1,523 research outputs found
Cosmic-ray propagation properties for an origin in SNRs
We have studied the impact of cosmic-ray acceleration in SNR on the spectra
of cosmic-ray nuclei in the Galaxy using a series expansion of the propagation
equation, which allows us to use analytical solutions for part of the problem
and an efficient numerical treatment of the remaining equations and thus
accurately describes the cosmic-ray propagation on small scales around their
sources in three spatial dimensions and time. We found strong variations of the
cosmic-ray nuclei flux by typically 20% with occasional spikes of much higher
amplitude, but only minor changes in the spectral distribution. The locally
measured spectra of primary cosmic rays fit well into the obtained range of
possible spectra. We further showed that the spectra of the secondary element
Boron show almost no variations, so that the above findings also imply
significant fluctuations of the Boron-to-Carbon ratio. Therefore the commonly
used method of determining CR propagation parameters by fitting
secondary-to-primary ratios appears flawed on account of the variations that
these ratios would show throughout the Galaxy.Comment: Accepted for publication in Ap
The Equation of State and the Hugoniot of Laser Shock-Compressed Deuterium
The equation of state and the shock Hugoniot of deuterium are calculated
using a first-principles approach, for the conditions of the recent shock
experiments. We use density functional theory within a classical mapping of the
quantum fluids [ Phys. Rev. Letters, {\bf 84}, 959 (2000) ]. The calculated
Hugoniot is close to the Path-Integral Monte Carlo (PIMC) result. We also
consider the {\it quasi-equilibrium} two-temperature case where the Deuterons
are hotter than the electrons; the resulting quasi-equilibrium Hugoniot mimics
the laser-shock data. The increased compressibility arises from hot
pairs occuring close to the zero of the electron chemical potential.Comment: Four pages; One Revtex manuscript, two postscipt figures; submitted
to PR
Correlation between the Proton Conductivity and Diffusion Coefficient of Sulfonic Acid Functionalized Chitosan and Nafion Composites via Impedance Spectroscopy Measurements
On the local birth place of Geminga
Using estimates of the distance and proper motion of Geminga and the
constraints on its radial velocity posed by the shape of its bow shock, we
investigate its birth place by tracing its space motion backwards in time. Our
results exclude the lambda Ori association as the origin site because of the
large distance between both objects at any time. Our simulations place the
birth region at approximately 90-240 pc from the Sun, between 197 degrees and
199 degrees in Galactic longitude and -16 degrees and -8 degrees in latitude,
most probably inside the Cas-Tau OB association or the Ori OB1a association. We
discard the possibility of the progenitor being a massive field star. The
association of Geminga with either stellar association implies an upper limit
of M = 15 Msun for the mass of its progenitor. We also propose new members for
the Cas-Tau and Ori OB1 associations.Comment: 6 pages, 5 figures. Accepted for publication in Astronomy &
Astrophysic
The 2-D electron gas at arbitrary spin polarizations and arbitrary coupling strengths: Exchange-correlation energies, distribution functions and spin-polarized phases
We use a recent approach [Phys. Rev. Letters, {\bf 84}, 959 (2000)] for
including Coulomb interactions in quantum systems via a classical mapping of
the pair-distribution functions (PDFs) for a study of the 2-D electron gas. As
in the 3-D case, the ``quantum temperature'' T_q of a classical 2-D Coulomb
fluid which has the same correlation energy as the quantum fluid is determined
as a function of the density parameter r_s. Spin-dependent exchange-correlation
energies are reported. Comparisons of the spin-dependent pair-distributions and
other calculated properties with any available 2-D quantum Monte Carlo (QMC)
results show excellent agreement, strongly favouring more recent QMC data. The
interesting novel physics brought to light by this study are: (a) the
independently determined quantum-temperatures for 3-D and 2-D are found to be
approximately the same, (i.e, universal) function of the classical coupling
constant Gamma. (b) the coupling constant Gamma increases rapidly with r_s in
2-D, making it comparatively more coupled than in 3-D; the stronger coupling in
2-D requires bridge corrections to the hyper- netted-chain method which is
adequate in 3-D; (c) the Helmholtz free energy of spin-polarized and
unpolarized phases have been calculated. The existence of a spin-polarized 2-D
liquid near r_s = 30, is found to be a marginal possibility. These results
pertain to clean uniform 2-D electron systems.Comment: This paper replaces the cond-mat/0109228 submision; the new version
include s more accurate numerical evaluation of the Helmholtz energies of the
para- and ferromagentic 2D fluides at finite temperatures. (Paper accepted
for publication in Phys. Rev. Lett.
Nucleon-nucleon elastic scattering analysis to 2.5 GeV
A partial-wave analysis of NN elastic scattering data has been completed.
This analysis covers an expanded energy range, from threshold to a laboratory
kinetic energy of 2.5 GeV, in order to include recent elastic pp scattering
data from the EDDA collaboration. The results of both single-energy and
energy-dependent analyses are described.Comment: 23 pages of text. Postscript files for the figures are available from
ftp://clsaid.phys.vt.edu/pub/said/n
First direct observation of two protons in the decay of Fe with a TPC
The decay of the ground-state two-proton emitter 45Fe was studied with a
time-projection chamber and the emission of two protons was unambiguously
identified. The total decay energy and the half-life measured in this work
agree with the results from previous experiments. The present result
constitutes the first direct observation of the individual protons in the
two-proton decay of a long-lived ground-state emitter. In parallel, we
identified for the first time directly two-proton emission from 43Cr, a known
beta-delayed two-proton emitter. The technique developped in the present work
opens the way to a detailed study of the mechanism of ground-state as well as
beta-delayed two-proton radioactivity.Comment: 4 pages, 5 figure
New pathway to bypass the 15O waiting point
We propose the sequential reaction process
O(,)O as a new pathway to bypass of the
O waiting point. This exotic reaction is found to have a surprisingly
high cross section, approximately 10 times higher than the
O(,)O. These cross sections were calculated after
precise measurements of energies and widths of the proton-unbound F low
lying states, obtained using the H(O,p)O reaction. The large
cross section can be understood to arise from the more
efficient feeding of the low energy wing of the ground state resonance by the
gamma decay. The implications of the new reaction in novae explosions and X-ray
bursts are discussed.Comment: submitte
Search for new resonant states in 10C and 11C and their impact on the cosmological lithium problem
The observed primordial 7Li abundance in metal-poor halo stars is found to be
lower than its Big-Bang nucleosynthesis (BBN) calculated value by a factor of
approximately three. Some recent works suggested the possibility that this
discrepancy originates from missing resonant reactions which would destroy the
7Be, parent of 7Li. The most promising candidate resonances which were found
include a possibly missed 1- or 2- narrow state around 15 MeV in the compound
nucleus 10C formed by 7Be+3He and a state close to 7.8 MeV in the compound
nucleus 11C formed by 7Be+4He. In this work, we studied the high excitation
energy region of 10C and the low excitation energy region in 11C via the
reactions 10B(3He,t)10C and 11B(3He,t)11C, respectively, at the incident energy
of 35 MeV. Our results for 10C do not support 7Be+3He as a possible solution
for the 7Li problem. Concerning 11C results, the data show no new resonances in
the excitation energy region of interest and this excludes 7Be+4He reaction
channel as an explanation for the 7Li deficit.Comment: Accepted for publication in Phys. Rev. C (Rapid Communication
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