arXiv subjects

Shinsuke Ota

Publications and source records attributed to Shinsuke Ota.

9 recordsLinked to original sources

arXiv2026

Ion-Optical Tuning of the Large Acceptance Spectrometer for Improved Angular Resolution and Acceptance

The trade-off between angular resolution and acceptance in scattering-angle measurements with a magnetic spectrometer is quantitatively evaluated for the Large Acceptance Spectrometer (LAS). The dependence on the multipole magnet field strength is investigated. Third-order transfer matrices were calculated with GICOSY, and particle transport was simulated with MOCADI. The vertical angular resolution is defined as the standard deviation between reconstructed and true angles, while the acceptance is determined from the transport efficiency within an elliptical gate in target angle space. The resolution improves with increasing field strength, reaching $\sigma_b \sim 5.5$ mrad at +20\%, consistent with $5.43 \pm 0.20$ mrad. In contrast, stronger fields reduce the vertical acceptance and solid angle. These results demonstrate a trade-off between resolution and acceptance. Enhanced vertical focusing shifts the focal condition away from the nominal focal plane, enabling high-precision reconstruction.

physics.ins-det↗

arXiv2026

Establishing the $^{40}$Ca$(p,p \alpha)$ reaction at 392 MeV under quasi-free scattering conditions

The $(p,p \alpha)$ reaction offers a direct means to probe preformed $\alpha$-cluster structures in nuclei under quasi-free scattering conditions. Previous studies around 100 MeV provided valuable insights into $\alpha$ clustering, but quantitative comparison with microscopic cluster wave functions remained limited due to strong distortion effects. At higher energies, the reaction mechanism becomes simpler and the distorted-wave impulse approximation (DWIA) provides a more reliable framework for quantitative analysis. In the present work, the $^{40}$Ca$(p,p\alpha)$ reaction was measured at an incident energy of 392 MeV using the high-resolution Grand Raiden and LAS spectrometers at RCNP. Despite the small cross section in this energy region, the achieved resolution allowed clear separation of the ground and excited states of the residual $^{36}$Ar nucleus, and corresponding momentum distributions were extracted. DWIA calculations using a Woods-Saxon $\alpha + ^{36}$Ar bound-state wave function yielded an experimental spectroscopic factor of $ S_{\mathrm{FAC}}^{\mathrm{WS}} = 0.51 \pm 0.05 $, consistent with the previous result at 101.5 MeV $(0.52 \pm 0.23 )$. This agreement demonstrates that the reaction mechanism is well described across a wide energy range. The present study establishes the feasibility of high-precision $(p,p\alpha)$ measurements at several hundred MeV and highlights their potential as a quantitative probe of $\alpha$ clustering in medium-mass nuclei, forming the basis for systematic studies in both stable and unstable systems.

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arXiv2025

Measurement of the isoscalar giant monopole resonance in $^{86}$Kr via deuteron inelastic scattering using an active target CAT-M

Deuteron inelastic scattering on $^{86}$Kr was measured in inverse kinematics with the gaseous active target CAT-M, as part of a systematic investigation aimed at determining the nuclear matter incompressibility. The isoscalar monopole strength distribution was extracted via multipole decomposition analysis, and the energy of the isoscalar giant monopole resonance was determined to be 17 $\pm$ 1 MeV. The nuclear incompressibility of $^{86}$Kr and the isospin-dependent term of the nuclear matter incompressibility are discussed.

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arXiv2025

The HYDRA pion-tracker for hypernuclei studies at R3B

The HYpernuclei-Decay at R3B Apparatus (HYDRA) tracker is a novel time projection chamber combined with a plastic scintillator wall for timing and trigger purposes. This detector is a low radiation length tracker dedicated to measuring pions from the weak decay of light hypernuclei produced from ion-ion collisions at few GeV/nucleon in the magnetic field of the large-acceptance dipole magnet GLAD at the Reactions with Relativistic Radioactive Beams (R3B) experiment at GSI-FAIR. In this paper, we describe the design of the detector and provide the results of its first characterizations.

physics.ins-det↗

arXiv2023

Development of fast-response PPAC with strip-readout for heavy-ion beams

A strip-readout parallel-plate avalanche counter (SR-PPAC) has been developed aiming at the high detection efficiency and good position resolution in high-intensity heavy-ion measurements. The performance was evaluated using 115 MeV/u $^{132}$Xe, 300 MeV/u $^{132}$Sn, and 300 MeV/u $^{48}$Ca beams. A detection efficiency beyond 99% for these beams is achieved even at an incident beam intensity of 0.7 billion particles per second. The best position resolution achieved is 235 um (FWHM).

