20,613 research outputs found
Comment on "138La-138Ce-136Ce nuclear cosmochronometer of the supernova neutrino process"
The nuclear chosmochronometer suggested by Hayakawa et al. [Phys. Rev.C 77,
065802 (2008)] based on the 138La-138Ce-136Ce abundance ratio in presolar
grains would be affected by the existence of a hitherto unknown low-energy 1+
state in 138La. Results of a recent high-resolution study of the 138Ba(3He,t)
reaction under kinematics selectively populating 1+ states in 138La through
Gamow-Teller transitions provides strong evidence against the existence of such
a hypothetical state.Comment: Comment on Phys. Rev. C 77, 065802 (2008), submittted to Phys. Rev.
Electric dipole response of 208Pb from proton inelastic scattering: constraints on neutron skin thickness and symmetry energy
The electric dipole (E1) response of 208Pb has been precisely determined by
measuring Coulomb excitation induced by proton scattering at very forward
angles. The electric dipole polarizability, defined as inverse energy-weighted
sum rule of the E1 strength, has been extracted as 20.1+-0.6 fm^3. The data can
be used to constrain the neutron skin thickness of 208Pb to
0.168(+-0.009)_expt(+-0.013)_theo(+-0.021)_est fm, where the subscript "expt"
refers to the experimental uncertainty, "theor" to the theoretical confidence
band and "est" to the uncertainty associated with the estimation of the
symmetry energy at the saturation density. In addition, a constraint band has
been extracted in the plane of the symmetry energy (J) and its slope parameter
(L) at the saturation density.Comment: 6 pages, 8 figures, revised manuscript submitted to special volume of
Eur. Phys. J. A on symmetry energ
Properties of the first excited state of 9Be derived from (gamma,n) and (e,e') reactions
Properties of the first excited state of the nucleus 9Be are discussed based
on recent (e,e') and (gamma,n) experiments. The parameters of an R-matrix
analysis of different data sets are consistent with a resonance rather than a
virtual state predicted by some model calculations. The energy and the width of
the resonance are deduced. Their values are rather similar for all data sets,
and the energy proves to be negative. It is argued that the disagreement
between the extracted B(E1) values may stem from different ways of integration
of the resonance. If corrected, fair agreement between the (e,e') and one of
the (gamma,n) data sets is found. A recent (gamma,n) experiment at the HIgS
facility exhibits larger cross sections close to the neutron threshold which
remain to be explained.Comment: 5 pages, accepted fro publication in Phys. Rev.
Multiple Scales in the Fine Structure of the Isoscalar Giant Quadrupole Resonance in ^{208}Pb
The fine structure of the isoscalar giant quadrupole resonance in ^{208}Pb,
observed in high-resolution (p,p') and (e,e') experiments, is studied using the
entropy index method. In a novel way, it enables to determine the number of
scales present in the spectra and their magnitude. We find intermediate scales
of fluctuations around 1.1 MeV, 460 keV and 125 keV for an excitation energy
region 0 - 12 MeV. A comparison with scales extracted from second RPA
calculations, which are in good agreement with experiment, shows that they
arise from the internal mixing of collective motion with two particle-two hole
components of the nuclear wavefunction.Comment: 14 pages including 6 figures (to be published in Phys. Lett. B
Solid-State Excitation Laser for Laser-Ultrasonics
The inspection speed of laser-ultrasonics compared with conventional ultrasonic testing is limited by the pulse repetition rate of the excitation laser. The maximum pulse repetition rate reported up to now for CO2-lasers, which are presently used for nearly all systems, is in the range of 400 Hz. In this paper a new approach based on a diode-pumped solid-state laser is discussed, which is currently being developed. This new excitation laser is designed for a repetition rate of 1 kHz and will operate at a mid-IR wavelength of 3.3 m. The higher repeti-tion rate enables a higher inspection speed, whereas the mid-IR wavelength anticipates a better coupling efficiency. The total power for pumping the laser crystals is transported via flexible optical fibres to the compact laser head, thus allowing operation on a robot arm. The laser head consists of a master oscillator feeding several lines of power amplifiers and in-cludes nonlinear optical wavelength conversion by an optical parametric process. It is char-acterized by a modular construction which provides optimal conditions for operation at high average power as well as for easy maintenance. These features will enable building reliable, long-lived, rugged, smart laser ultrasonic systems in futur
Resonance parameters of the first 1/2+ state in 9Be and astrophysical implications
Spectra of the 9Be(e,e') reaction have been measured at the S-DALINAC at an
electron energy E_0 = 73 MeV and scattering angles of 93{\deg} and 141{\deg}
with high energy resolution up to excitation energies E_x = 8 MeV. The
astrophysically relevant resonance parameters of the first excited 1/2+ state
of 9Be have been extracted in a one-level approximation of R-matrix theory
resulting in a resonance energy E_R = 1.748(6) MeV and width Gamma_R = 274(8)
keV in good agreement with the latest 9Be(gamma,n) experiment but with
considerably improved uncertainties. However, the reduced B(E1) transition
strength deduced from an extrapolation of the (e,e') data to the photon point
is a factor of two smaller. Implications of the new results for a possible
production of 12C in neutron-rich astrophysical scenarios are discussed.Comment: 8 pages, 7 figures, accepted for publication in Phys. Rev.
Reduction of Effective Terahertz Focal Spot Size By Means Of Nested Concentric Parabolic Reflectors
An ongoing limitation of terahertz spectroscopy is that the technique is
generally limited to the study of relatively large samples of order 4 mm across
due to the generally large size of the focal beam spot. We present a nested
concentric parabolic reflector design which can reduce the terahertz focal spot
size. This parabolic reflector design takes advantage of the feature that
reflected rays experience a relative time delay which is the same for all
paths. The increase in effective optical path for reflected light is equivalent
to the aperture diameter itself. We have shown that the light throughput of an
aperture of 2 mm can be increased by a factor 15 as compared to a regular
aperture of the same size at low frequencies. This technique can potentially be
used to reduce the focal spot size in terahertz spectroscopy and enable the
study of smaller samples
First-principles phase diagram calculations for the HfC–TiC, ZrC–TiC, and HfC–ZrC solid solutions
We report first-principles phase diagram calculations for the binary systems HfC–TiC, TiC–ZrC, and HfC–ZrC. Formation energies for superstructures of various bulk compositions were computed with a plane-wave pseudopotential method. They in turn were used as a basis for fitting cluster expansion Hamiltonians, both with and without approximations for excess vibrational free energies. Significant miscibility gaps are predicted for the systems TiC–ZrC and HfC–TiC, with consolute temperatures in excess of 2000 K. The HfC–ZrC system is predicted to be completely miscibile down to 185 K. Reductions in consolute temperature due to excess vibrational free energy are estimated to be ~7%, ~20%, and ~0%, for HfC–TiC, TiC–ZrC, and HfC–ZrC, respectively. Predicted miscibility gaps are symmetric for HfC–ZrC, almost symmetric for HfC–TiC and asymmetric for TiC–ZrC
Performance of a centrifugal pump running in inverse mode
This paper presents the functional characterization of a centrifugal pump used as a turbine. It shows the characteristics of the machine involved at several rotational speeds, comparing the respective flows and heads. In this way, it is possible to observe the influence of the rotational speed on efficiency, as well as obtaining the characteristics at constant head and runaway speed. Also, the forces actuating on the impeller were studied. An uncertainty analysis was made to assess the accuracy of the results. The research results indicate that the turbine characteristics can be predicted to some extent from the pump characteristics, that water flows out of the runner free of swirl flow at the best efficiency point, and that radial stresses are lower than in pump mode
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