arXiv subjects

T. Kawabata

Publications and source records attributed to T. Kawabata.

At least 37 records · Page 2Linked to original sources

arXiv2019

Measurement of $γ$ rays from the giant resonances excited by $^{12}$C$(p,p')$ reaction at 392 MeV and 0$^{\circ}$

We measured both the differential cross section ($σ_{p,p^\prime}$ $=d^2σ/dΩdE_{x}$) and the $γ$-ray emission probability ($R_γ(E_x)$ $=σ_{p,p^\primeγ}$/$σ_{p,p^\prime}$) from the giant resonances excited by $\rm^{12}C$(\textit{p,p}$^\prime$) reaction at 392 MeV and 0$^\circ$, using a magnetic spectrometer and an array of NaI(Tl) counters. The absolute value of $R_γ(E_x)$ was calibrated by using the well-known $γ$-ray emission probability from $\rm^{12}C^* ( 15.11$ MeV, $ 1^+$, $T=1$) and $\rm^{16}O^* ( 6.9$ MeV, $2^+$, $T=0$) states within 5\% uncertainty. We found that $R_γ(E_x)$ starts from zero at $E_x=16$ MeV, increases to a maximum of 53.3$\pm$0.4$\pm$3.9\% at $E_x=27$ MeV and then decreases. We also compared the measured values of $R_γ(E_x)$ with statistical model calculation based on the Hauser-Feshbach formalism in the energy region $E_x=$ 16-32 MeV and discussed the features of $γ$-ray emission probability quantitatively.

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arXiv2018

Dominance of tensor correlations in high-momentum nucleon pairs studied by (p,pd) reaction

The isospin character of p-n pairs at large relative momentum has been observed for the first time in the 16O ground state. A strong population of the J,T=1,0 state and a very weak population of the J,T=0,1 state were observed in neutron pick up domain of 16O(p,pd) at 392 MeV. This strong isospin dependence at large momentum transfer is not reproduced by the distorted-wave impulse approximation calculations with known spectroscopic amplitudes. The results indicate the presence of high-momentum protons and neutrons induced by the tensor interactions in ground state of 16O.

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arXiv2018

Performance test of the MAIKo active target

A new active target named MAIKo (Mu-PIC based Active target for Inverse Kinematics$_{\circ}$) has been developed at Kyoto University and Research Center for Nuclear Physics (RCNP), Osaka University. MAIKo is suited for missing-mass spectroscopy of unstable nuclei at forward scattering angles in inverse kinematics. MAIKo consists of a time projection chamber (TPC), which incorporates a micro-pixel chamber ($μ$-PIC) as the electron multiplication and collection system. In MAIKo, the medium gas also plays the role of a reaction target, thus allowing detection of low-energy recoil particles with high position resolution. The MAIKo TPC was commissioned with He(93%)+iso-C$_{4}$H$_{10}$(7%) and He(93%)+CO$_{2}$(7%) mixture gasses at 430 hPa. The gas gain and the angular resolution of MAIKo were evaluated with an alpha source and a $^{4}$He beam at 56 MeV. The TPC was stably operated up to 1000-kcps beam intensity. A tracking algorithm using the Hough transform method has been developed to analyze scattering events. An angular resolution of 1.3$^{\circ}$ was achieved for scattered $^{4}$He particles.

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arXiv2018

Isoscalar Giant Monopole, Dipole, and Quadrupole Resonances in $^{90,92}$Zr and $^{92}$Mo

The isoscalar giant monopole, dipole, and quadrupole strength distributions have been deduced in $^{90, 92}$Zr, and $^{92}$Mo from "background-free" spectra of inelastic $α$-particle scattering at a beam energy of 385 MeV at extremely forward angles, including 0$^{\circ}$. These strength distributions were extracted by a multipole-decomposition analysis based on the expected angular distributions of the respective multipoles. All these strength distributions for the three nuclei practically coincide with each other, affirming that giant resonances, being collective phenomena, are not influenced by nuclear shell structure near $A\sim$90, contrary to the claim in a recent measurement.

