arXiv subjects

N. Tanaka

Publications and source records attributed to N. Tanaka.

8 recordsLinked to original sources

arXiv2023

Bulk superconductivity in Pb-substituted BiS$_{\bf 2}$-based compounds studied by hard-x-ray spectroscopy

In this study, we investigate the bulk electronic structure of Pb-substituted LaO$_{0.5}$F$_{0.5}$BiS$_2$ single crystals, using two types of hard-x-ray spectroscopy. High-energy-resolution fluorescence-detected x-ray absorption spectroscopy revealed a spectral change at low temperatures. Using density functional theory (DFT) simulations, we find that the temperature-induced change originates from a structural phase transition, similar to the pressure-induced transition in LaO$_{0.5}$F$_{0.5}$BiS$_2$. This finding suggests that the mechanism of bulk superconductivity induced by Pb substitution is the same as that under high pressure. Furthermore, a novel low-valence state with a mixture of divalent and trivalent Bi ions is discovered using hard x-ray photoemission spectroscopy with the aid of DFT calculations.

cond-mat.supr-con↗

arXiv2017

X-ray Studies of the Extended TeV Gamma-Ray Source VER J2019+368

This article reports the results of X-ray studies of the extended TeV $γ$-ray source VER J2019+368. Suzaku observations conducted to examine properties of the X-ray pulsar wind nebula (PWN) around PSR J2021+3651 revealed that the western region of the X-ray PWN has a source extent of $15' \times 10'$ with the major axis oriented to that of the TeV emission. The PWN-west spectrum was closely fitted by a power-law for absorption at $N({\rm H}) = (8.2^{+1.3}_{-1.1}) \times 10^{21}~{\rm cm^{-2}}$ and a photon index of $Γ= 2.05\pm0.12$, with no obvious change in the index within the X-ray PWN. The measured X-ray absorption indicates that the distance to the source is much less than $10~{\rm kpc}$ inferred by radio data. Aside from the PWN, no extended emission was observed around PSR J2021+3651 even by Suzaku. Archival data from the XMM-Newton were also analyzed to complement the Suzaku observations, indicating that the eastern region of the X-ray PWN has a similar spectrum ($N(\rm H)=(7.5 \pm 0.9) \times 10^{21}~{\rm cm^{-2}}$ and $Γ=2.03 \pm 0.10$) and source extent up to at least $12'$ along the major axis. The lack of significant change in the photon index and the source extent in X-ray are used to constrain the advection velocity or the diffusion coefficient for accelerated X-ray-producing electrons. A mean magnetic field of ${\sim}3~μ{\rm G}$ is required to account for the measured X-ray spectrum and reported TeV $γ$-ray spectrum. A model calculation of synchrotron radiation and inverse Compton scattering was able to explain ${\sim}80\%$ of the reported TeV flux, indicating that the X-ray PWN is a major contributor of VER J2019+368.

astro-ph.HE↗

arXiv2014

Neutron occupancy of the 0d5/2 orbital and the N=16 shell closure in 24O

One-neutron knockout from 24O leading to the first excited state in 23O has been measured for a proton target at a beam energy of 62 MeV/nucleon. The decay energy spectrum of the neutron unbound state of 23O was reconstructed from the measured four momenta of the 22O fragment and emitted neutron. A sharp peak was found at Edecay=50$\pm$3 keV, corresponding to an excited state in 23O at 2.78$\pm$0.11 MeV, as observed in previous measurements. The longitudinal momentum distribution for this state was consistent with d -wave neutron knockout, providing support for a Jπ assignment of 5/2+. The associated spectroscopic factor was deduced to be C2S(0d5/2)=4.1$\pm$0.4 by comparing the measured cross section (View the MathML source) with a distorted wave impulse approximation calculation. Such a large occupancy for the neutron 0d5/2 orbital is in line with the N=16 shell closure in 24O.

nucl-ex↗

arXiv2012

Observation of new microsecond isomers among fission products of 345 MeV/nucleon 238U

