199 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
Impact of solid waste disposal on nutrient dynamics in a sandy catchment
Groundwaters impacted by mature landfill leachate are generally enriched in ammonium. In order to assess the dynamics of ammonium exchanges between leachates and the water system inside a sandy permeable catchment we measured ammonium, nitrate and chloride concentrations in the stream and in sediment pore waters of the streambed of a landfill impacted aquifer. Geophysical investigation methods complemented the biogeochemical survey. The studied zone is a 23 kmÂČ catchment located in a coastal lagoon area sensitive to eutrophication risk. Ammonium concentrations in the river were up to 800 ”mol lâ1 during low water period in summer. Three surveys of the river chemistry showed a regular increase in ammonium, nitrate and chloride concentrations along a 1 km section of the watercourse, downstream the landfill, implying that the leachate plume exfiltrates along this section. Sediment cores collected within this section showed all an increase in ammonium concentrations with depth in pore waters as a consequence of the landfill leachate dispersion, as attested by a simultaneous increase in chloride concentrations. Nitrate enrichment in the river water was due to nitrification of ammonium at the interface between groundwater and streamwater. The apparent nitrification rate obtained was within values reported for turbid estuaries, although the river contained very little suspended particulate matter. Actually, pore water chemistry suggests that nitrification occurred for the most part in subsurface permeable sediments, rather than in stream water. The overall topographic, hydrological, geochemical, and geoelectrical data set permit to estimate the extension of the chloride and ammonium plume. The estimation of the apparent ammonium plume velocity is 23 m yearâ1 whereas the chloride plume velocity should be 50 m yearâ1. The river is the outlet of the impacted groundwaters. Considering that the input of ammonium from the landfill is balanced by the present day output via the river, the residence time of ammonium in the aquifer is between 7 and 18 years
Treatment of major depression
Stress and Psychopatholog
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
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
Review of Psychedelic drugs: more emphasis on safety issues
Stress and Psychopatholog
Status of ANITA and ANITA-lite
We describe a new experiment to search for neutrinos with energies above 3 x
10^18 eV based on the observation of short duration radio pulses that are
emitted from neutrino-initiated cascades. The primary objective of the
ANtarctic Impulse Transient Antenna (ANITA) mission is to measure the flux of
Greisen-Zatsepin-Kuzmin (GZK) neutrinos and search for neutrinos from Active
Galactic Nuclei (AGN). We present first results obtained from the successful
launch of a 2-antenna prototype instrument (called ANITA-lite) that circled
Antarctica for 18 days during the 03/04 Antarctic campaign and show preliminary
results from attenuation length studies of electromagnetic waves at radio
frequencies in Antarctic ice. The ANITA detector is funded by NASA, and the
first flight is scheduled for December 2006.Comment: 9 pages, 8 figures, to be published in Proceedings of International
School of Cosmic Ray Astrophysics, 14th Course: "Neutrinos and Explosive
Events in the Universe", Erice, Italy, 2-13 July 200
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
A 1-D modelling of streaming potential dependence on water content during drainage experiment in sand
The understanding of electrokinetics for unsaturated conditions is crucial
for numerous of geophysical data interpretation. Nevertheless, the behaviour of
the streaming potential coefficient C as a function of the water saturation Sw
is still discussed. We propose here to model both the Richards' equation for
hydrodynamics and the Poisson's equation for electrical potential for
unsaturated conditions using 1-D finite element method. The equations are first
presented and the numerical scheme is then detailed for the Poisson's equation.
Then, computed streaming potentials (SPs) are compared to recently published SP
measurements carried out during drainage experiment in a sand column. We show
that the apparent measurement of DV / DP for the dipoles can provide the SP
coefficient in these conditions. Two tests have been performed using existing
models for the SP coefficient and a third one using a new relation. The results
show that existing models of unsaturated SP coefficients C(Sw) provide poor
results in terms of SP magnitude and behaviour. We demonstrate that the
unsaturated SP coefficient can be until one order of magnitude larger than
Csat, its value at saturation. We finally prove that the SP coefficient follows
a non-monotonous behaviour with respect to water saturation. Key words:
Electrical properties; Electromagnetic theory; Hydrogeophysics; Hydrology;
Permeability and porosity; electrokinetic; streaming potential; self-potential;
water content; water saturation; unsaturated condition; finite element modelin
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