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arXiv2017

Evidence for Z=6 `magic number' in neutron-rich carbon isotopes

The nuclear shell structure, which originates in the nearly independent motion of nucleons in an average potential, provides an important guide for our understanding of nuclear structure and the underlying nuclear forces. Its most remarkable fingerprint is the existence of the so-called `magic numbers' of protons and neutrons associated with extra stability. Although the introduction of a phenomenological spin-orbit (SO) coupling force in 1949 helped explain the nuclear magic numbers, its origins are still open questions. Here, we present experimental evidence for the smallest SO-originated magic number (subshell closure) at the proton number 6 in 13-20C obtained from systematic analysis of point-proton distribution radii, electromagnetic transition rates and atomic masses of light nuclei. Performing ab initio calculations on 14,15C, we show that the observed proton distribution radii and subshell closure can be explained by the state-of-the-art nuclear theory with chiral nucleon-nucleon and three-nucleon forces, which are rooted in the quantum chromodynamics.

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arXiv2016

Proton-proton correlations in distinguishing the two-proton emission mechanism of $^{23}$Al and $^{22}$Mg

The proton-proton momentum correlation functions ($C_{pp}(q)$) for kinematically complete decay channels of $^{23}$Al $\rightarrow$ p + p + $^{21}$Na and $^{22}$Mg $\rightarrow$ p + p + $^{20}$Ne have been measured at the RIKEN RI Beam Factory. From the very different correlation strength of $C_{pp}(q)$ for $^{23}$Al and $^{22}$Mg, the source size and emission time information were extracted from the $C_{pp}(q)$ data by assuming a Gaussian source profile in the correlation function calculation code (CRAB). The results indicated that the mechanism of two-proton emission from $^{23}$Al was mainly sequential emission, while that of $^{22}$Mg was mainly three-body simultaneous emission. By combining our earlier results of the two-proton relative momentum and the opening angle, it is pointed out that the mechanism of two-proton emission could be distinguished clearly.

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arXiv2016

Are There Nuclear Structure Effects on the Isoscalar Giant Monopole Resonance and Nuclear Incompressibility near A~90?

"Background-free" spectra of inelastic $α$-particle scattering have been measured at a beam energy of 385 MeV in $^{90, 92}$Zr and $^{92}$Mo at extremely forward angles, including 0$^{\circ}$. The ISGMR strength distributions for the three nuclei coincide with each other, establishing clearly that nuclear incompressibility is not influenced by nuclear shell structure near $A\sim$90 as was claimed in recent measurements.

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arXiv2016

Charge-changing-cross-section measurements of $^{12-16}$C at around $45A$ MeV and development of a Glauber model for incident energies $10A-2100A$ MeV

We have measured for the first time the charge-changing cross sections ($σ_{\text{CC}}$) of $^{12-16}$C on a $^{12}$C target at energies below $100A$ MeV. To analyze these low-energy data, we have developed a finite-range Glauber model with a global parameter set within the optical-limit approximation which is applicable to reaction cross section ($σ_{\text{R}}$) and $σ_{\text{CC}}$ measurements at incident energies from 10$A$ to $2100A$ MeV. Adopting the proton-density distribution of $^{12}$C known from the electron-scattering data, as well as the bare total nucleon-nucleon cross sections, and the real-to-imaginary-part ratios of the forward proton-proton elastic scattering amplitude available in the literatures, we determine the energy-dependent slope parameter $β_{\rm pn}$ of the proton-neutron elastic differential cross section so as to reproduce the existing $σ_{\text{R}}$ and interaction-cross-section data for $^{12}$C+$^{12}$C over a wide range of incident energies. The Glauber model thus formulated is applied to calculate the $σ_{\text{\tiny R}}$'s of $^{12}$C on a $^9$Be and $^{27}$Al targets at various incident energies. Our calculations show excellent agreement with the experimental data. Applying our model to the $σ_{\text{\tiny R}}$ and $σ_{\text{\tiny CC}}$ for the "neutron-skin" $^{16}$C nucleus, we reconfirm the importance of measurements at incident energies below $100A$ MeV. The proton root-mean-square radii of $^{12-16}$C are extracted using the measured $σ_{\text{CC}}$'s and the existing $σ_{\text{R}}$ data. The results for $^{12-14}$C are consistent with the values from the electron scatterings, demonstrating the feasibility, usefulness of the $σ_{\text{CC}}$ measurement and the present Glauber model.