A search for isomeric gamma-decays among fission fragments from 345 MeV/nucleon 238U has been performed at the RIKEN Nishina Center RI Beam Factory. Fission fragments were selected and identified using the superconducting in-flight separator BigRIPS and were implanted in an aluminum stopper. Delayed gamma-rays were detected using three clover-type high-purity germanium detectors located at the focal plane within a time window of 20 microseconds following the implantation. We identified a total of 54 microsecond isomers with half-lives of ~0.1 - 10 microseconds, including discovery of 18 new isomers in very neutron-rich nuclei: 59Tim, 90Asm, 92Sem, 93Sem, 94Brm, 95Brm, 96Brm, 97Rbm, 108Nbm, 109Mom, 117Rum, 119Rum, 120Rhm, 122Rhm, 121Pdm, 124Pdm, 124Agm and 126Agm, and obtained a wealth of spectroscopic information such as half-lives, gamma-ray energies, gamma-ray relative intensities and gamma-gamma coincidences over a wide range of neutron-rich exotic nuclei. Proposed level schemes are presented for 59Tim, 82Gam, 92Brm, 94Brm, 95Brm, 97Rbm, 98Rbm, 108Nbm, 108Zrm, 109Mom, 117Rum, 119Rum, 120Rhm, 122Rhm, 121Pdm, 124Agm and 125Agm, based on the obtained spectroscopic information and the systematics in neighboring nuclei. Nature of the nuclear isomerism is discussed in relation to evolution of nuclear structure.

nucl-ex↗

arXiv2012

The N = 16 spherical shell closure in 24O

The unbound excited states of the neutron drip-line isotope 24O have been investigated via the 24O(p,p')23O+n reaction in inverse kinematics at a beam energy of 62 MeV/nucleon. The decay energy spectrum of 24O* was reconstructed from the momenta of 23O and the neutron. The spin-parity of the first excited state, observed at Ex = 4.65 +/- 0.14 MeV, was determined to be Jpi = 2+ from the angular distribution of the cross section. Higher lying states were also observed. The quadrupole transition parameter beta2 of the 2+ state was deduced, for the first time, to be 0.15 +/- 0.04. The relatively high excitation energy and small beta2 value are indicative of the N = 16 shell closure in 24O.

nucl-ex↗

arXiv2011

One- and two-neutron removal reactions from the most neutron-rich carbon isotopes

The structure of $^{19,20,22}$C has been investigated using high-energy (about 240 MeV/nucleon) one- and two-neutron removal reactions on a carbon target. Measurements were made of the inclusive cross sections and momentum distributions for the charged residues. Narrow momentum distributions were observed for one-neutron removal from $^{19}$C and $^{20}$C and two-neutron removal from $^{22}$C. Two-neutron removal from $^{20}$C resulted in a relatively broad momentum distribution. The results are compared with eikonal-model calculations combined with shell-model structure information. The neutron-removal cross sections and associated momentum distributions are calculated for transitions to both the particle-bound and particle-unbound final states. The calculations take into account the population of the mass $A-1$ reaction residues, $^{A-1}$C, and, following one-neutron emission after one-neutron removal, the mass $A-2$ two-neutron removal residues, $^{A-2}$C. The smaller contributions of direct two-neutron removal, that populate the $^{A-2}$C residues in a single step, are also computed. The data and calculations are shown to be in good overall agreement and consistent with the predicted shell-model ground state configurations and the one-neutron overlaps with low-lying states in $^{18-21}$C. These suggest significant $ν{s}_{1/2}^2$ valence neutron configurations in both $^{20}$C and $^{22}$C. The results for $^{22}$C strongly support the picture of $^{22}$C as a two-neutron halo nucleus with a dominant $ν{s}_{1/2}^2$ ground state configuration.

nucl-ex↗

arXiv2005

ASCA and XMM-Newton Observations of A2029

The X-ray data of A2029 obtained with XMM-Newton show no evidence of an embedded AGN in the central region of this cluster, which was suggested from the analysis of restored ASCA image data, although some hot spots are seen within or around the central cD galaxy. The absence of AGN at the cluster center is consistentent with the result of Chandra observations. Radial profiles of the iron abundance and the 2D (surface) temperature obtained from the XMM-Newton data are in good agreement with the Chandra data as a whole.

astro-ph↗

arXiv1997

Exploration of resonances by analytic continuation in the coupling constant

The energy and the width of resonance states are determined by analytic continuation of bound-state energies as a function of the coupling constant (potential strength). The advantage of the method is that the existing techniques for calculation of bound states can be applied, without any modifications, to determine the position of resonances. Various numerical examples show the applicability of the method for three-body systems, including the excited states of the $^6$He and $^6$Li.

nucl-th↗