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arXiv2023

Performance of prototype Dual Gain Multilayer Thick GEM with high-intensity heavy-ion beam injections in low-pressure hydrogen gas

A prototype Dual Gain Multilayer Thick Gas Electron Multilyer (DG-M-THGEM) with an active area of 10 cm $\times$ 10 cm was manufactured aiming at the production of a large-volume active-target time projection chamber which can work under the condition of high-intensity heavy-ion beam injections. The DG-M-THGEM has a alternating structure of electrodes and insulators. Effective gas gains of two regions, which are called beam and recoil regions, are separately controlled. Performance of the prototype DG-M-THGEM in hydrogen gas at a pressure of 40 kPa was evaluated. Irradiating a $^{132}$Xe beam, an effective gas gain lower than 100 with a charge resolution of 3% was achieved in the beam region while the effective gas gain of 2000 was maintained in the recoil region. Position distributions of measured charges along the beam axis were investigated in order to evaluate gain uniformity in the high intensity beam injection. The gain shift was estimated by simulations considering space charges in the drift region. The gain shift was suppressed within 3% even at the beam intensity of 2.5 $\times$ 10$^{6}$ particles per second.

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arXiv2022

Neutron star mass formula with nuclear saturation parameters for asymmetric nuclear matter

Low-mass neutron stars are directly associated with the nuclear saturation parameters because their central density is definitely low. We have already found a suitable combination of nuclear saturation parameters for expressing the neutron star mass and gravitational redshift, i.e., $η\equiv (K_0L^2)^{1/3}$ with the incompressibility for symmetric nuclear matter, $K_0$, and the density-dependent nuclear symmetry energy, $L$. In this study, we newly find another suitable combination given by $η_τ\equiv (-K_τL^5)^{1/6}$ with the isospin dependence of incompressibility for asymmetric nuclear matter, $K_τ$, and derive the empirical relations for the neutron star mass and gravitational redshift as a function of $η_τ$ and the normalized central number density. With these empirical relations, one can evaluate the mass and gravitational redshift of the neutron star, whose central number density is less than threefold the saturation density, within $\sim 10\%$ accuracy, and the radius within a few \% accuracies. In addition, we discuss the neutron star mass and radius constraints from the terrestrial experiments, using the empirical relations, together with those from the astronomical observations. Furthermore, we find a tight correlation between $η_τ$ and $η$. With this correlation, we derive the constraint on $K_τ$ as $-348\le K_τ\le -237$ MeV, assuming that $L=60\pm 20$ and $K_0=240\pm 20$ MeV.

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arXiv2022

Chiral symmetry restoration at high matter density observed in pionic atoms

Modern theories of physics tell that the vacuum is not an empty space. Hidden in the vacuum is a structure of anti-quarks $\bar{q}$ and quarks $q$. The $\bar{q}$ and $q$ pair has the same quantum number as the vacuum and condensates in it since the strong interaction of the quantum chromodynamics (QCD) is too strong to leave it empty. The $\bar{q}q$ condensation breaks the chiral symmetry of the vacuum. The expectation value $<\bar{q}q>$ is an order parameter. For higher temperature or higher matter-density, $|<\bar{q}q>|$ decreases reflecting the restoration of the symmetry. In contrast to these clear-cut arguments, experimental evidence is so far limited. First of all, the $\bar{q}q$ is nothing but the vacuum itself. It is neither visible nor perceptible. In this article, we unravel this invisible existence by high precision measurement of pionic atoms, $\pi^-$-meson-nucleus bound systems. Using the $\pi^-$ as a probe, we demonstrate that $|<\bar{q}q>|$ is reduced in the nucleus at 58% of the normal nuclear density by a factor of 77 $\pm$ 2% compared with that in the vacuum. This reduction indicates that the chiral symmetry is partially restored due to the extremely high density of the nucleus. The present experimental result clearly exhibits the existence of the hidden structure, the chiral condensate, in the vacuum.

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arXiv2019

Compression-mode resonances in the calcium isotopes and implications for the asymmetry term in nuclear incompressibility

Recent data on isoscalar giant monopole resonance (ISGMR) in the calcium isotopes $^{40,44,48}$Ca have suggested that $K_τ$, the asymmetry term in the nuclear incompressibility, has a positive value. A value of $K_τ> 0$ is entirely incompatible with present theoretical frameworks and, if correct, would have far-reaching implications on our understanding of myriad nuclear and astrophysical phenomena. This paper presents results of an independent ISGMR measurement with the $^{40,42,44,48}$Ca($α,α^\prime$) reaction at $E_α= 386$ MeV. These results conclusively discount the possibility of a positive value for $K_τ$, and are consistent with the previously-obtained values for this quantity.

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