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arXiv2016

Change of nuclear configurations in the neutrinoless double-$β$ decay of $^{130}$Te $\rightarrow$ $^{130}$Xe and $^{136}$Xe $\rightarrow$ $^{136}$Ba

The change in the configuration of valence protons between the initial and final states in the neutrinoless double-$β$ decay of $^{130}$Te $\rightarrow$ $^{130}$Xe and of $^{136}$Xe $\rightarrow$ $^{136}$Ba has been determined by measuring the cross sections of the ($d$,$^3$He) reaction with 101-MeV deuterons. Together with our recent determination of the relevant neutron configurations involved in the process, a quantitative comparison with the latest shell-model and interacting-boson-model calculations reveals significant discrepancies. These are the same calculations used to determine the nuclear matrix elements governing the rate of neutrinoless double-$β$ decay in these systems.

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arXiv2015

Dipole polarizability of 120Sn and nuclear energy density functionals

The electric dipole strength distribution in 120Sn between 5 and 22 MeV has been determined at RCNP Osaka from a polarization transfer analysis of proton inelastic scattering at E_0 = 295 MeV and forward angles including 0°. Combined with photoabsorption data an electric dipole polarizability α_D(120Sn) = 8.93(36) fm^3 is extracted. The dipole polarizability as isovector observable par excellence carries direct information on the nuclear symmetry energy and its density dependence. The correlation of the new value with the well established α_D(208Pb) serves as a test of its prediction by nuclear energy density functionals (EDFs). Models based on modern Skyrme interactions describe the data fairly well while most calculations based on relativistic Hamiltonians cannot.

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arXiv2015

Splitting of ISGMR strength in the light-mass nucleus $^{24}$Mg due to ground-state deformation

The isoscalar giant monopole resonance (ISGMR) strength distribution in $^{24}$Mg has been determined from background-free inelastic scattering of 386-MeV $α$ particles at extreme forward angles, including 0$^{\circ}$. The ISGMR strength distribution has been observed for the first time to have a two-peak structure in a light-mass nucleus. This splitting of ISGMR strength is explained well by microscopic theory in terms of the prolate deformation of the ground state of $^{24}$Mg.

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arXiv2015

Different mechanism of two-proton emission from proton-rich nuclei $^{23}$Al and $^{22}$Mg

Two-proton relative momentum ($q_{pp}$) and opening angle ($θ_{pp}$) distributions from the three-body decay of two excited proton-rich nuclei, namely $^{23}$Al $\rightarrow$ p + p + $^{21}$Na and $^{22}$Mg $\rightarrow$ p + p + $^{20}$Ne, have been measured with the projectile fragment separator (RIPS) at the RIKEN RI Beam Factory. An evident peak at $q_{pp}\sim20$ MeV/c as well as a peak in $θ_{pp}$ around 30$^\circ$ are seen in the two-proton break-up channel from a highly-excited $^{22}$Mg. In contrast, such peaks are absent for the $^{23}$Al case. It is concluded that the two-proton emission mechanism of excited $^{22}$Mg is quite different from the $^{23}$Al case, with the former having a favorable diproton emission component at a highly excited state and the latter dominated by the sequential decay process.

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arXiv2015

Low-energy electric dipole response in 120Sn

The electric dipole strength in 120Sn has been extracted from proton inelastic scattering experiments at E_p = 295 MeV and at forward angles including 0 degree. Below neutron threshoild it differs from the results of a 120Sn(gamma,gamma') experiment and peaks at an excitation energy of 8.3 MeV. The total strength corresponds to 2.3(2)% of the energy-weighted sum rule and is more than three times larger than what is observed with the (gamma,gamma') reaction. This implies a strong fragmentation of the E1 strength and/or small ground state branching ratios of the excited 1- states.

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arXiv2014

Excitation of Giant Monopole Resonance in $^{208}$Pb and $^{116}$Sn Using Inelastic Deuteron Scattering

The excitation of the isoscalar giant monopole resonance (ISGMR) in $^{116}$Sn and $^{208}$Pb has been investigated using small-angle (including $0^\circ$) inelastic scattering of 100 MeV/u deuteron and multipole-decomposition analysis (MDA). The extracted strength distributions agree well with those from inelastic scattering of 100 MeV/u $α$ particles. These measurements establish deuteron inelastic scattering at E$_d \sim$ 100 MeV/u as a suitable probe for extraction of the ISGMR strength with MDA, making feasible the investigation of this resonance in radioactive isotopes in inverse kinematics.

