arXiv subjects

Y. Fujita

Publications and source records attributed to Y. Fujita.

68 records · Page 4Linked to original sources

arXiv2008

Spectroscopy of 24Al and extraction of Gamow-Teller strengths with the 24Mg(3He,t) reaction at 420 MeV

The 24Mg(3He,t)24Al reaction has been studied at E(3He)=420 MeV. An energy resolution of 35 keV was achieved. Gamow-Teller strengths to discrete levels in 24Al are extracted by using a recently developed empirical relationship for the proportionality between Gamow-Teller strengths and differential cross sections at zero momentum transfer. Except for small discrepancies for a few weak excitations, good agreement with previous 24Mg(p,n) data and nuclear-structure calculations using the USDA/B interactions in the sd shell-model space is found. The excitation energy of several levels in 24Al of significance for determination of the 23Mg(p,gamma)24Al thermonuclear reaction rate were measured. Results are consistent with two of the three previous (3He,t) measurements, performed at much lower beam energies. However, a new state at Ex(24Al)=2.605(10) MeV was found and is the third state above the proton separation energy.

nucl-ex↗

arXiv2008

On the extraction of weak transition strengths via the (3He,t) reaction at 420 MeV

Differential cross sections for transitions of known weak strength were measured with the (3He,t) reaction at 420 MeV on targets of 12C, 13C, 18O, 26Mg, 58Ni, 60Ni, 90Zr, 118Sn, 120Sn and 208Pb. Using this data, it is shown the proportionalities between strengths and cross sections for this probe follow simple trends as a function of mass number. These trends can be used to confidently determine Gamow-Teller strength distributions in nuclei for which the proportionality cannot be calibrated via beta-decay strengths. Although theoretical calculations in distorted-wave Born approximation overestimate the data, they allow one to understand the main experimental features and to predict deviations from the simple trends observed in some of the transitions.

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arXiv2006

$2α+t$ cluster structure in $^{11}$B

The cluster structures of the excited states in $^{11}$B are studied by analyzing the isoscalar monopole and quadrupole strengths in the $^{11}$B($d$,$d'$) reaction at $E_d=200$ MeV. The excitation strengths are compared with the predictions by the shell-model and antisymmetrized molecular-dynamics (AMD) calculations. It is found that the large monopole strength for the $3/2^-_3$ state at $E_x=8.56$ MeV is well described by the AMD calculation and is an evidence for a developed $2α+t$ cluster structure.

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arXiv2006

The (t,3He) and (3He,t) reactions as probes of Gamow-Teller strength

Charge-exchange reactions are an important tool for determining weak-interaction rates. They provide stringent tests for nuclear structure models necessary for modeling astrophysical environments such as neutron stars and core-collapse supernovae. In anticipation of (t,3He) experiments at 115 MeV/nucleon on nuclei of relevance (A~40-120) in the late evolution of stars, it is shown via a study of the 26Mg(t,3He) reaction that this probe is an accurate tool for extracting Gamow-Teller transition strengths. To do so, the data are complemented by results from the 26Mg(3He,t) reaction at 140 MeV/nucleon which allows for a comparison of T=2 analog states excited via the mirror reactions. Extracted Gamow-Teller strengths from 26Mg(t,3He) and 26Mg(3He,t) are compared with those from 26Mg(d,2He) and 26Mg(p,n) studies, respectively. A good correspondence is found, indicating probe-independence of the strength extraction. Furthermore, we test shell-model calculations using the new USD-05B interaction in the sd-model space and show that it reproduces the experimental Gamow-Teller strength distributions well. A second goal of this work is to improve the understanding of the (t,3He) and (3He,t) reaction mechanisms at intermediate energies since detailed studies are scarce. The Distorted-Wave Born Approximation is employed, taking into account the composite structures of the 3He and triton particles. The reaction model provides the means to explain systematic uncertainties at the 10-20% level in the extraction of Gamow-Teller strengths as being due to interference between Gamow-Teller dL=0, dS=1 and dL=2, dS=1 amplitudes that both contribute to transitions from 0+ to 1+ states.

