1,453 research outputs found
Self-organized criticality in a rice-pile model
We present a new model for relaxations in piles of granular material. The
relaxations are determined by a stochastic rule which models the effect of
friction between the grains. We find power-law distributions for avalanche
sizes and lifetimes characterized by the exponents and
, respectively. For the discharge events, we find a
characteristic size that scales with the system size as , with . We also find that the frequency of the discharge events
decrease with the system size as with .Comment: 4 pages, RevTex, multicol, epsf, rotate (sty files provided). To
appear Phys. Rev. E Rapid Communication (Nov or Dec 96
Universality classes for rice-pile models
We investigate sandpile models where the updating of unstable columns is done
according to a stochastic rule. We examine the effect of introducing nonlocal
relaxation mechanisms. We find that the models self-organize into critical
states that belong to three different universality classes. The models with
local relaxation rules belong to a known universality class that is
characterized by an avalanche exponent , whereas the models
with nonlocal relaxation rules belong to new universality classes characterized
by exponents and . We discuss the values
of the exponents in terms of scaling relations and a mapping of the sandpile
models to interface models.Comment: 4 pages, including 3 figure
Does field application of manure from sulfadiazine treated pigs affect the abundance and transferability of resistance genes?
Measurement of the Total Active 8B Solar Neutrino Flux at the Sudbury Neutrino Observatory with Enhanced Neutral Current Sensitivity
The Sudbury Neutrino Observatory (SNO) has precisely determined the total
active (nu_x) 8B solar neutrino flux without assumptions about the energy
dependence of the nu_e survival probability. The measurements were made with
dissolved NaCl in the heavy water to enhance the sensitivity and signature for
neutral-current interactions. The flux is found to be 5.21 +/- 0.27 (stat) +/-
0.38 (syst) x10^6 cm^{-2}s^{-1}, in agreement with previous measurements and
standard solar models. A global analysis of these and other solar and reactor
neutrino results yields Delta m^{2} = 7.1^{+1.2}_{-0.6}x10^{-5} ev^2 and theta
= 32.5^{+2.4}_{-2.3} degrees. Maximal mixing is rejected at the equivalent of
5.4 standard deviations.Comment: Submitted to Phys. Rev. Let
Electron Antineutrino Search at the Sudbury Neutrino Observatory
Upper limits on the \nuebar flux at the Sudbury Neutrino Observatory have
been set based on the \nuebar charged-current reaction on deuterium. The
reaction produces a positron and two neutrons in coincidence. This distinctive
signature allows a search with very low background for \nuebar's from the Sun
and other potential sources. Both differential and integral limits on the
\nuebar flux have been placed in the energy range from 4 -- 14.8 MeV. For an
energy-independent \nu_e --> \nuebar conversion mechanism, the integral limit
on the flux of solar \nuebar's in the energy range from 4 -- 14.8 MeV is found
to be \Phi_\nuebar <= 3.4 x 10^4 cm^{-2} s^{-1} (90% C.L.), which corresponds
to 0.81% of the standard solar model 8B \nu_e flux of 5.05 x 10^6 cm^{-2}
s^{-1}, and is consistent with the more sensitive limit from KamLAND in the 8.3
-- 14.8 MeV range of 3.7 x 10^2 cm^{-2} s^{-1} (90% C.L.). In the energy range
from 4 -- 8 MeV, a search for \nuebar's is conducted using coincidences in
which only the two neutrons are detected. Assuming a \nuebar spectrum for the
neutron induced fission of naturally occurring elements, a flux limit of
Phi_\nuebar <= 2.0 x 10^6 cm^{-2} s^{-1}(90% C.L.) is obtained.Comment: submitted to Phys. Rev.
