262 research outputs found
Natural Nuclear Reactor Oklo and Variation of Fundamental Constants Part 1: Computation of Neutronics of Fresh Core
Using modern methods of reactor physics we have performed full-scale
calculations of the natural reactor Oklo. For reliability we have used recent
version of two Monte Carlo codes: Russian code MCU REA and world wide known
code MCNP (USA). Both codes produce similar results. We have constructed a
computer model of the reactor Oklo zone RZ2 which takes into account all
details of design and composition. The calculations were performed for three
fresh cores with different uranium contents. Multiplication factors,
reactivities and neutron fluxes were calculated. We have estimated also the
temperature and void effects for the fresh core. As would be expected, we have
found for the fresh core a significant difference between reactor and Maxwell
spectra, which was used before for averaging cross sections in the Oklo
reactor. The averaged cross section of Sm-149 and its dependence on the shift
of resonance position (due to variation of fundamental constants) are
significantly different from previous results.
Contrary to results of some previous papers we find no evidence for the
change of the fine structure constant in the past and obtain new, most accurate
limits on its variation with time:
-4 10^{-17}year^{-1} < d alpha/dt/alpha < 3 10^{-17} year^{-1}
A further improvement in the accuracy of the limits can be achieved by taking
account of the core burnup. These calculations are in progress.Comment: 25 pages, 14 figures, 12 tables, minor corrections, typos correcte
Relocation of Advanced Water Vapor Radiometer 1 to Deep Space Station 55
In June of 2004, the Advanced Water Vapor Radiometer (AWVR) unit no. 1 was relocated to the Deep Space Station (DSS) 55 site in Madrid, Spain, from DSS 25 in Goldstone, California. This article summarizes the relocation activity and the subsequent operation and data acquisition. This activity also relocated the associated Microwave Temperature Profiler (MTP) and Surface Meteorology (SurfMET) package that collectively comprise the Cassini Media Calibration System (MCS)
The Celestial Reference Frame at 24 and 43 GHz. II. Imaging
We have measured the sub-milli-arcsecond structure of 274 extragalactic
sources at 24 and 43 GHz in order to assess their astrometric suitability for
use in a high frequency celestial reference frame (CRF). Ten sessions of
observations with the Very Long Baseline Array have been conducted over the
course of 5 years, with a total of 1339 images produced for the 274
sources. There are several quantities that can be used to characterize the
impact of intrinsic source structure on astrometric observations including the
source flux density, the flux density variability, the source structure index,
the source compactness, and the compactness variability. A detailed analysis of
these imaging quantities shows that (1) our selection of compact sources from
8.4 GHz catalogs yielded sources with flux densities, averaged over the
sessions in which each source was observed, of about 1 Jy at both 24 and 43
GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25%
relative to their mean values, with variations in the session-to-session flux
density scale being less than 10%, (3) sources were found to be more compact
with less intrinsic structure at higher frequencies, and (4) variations of the
core radio emission relative to the total flux density of the source are less
than 8% on average at 24 GHz. We conclude that the reduction in the effects due
to source structure gained by observing at higher frequencies will result in an
improved CRF and a pool of high-quality fiducial reference points for use in
spacecraft navigation over the next decade.Comment: 63 pages, 18 figures, 6 tables, accepted by the Astronomical Journa
Experimental Limit on the Cosmic Diffuse Ultra-high Energy Neutrino Flux
We report results from 120 hours of livetime with the Goldstone Lunar
Ultra-high energy neutrino Experiment (GLUE). The experiment searches for <10
ns microwave pulses from the lunar regolith, appearing in coincidence at two
large radio telescopes separated by 22 km and linked by optical fiber. Such
pulses would arise from subsurface electromagnetic cascades induced by
interactions of >= 100 EeV neutrinos in the lunar regolith. No candidates are
yet seen, and the implied limits constrain several current models for
ultra-high energy neutrino fluxes.Comment: 4 pages, 4 figures, revtex4 style. New intro section, Fig. 2, Fig 4;
in final PRL revie
Accelerator measurements of magnetically-induced radio emission from particle cascades with applications to cosmic-ray air showers
For fifty years, cosmic-ray air showers have been detected by their radio
emission. We present the first laboratory measurements that validate
electrodynamics simulations used in air shower modeling. An experiment at SLAC
provides a beam test of radio-frequency (RF) radiation from charged particle
cascades in the presence of a magnetic field, a model system of a cosmic-ray
air shower. This experiment provides a suite of controlled laboratory
measurements to compare to particle-level simulations of RF emission, which are
relied upon in ultra-high-energy cosmic-ray air shower detection. We compare
simulations to data for intensity, linearity with magnetic field, angular
distribution, polarization, and spectral content. In particular, we confirm
modern predictions that the magnetically induced emission in a dielectric forms
a cone that peaks at the Cherenkov angle and show that the simulations
reproduce the data within systematic uncertainties.Comment: 5 pages, 7 figure
Dielectron Cross Section Measurements in Nucleus-Nucleus Reactions at 1.0 A GeV
We present measured dielectron production cross sections for Ca+Ca, C+C,
He+Ca, and d+Ca reactions at 1.0 A GeV. Statistical uncertainties and
systematic effects are smaller than in previous DLS nucleus-nucleus data. For
pair mass < 0.35 GeV/c2 : 1) the Ca+Ca cross section is larger than the
previous DLS measurement and current model results, 2) the mass spectra suggest
large contributions from pi0 and eta Dalitz decays, and 3) dsigma/dM is
proportional to ApAt. For M > 0.5 GeV/c2 the Ca+Ca to C+C cross section ratio
is significantly larger than the ratio of ApAt values.Comment: Submitted to Physical Review Letters. Further analysis information
will be posted on our web pages -- http://macdls.lbl.gov Figure 1 has been
redrawn to make more legible. Text modified to support redrawn figur
Observations of the Askaryan Effect in Ice
We report on the first observations of the Askaryan effect in ice: coherent impulsive radio Cherenkov radiation from the charge asymmetry in an electromagnetic (EM) shower. Such radiation has been observed in silica sand and rock salt, but this is the first direct observation from an EM shower in ice. These measurements are important since the majority of experiments to date that rely on the effect for ultra-high energy neutrino detection are being performed using ice as the target medium. As part of the complete validation process for the Antarctic Impulsive Transient Antenna (ANITA) experiment, we performed an experiment at the Stanford Linear Accelerator Center (SLAC) in June 2006 using a 7.5 metric ton ice target, yielding results fully consistent with theoretical expectations
Probing the Rho Spectral Function in Hot and Dense Nuclear Matter by Dileptons
We present a dynamical study of and production in
proton-nucleus and nucleus-nucleus collisions at CERN-SPS energies on the basis
of the covariant transport approach HSD employing a momentum-dependent
-meson spectral function that includes the pion modifications in the
nuclear medium as well as the polarization of the -meson due to resonant
scattering. We find that the experimental data from the CERES and
HELIOS-3 Collaborations can be described equally well as within the dropping
-mass scenario. Whereas corresponding dilepton -spectra are found to
be very similar, the inclusive dilepton yield in the invariant mass range GeV should allow to disentangle the two scenarios
experimentally.Comment: 13 pages RevTeX slightly revised, 6 eps-figure
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