfies the requirement except at the lower edge of the frequency range ($\sim$20 GHz). We also estimate the reflectance of a larger pyramidal structure on the surface. We find a design to satisfy our requirement.

astro-ph.IM↗

arXiv2021

Complete set of bound negative-parity states in the neutron-rich 18N nucleus

High-resolution gamma-ray spectroscopy of 18N is performed with the Advanced GAmma Tracking Array AGATA, following deep-inelastic processes induced by an 18O beam on a 181Ta target. Six states are newly identified, which together with the three known excitations exhaust all negative-parity excited states expected in 18N below the neutron threshold. Spin and parities are proposed for all located states on the basis of decay branchings and comparison with large-scale shell-model calculations performed in the p-sd space, with the YSOX interaction. Of particular interest is the location of the 0^-_1 and 1^-_2 excitations, which provide strong constrains for cross-shell p-sd matrix elements based on realistic interactions, and help to simultaneously reproduce the ground and first-excited states in 16N and 18N, for the first time. Understanding the 18N structure may also have significant impact on neutron-capture cross-section calculations in r-process modeling including light neutron-rich nuclei.

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