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arXiv2006

Measurement of the Gamow-Teller Strength Distribution in 58Co via the 58Ni(t,3He) reaction at 115 MeV/nucleon

Electron capture and beta decay play important roles in the evolution of pre-supernovae stars and their eventual core collapse. These rates are normally predicted through shell-model calculations. Experimentally determined strength distributions from charge-exchange reactions are needed to test modern shell-model calculations. We report on the measurement of the Gamow-Teller strength distribution in 58Co from the 58Ni(t,3He) reaction with a secondary triton beam of an intensity of ~10^6 pps at 115 MeV/nucleon and a resolution of \~250 keV. Previous measurements with the 58Ni(n,p) and the 58Ni(d,2He) reactions were inconsistent with each other. Our results support the latter. We also compare the results to predictions of large-scale shell model calculations using the KB3G and GXPF1 interactions and investigate the impact of differences between the various experiments and theories in terms of the weak rates in the stellar environment. Finally, the systematic uncertainties in the normalization of the strength distribution extracted from 58Ni(3He,t) are described and turn out to be non-negligible due to large interferences between the dL=0, dS=1 Gamow-Teller amplitude and the dL=2, dS=1 amplitude.

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arXiv2004

High-resolution soft x-ray photoemission study of a Kondo semiconductor and related compounds

We have performed the bulk-sensitive high-resolution soft x-ray photoemission study of a Kondo semiconductor CeRhAs and related compounds CeNiSn and CePdSn. The comparison of the spectra of polycrystalline CePdSn on the fractured and scraped surfaces shows that the fracturing of the samples is much better than the scraping in order to obtain intrinsic photoemission spectra. The Ce 4d core-level spectra show clear differences in the electronic states among the materials.

cond-mat.str-el↗

arXiv2004

Isovector and isoscalar spin-flip M1 strengths in $^{11}$B

The $^{11}$B($^3$He$, t$), $^{11}$B($d, d'$), and $^{11}$B($p, p'$) reactions were measured at forward scattering angles including $0^\circ$ to study the isovector and isoscalar spin-flip M1 strengths in $^{11}$B. The measured $^{11}$B($^3$He$, t$) cross sections were compared with the results of the distorted-wave impulse-approximation (DWIA) calculation, and the Gamow-Teller (GT) strengths for low-lying states in $^{11}$C were determined. The GT strengths were converted to the isovector spin-flip M1 strengths using the isobaric analog relations under the assumption of the isospin symmetry. The isoscalar spin-flip M1 strengths were obtained from the ($d, d'$) analysis by assuming that the shape of the collective transition form factor with the same $ΔJ^π$ is similar in the $^{11}$B($d, d'$) and $^{12}$C($d, d'$) reactions. The obtained isovector and isoscalar strengths were used in the DWIA calculations for the $^{11}$B($p, p'$) reaction. The DWIA calculation reasonably well explains the present $^{11}$B($p, p'$) result. However, the calculated cross section for the 8.92-MeV 3/2$^-_2$ state was significantly smaller than the experimental values. The transition strengths obtained in the shell-model calculations were found to be 20-50% larger than the experimental strengths. The transition strengths for the neutrino induced reactions were estimated by using the isovector and isoscalar spin-flip M1 strengths. The present results are quantitatively in agreement with the theoretical estimation discussing the axial isoscalar coupling in the neutrino scattering process, and are useful in the measurement of the stellar neutrinos using the neutral- and charged-current reactions on $^{11}$B.

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arXiv2002

Polarization transfer in the $^{16}$O$(p,p')$ reaction at forward angles and structure of the spin-dipole resonances

Cross sections and polarization transfer observables in the $^{16}$O$(p,p')$ reactions at 392 MeV were measured at several angles between $θ_{lab}=$ 0$^\circ$ and 14$^\circ$. The non-spin-flip ($ΔS=0$) and spin-flip ($ΔS=1$) strengths in transitions to several discrete states and broad resonances in $^{16}$O were extracted using a model-independent method. The giant resonances in the energy region of $E_x=19-$27 MeV were found to be predominantly excited by $ΔL=1$ transitions. The strength distribution of spin-dipole transitions with $ΔS=1$ and $ΔL=1$ were deduced. The obtained distribution was compared with a recent shell model calculation. Experimental results are reasonably explained by distorted-wave impulse approximation calculations with the shell model wave functions.

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arXiv2000

Thin Ice Target for $^{16}$O(p,p') experiment

A windowless and self-supporting ice target is described. An ice sheet with a thickness of 29.7 mg/cm$^2$ cooled by liquid nitrogen was placed at the target position of a magnetic spectrometer and worked stably in the $^{16}$O$(p,p')$ experiment at $E_{p}=392$ MeV. Background-free spectra were obtained.