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arXiv2013

Isoscalar Giant Resonance Strengths in $^{32}$S and possible excitations of superdeformed and $^{28}$Si + $α$ cluster bandheads

Isoscalar giant resonances and low spin states in $^{32}$S have been measured with inelastic $α$ scattering at extremely forward angles including zero degrees at E$_α$ = 386 MeV. By applying the multipole decomposition analysis, various excited states are classified according to their spin and parities (J$^π$), and are discussed in relation to the super deformed and $^{28}$Si + $α$ cluster bands.

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arXiv2013

Shallow and diffuse spin-orbit potential for proton elastic scattering from neutron-rich helium isotopes at 71 MeV/nucleon

Vector analyzing powers for proton elastic scattering from 8He at 71 MeV/nucleon have been measured using a solid polarized proton target operated in a low magnetic field of 0.1 T. The spin-orbit potential obtained from a phenomenological optical model analysis is found to be significantly shallower and more diffuse than the global systematics of stable nuclei, which is an indication that the spin-orbit potential is modified for scattering involving neutron-rich nuclei. A close similarity between the matter radius and the root-mean-square radius of the spin-orbit potential is also identified.

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arXiv2012

Alpha-resonance structure in $^{11}$C studied via resonant scattering of $^{7}$Be+$α$ and $^{7}$Be($α$, $p$) reaction

The resonance structure in $^{11}$C is particularly of interest with regard to the astrophysical $^{7}$Be($α$, $γ$) reaction, relevant at high temperature, and to the $α$-cluster structure in $^{11}$C. The measurement was to determine unknown resonance parameters for the high excited states of $^{11}$C. In particular, the $α$ decay width can be useful information to discuss $α$ cluster structure in $^{11}$C. New measurements of the $^{7}$Be+$α$ resonant scattering and the $^{7}$Be($α$, $p$)$^{10}$B reaction in inverse kinematics were performed for center-of-mass energy up to 5.5 MeV, and the resonances at excitation energies of 8.9--12.7 MeV in the compound $^{11}$C nucleus were studied. Inelastic scattering of $^{7}$Be+$α$ and the $^{7}$Be($α$, $p_1$)$^{10}$B$^*$ reaction were also studied with a simultaneous $γ$-ray measurement. The measurements were performed at the low-energy RI beam facility CRIB (CNS Radioactive Ion Beam separator) of the Center for Nuclear Study (CNS), the University of Tokyo. We obtained excitation functions of $^{7}$Be($α$, $α_0$)$^{7}$Be (elastic scattering), $^{7}$Be($α$, $α_1$)$^{7}$Be$^*$ (inelastic scattering), $^{7}$Be($α$, $p_0$)$^{10}$B, and $^{7}$Be($α$, $p_1$)$^{10}$B$^*$. Many resonances including a new one were observed and their parameters were determined by an R-matrix analysis. The resonances we observed possibly enhance the $^{7}$Be($α$, $γ$) reaction rate but in a smaller magnitude than the lower-lying resonances. A new negative-parity cluster band, similar to the one previously suggested in the mirror nucleus $^{11}$B, is proposed.

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arXiv2012

Giant Monopole Resonance in even-A Cd isotopes, the asymmetry term in nuclear incompressibility, and the "softness" of Sn and Cd nuclei

The isoscalar giant monopole resonance (ISGMR) in even-A Cd isotopes has been studied by inelastic $α$-scattering at 100 MeV/u and at extremely forward angles, including 0deg. The asymmetry term in the nuclear incompressibility extracted from the ISGMR in Cd isotopes is found to be $K_τ = -555 \pm 75$ MeV, confirming the value previously obtained from the Sn isotopes. ISGMR strength has been computed in relativistic RPA using NL3 and FSUGold effective interactions. Both models significantly overestimate the centroids of the ISGMR strength in the Cd isotopes. Combined with other recent theoretical effort, the question of the "softness" of the open-shell nuclei in the tin region remains open still.

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