Temporal variability in shell mound formation at Albatross Bay, northern Australia
We report the results of 212 radiocarbon determinations from the archaeological excavation of 70 shell mound deposits in the Wathayn region of Albatross Bay, Australia. This is an intensive study of a closely co-located group of mounds within a geographically restricted area in a wider region where many more shell mounds have been reported. Valves from the bivalve Tegillarcca granosa were dated. The dates obtained are used to calculate rates of accumulation for the shell mound deposits. These demonstrate highly variable rates of accumulation both within and between mounds. We assess these results in relation to likely mechanisms of shell deposition and show that rates of deposition are affected by time-dependent processes both during the accumulation of shell deposits and during their subsequent deformation. This complicates the interpretation of the rates at which shell mound deposits appear to have accumulated. At Wathayn, there is little temporal or spatial consistency in the rates at which mounds accumulated. Comparisons between the Wathayn results and those obtained from shell deposits elsewhere, both in the wider Albatross Bay region and worldwide, suggest the need for caution when deriving behavioural inferences from shell mound deposition rates, and the need for more comprehensive sampling of individual mounds and groups of mounds
The role of highly oxygenated molecules (HOMs) in determining the composition of ambient ions in the boreal forest
In order to investigate the negative ions in the boreal forest we have performed measurements to chemically characterise the composition of negatively charged clusters containing highly oxygenated molecules (HOMs). Additionally, we compared this information with the chemical composition of the neutral gas-phase molecules detected in the ambient atmosphere during the same period. The chemical composition of the ions was retrieved using an atmospheric pressure interface time-of-flight mass spectrometer (APiTOF-MS) while the gas-phase neutral molecules (mainly sulfuric acid and HOMs) were characterised using the same mass spectrometer coupled to a nitrate-based chemical ionisation unit (CI-APi-TOF). Overall, we divided the identified HOMs in two classes: HOMs containing only carbon, hydrogen and oxygen and nitrogen-containing HOMs or organonitrates (ONs). During the day, among the ions, in addition to the well-known pure sulfuric acid clusters, we found a large number of HOMs clustered with nitrate (NO3-) or bisulfate (HSO4-), with the first one being more abundant. During the night, the distribution of ions, mainly composed of HOM clustered with NO3-, was very similar to the neutral compounds that are detected in the CI-APi-TOF as adducts with the artificially introduced primary ion (NO3-). For the first time, we identified several clusters containing up to 40 carbon atoms. These ions are formed by up to four oxidised alpha-pinene units clustered with NO3-. While we know that dimers (16-20 carbon atoms) are probably formed by a covalent bond between two alpha-pinene oxidised units, it is still unclear what bonding formed larger clusters. Finally, diurnal profiles of the negative ions were consistent with the neutral compounds revealing that ONs peak during the day while HOMs are more abundant at night-time. However, during the day, a large fraction of the negative charge is taken up by the pure sulfuric acid clusters causing differences between ambient ions and neutral compounds (i.e. less available charge for HOM and ON).Peer reviewe
Comment on "On the subtleties of searching for dark matter with liquid xenon detectors"
In a recent manuscript (arXiv:1208.5046) Peter Sorensen claims that
XENON100's upper limits on spin-independent WIMP-nucleon cross sections for
WIMP masses below 10 GeV "may be understated by one order of magnitude or
more". Having performed a similar, though more detailed analysis prior to the
submission of our new result (arXiv:1207.5988), we do not confirm these
findings. We point out the rationale for not considering the described effect
in our final analysis and list several potential problems with his study.Comment: 3 pages, no figure
Removing krypton from xenon by cryogenic distillation to the ppq level
The XENON1T experiment aims for the direct detection of dark matter in a
cryostat filled with 3.3 tons of liquid xenon. In order to achieve the desired
sensitivity, the background induced by radioactive decays inside the detector
has to be sufficiently low. One major contributor is the -emitter
Kr which is an intrinsic contamination of the xenon. For the XENON1T
experiment a concentration of natural krypton in xenon Kr/Xe < 200
ppq (parts per quadrillion, 1 ppq = 10 mol/mol) is required. In this
work, the design of a novel cryogenic distillation column using the common
McCabe-Thiele approach is described. The system demonstrated a krypton
reduction factor of 6.410 with thermodynamic stability at process
speeds above 3 kg/h. The resulting concentration of Kr/Xe < 26 ppq
is the lowest ever achieved, almost one order of magnitude below the
requirements for XENON1T and even sufficient for future dark matter experiments
using liquid xenon, such as XENONnT and DARWIN
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