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arXiv2000

Structure of isobaric analog states in 91Nb populated by the 90Zr(a,t) reaction

Decay via proton emission of isobaric analog states (IAS's) in $^{91}{Nb}$ was studied using the $^{90}{Zr}(α,t)$ reaction at $E_α$=180 MeV. This study provides information about the damping mechanism of these states. Decay to the ground state and low-lying phonon states in $^{90}{Zr}$ was observed. The experimental data are compared with theoretical predictions wherein the IAS `single-particle' proton escape widths are calculated in a continuum RPA approach. The branching ratios for decay to the phonon states are explained using a simple model.

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arXiv2000

Gamow-Teller Strengths of the Inverse-Beta Transition 176Yb --> 176Lu for Spectroscopy of Proton-Proton and other sub-MeV Solar Neutrinos

Discrete Gamow-Teller (GT) transitions, 176Yb-->176Lu at low excitation energies have been measured via the (3He,t) reaction at 450 MeV and at 0 degrees. For 176Yb, two low-lying states are observed, setting low thresholds Q(neutrino)=301 and 445 keV for neutrino capture. Capture rates estimated from the measured GT strengths, the simple two-state excitation structure, and the low Q(neutrino) in Yb--Lu indicate that Yb-based neutrino-detectors are well suited for a direct measurement of the complete sub-MeV solar electron-neutrino spectrum (including pp neutrinos) where definitive effects of flavor conversion are expected.

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arXiv1997

Emission from Isolated Black Holes and MACHOs Accreting from the Interstellar Medium

Adopting the advection dominated accretion flow (ADAF) model, we have re-examined the radiation emitted by isolated stellar-mass black holes accreting from the interstellar medium of our Galaxy. Two distinct types of black holes are given consideration, 1) remnants of conventional massive star evolution (RBHs, mass $\sim 10 M_{\sun}$), and 2) low-mass objects which possibly constitutes the dark halo and cause the observed microlensing events (MACHOBHs, mass $\sim 0.5 M_{\sun}$). In contrast to previous studies incorporating spherical accretion models, the characteristic hard X-ray emission from ADAFs serves as an important observational signature. Standard estimates for the present density of RBHs suggest that a fair number of objects, either in nearby molecular clouds (such as Orion) or in the general Galactic disk, may be detected by near future X-ray instruments. Their observation may also be feasible at lower frequencies, particularly in the infrared and optical bands. On the other hand, the smaller mass and larger velocity on average of MACHOBHs compared to RBHs make them unlikely to be observable in the foreseeable future.

astro-ph↗

arXiv1996

Multiphase Chemical Evolution in Elliptical Galaxies

The recent {\em ASCA} results show that the iron abundance of the X-ray gas of elliptical galaxies is less than the solar abundance ($Z_{\sun}$). The observed low iron abundance is inconsistent with the predictions of the previous chemical evolution models. In order to solve this problem, we present a simple model of chemical evolution for elliptical galaxies after the galactic wind period under the assumption that the gases ejected from stars do not mix with the circumferential gas. The ejected gas components evolve separately according to their birth time and origin. We have investigated their evolution qualitatively. The gas components originated from supernova remnant shells cool and drop out of the hot gas faster than the other components because of their high density and metal abundance. As a result, supernovae cannot heat the whole gas of the elliptical galaxies effectively, in contrary to the previous results. If the metal abundance of mass-loss gas is not uniform, the mass-loss gas with higher abundance also easily drops out and the average abundance can decrease. We believe that this is a hint of solving the low abundance problem.

astro-ph↗

arXiv1996

The X-ray Structure of A399 and A401: A Pre-Merging Cluster Pair

We present {\em ASCA} results of the pair clusters A399 and A401. The region between the two clusters exhibits excess X-rays over the value expected with a simple superposition of the two clusters. We see, however, no temperature rise at the merging front; the temperature is near the average of those in the inner regions of the two clusters. These indicate that the two clusters are really interacting but it is not strong at present. The inner regions of the two clusters show no radial variations of temperature, abundance and absorption values. We set upper-limits of mass deposition rate of cooling flow to be $\dot{M}<35\;\rm M_{\odot}yr^{-1}$ and $\dot{M}<59\;\rm M_{\odot}yr^{-1}$ for A399 and A401, respectively. A hint of azimuthal variation of the temperature is also found.

astro-ph↗