349 research outputs found
Ozonolysis of surface-adsorbed methoxyphenols: kinetics of aromatic ring cleavage vs. alkene side-chain oxidation
Lignin pyrolysis products, which include a variety of substituted
methoxyphenols, constitute a major component of organics released by biomass
combustion, and may play a central role in the formation of atmospheric brown
carbon. Understanding the atmospheric fate of these compounds upon exposure
to trace gases is therefore critical to predicting the chemical and physical
properties of biomass burning aerosol. We used diffuse reflectance infrared
spectroscopy to monitor the heterogeneous ozonolysis of 4-propylguaiacol,
eugenol, and isoeugenol adsorbed on NaCl and α-Al<sub>2</sub>O<sub>3</sub> substrates.
Adsorption of gaseous methoxyphenols onto these substrates produced near-monolayer surface concentrations of
3 × 10<sup>18</sup> molecules m<sup>−2</sup>. The subsequent dark
heterogeneous ozonolysis of adsorbed 4-propylguaiacol cleaved the aromatic
ring between the methoxy and phenol groups with the product conclusively
identified by GC-MS and <sup>1</sup>H-NMR. Kinetic analysis of eugenol and
isoeugenol dark ozonolysis also suggested the formation of ring-cleaved
products, although ozonolysis of the unsaturated substituent groups forming
carboxylic acids and aldehydes was an order of magnitude faster. Average
uptake coefficients for NaCl-adsorbed methoxyphenols were
γ = 2.3 (± 0.8) × 10<sup>−7</sup> and
2 (± 1) × 10<sup>−6</sup> for ozonolysis of the aromatic ring and
the unsaturated side chain, respectively, and reactions on
α-Al<sub>2</sub>O<sub>3</sub> were approximately two times slower. UV–visible
radiation (λ > 300 nm) enhanced eugenol ozonolysis of the
aromatic ring by a factor of 4(± 1) but had no effect on ozonolysis of
the alkene side chain
Target Mass Monitoring and Instrumentation in the Daya Bay Antineutrino Detectors
The Daya Bay experiment measures sin^2 2{\theta}_13 using functionally
identical antineutrino detectors located at distances of 300 to 2000 meters
from the Daya Bay nuclear power complex. Each detector consists of three nested
fluid volumes surrounded by photomultiplier tubes. These volumes are coupled to
overflow tanks on top of the detector to allow for thermal expansion of the
liquid. Antineutrinos are detected through the inverse beta decay reaction on
the proton-rich scintillator target. A precise and continuous measurement of
the detector's central target mass is achieved by monitoring the the fluid
level in the overflow tanks with cameras and ultrasonic and capacitive sensors.
In addition, the monitoring system records detector temperature and levelness
at multiple positions. This monitoring information allows the precise
determination of the detectors' effective number of target protons during data
taking. We present the design, calibration, installation and in-situ tests of
the Daya Bay real-time antineutrino detector monitoring sensors and readout
electronics.Comment: 22 pages, 20 figures; accepted by JINST. Changes in v2: minor
revisions to incorporate editorial feedback from JINS
A side-by-side comparison of Daya Bay antineutrino detectors
The Daya Bay Reactor Neutrino Experiment is designed to determine precisely
the neutrino mixing angle with a sensitivity better than 0.01 in
the parameter sin at the 90% confidence level. To achieve this
goal, the collaboration will build eight functionally identical antineutrino
detectors. The first two detectors have been constructed, installed and
commissioned in Experimental Hall 1, with steady data-taking beginning
September 23, 2011. A comparison of the data collected over the subsequent
three months indicates that the detectors are functionally identical, and that
detector-related systematic uncertainties exceed requirements.Comment: 24 pages, 36 figure
Observation of electron-antineutrino disappearance at Daya Bay
The Daya Bay Reactor Neutrino Experiment has measured a non-zero value for
the neutrino mixing angle with a significance of 5.2 standard
deviations. Antineutrinos from six 2.9 GW reactors were detected in
six antineutrino detectors deployed in two near (flux-weighted baseline 470 m
and 576 m) and one far (1648 m) underground experimental halls. With a 43,000
ton-GW_{\rm th}-day livetime exposure in 55 days, 10416 (80376) electron
antineutrino candidates were detected at the far hall (near halls). The ratio
of the observed to expected number of antineutrinos at the far hall is
. A rate-only analysis
finds in a
three-neutrino framework.Comment: 5 figures. Version to appear in Phys. Rev. Let
Approaching the Gamow Window with Stored Ions : Direct Measurement of Xe 124 (p,γ) in the ESR Storage Ring
© 2019 American Physical Society. All rights reserved.We report the first measurement of low-energy proton-capture cross sections of Xe124 in a heavy-ion storage ring. Xe12454+ ions of five different beam energies between 5.5 and 8 AMeV were stored to collide with a windowless hydrogen target. The Cs125 reaction products were directly detected. The interaction energies are located on the high energy tail of the Gamow window for hot, explosive scenarios such as supernovae and x-ray binaries. The results serve as an important test of predicted astrophysical reaction rates in this mass range. Good agreement in the prediction of the astrophysically important proton width at low energy is found, with only a 30% difference between measurement and theory. Larger deviations are found above the neutron emission threshold, where also neutron and γ widths significantly impact the cross sections. The newly established experimental method is a very powerful tool to investigate nuclear reactions on rare ion beams at low center-of-mass energies.Peer reviewedFinal Published versio
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