146 research outputs found
G Electronics and Data Acquisition (Forward-Angle Measurements)
The G parity-violation experiment at Jefferson Lab (Newport News, VA) is
designed to determine the contribution of strange/anti-strange quark pairs to
the intrinsic properties of the proton. In the forward-angle part of the
experiment, the asymmetry in the cross section was measured for
elastic scattering by counting the recoil protons corresponding to the two
beam-helicity states. Due to the high accuracy required on the asymmetry, the
G experiment was based on a custom experimental setup with its own
associated electronics and data acquisition (DAQ) system. Highly specialized
time-encoding electronics provided time-of-flight spectra for each detector for
each helicity state. More conventional electronics was used for monitoring
(mainly FastBus). The time-encoding electronics and the DAQ system have been
designed to handle events at a mean rate of 2 MHz per detector with low
deadtime and to minimize helicity-correlated systematic errors. In this paper,
we outline the general architecture and the main features of the electronics
and the DAQ system dedicated to G forward-angle measurements.Comment: 35 pages. 17 figures. This article is to be submitted to NIM section
A. It has been written with Latex using \documentclass{elsart}. Nuclear
Instruments and Methods in Physics Research Section A: Accelerators,
Spectrometers, Detectors and Associated Equipment In Press (2007
Terrestrial exposure of a fresh Martian meteorite causes rapid changes in hydrogen isotopes and water concentrations
Determining the hydrogen isotopic compositions and H2O contents of meteorites and their components is important for addressing key cosmochemical questions about the abundance and source(s) of water in planetary bodies. However, deconvolving the effects of terrestrial contamination from the indigenous hydrogen isotopic compositions of these extraterrestrial materials is not trivial, because chondrites and some achondrites show only small deviations from terrestrial values such that even minor contamination can mask the indigenous values. Here we assess the effects of terrestrial weathering and contamination on the hydrogen isotope ratios and H2O contents of meteoritic minerals through monitored terrestrial weathering of Tissint, a recent Martian fall. Our findings reveal the rapidity with which this weathering affects nominally anhydrous phases in extraterrestrial materials, which illustrates the necessity of sampling the interiors of even relatively fresh meteorite falls and underlines the importance of sample return missions
The <i>Rosetta</i> Mission and the Chemistry of Organic Species in Comet 67P/ChuryumovâGerasimenko
Comets are regarded as probably the most primitive of solar system objects, preserving a record of the materials from which the solar system aggregated. Key amongst their components are organic compounds â molecules that may trace their heritage to the interstellar medium from which the protosolar nebula eventually emerged. The most recent cometary space mission, Rosetta, carried instruments designed to characterize, in unprecedented detail, the organic species in comet 67P/ChuryumovâGerasimenko (67P). Rosetta was the first mission to match orbits with a comet and follow its evolution over time, and also the first mission to land scientific instruments on a comet surface. Results from the mission revealed a greater variety of molecules than previously identified and indicated that 67P contained both primitive and processed organic entities
Cross sections relevant to gamma-ray line emission in solar flares:He-induced reactions on O nuclei
Gamma-ray production cross sections have been measured for gamma-ray lines
copiously emitted in the He bombardment of O nuclei: the 937, 1042
and 1081 keV lines of F and the 1887 keV line of Ne. Four Ge
detectors with BGO shielding for Compton suppression were used to measure the
angular distributions of the gamma-rays. The excitation functions have been
obtained for He bombarding energies from 3.7 to 36 MeV. Total cross
sections are tabulated for calculations relevant to gamma-ray astronomy. The
importance of these lines as diagnosis for the presence and properties of
accelerated He in solar flares is discussed in light of the measured cross
sections.Comment: Phys. Rev. C68 (2003) 0258XX, in pres
The micrometeorite flux at Dome C (Antarctica), monitoring the accretion of extraterrestrial dust on Earth
The annual flux of extraterrestrial material on Earth is largely dominated by sub-millimetre particles. The mass distribution and absolute value of this cosmic dust flux at the Earthâs surface is however still uncertain due to the difficulty in monitoring both the collection efficiency and the exposure parameter (i.e. the area-time product in m2.yr). In this paper, we present results from micrometeorite collections originating from the vicinity of the CONCORDIA Station located at Dome C (Antarctica), where we performed several independent melts of large volumes of ultra-clean snow. The regular precipitation rate and the exceptional cleanliness of the snow from central Antarctica allow a unique control on both the exposure parameter and the collection efficiency. A total of 1280 unmelted micrometeorites (uMMs) and 808 cosmic spherules (CSs) with diameters ranging from 30 to 350 ÎŒm were identified. Within that size range, we measured mass fluxes of 3.0 ÎŒg.mâ2.yrâ1 for uMMs and 5.6 ÎŒg.mâ2.yrâ1 for CSs. Extrapolated to the global flux of particles in the 12-700 ÎŒm diameter range, the mass flux of dust at Earthâs surface is 5, 200 ± 1500 1200 tons.yrâ1 (1, 600 ± 500 and 3, 600 ± 1000 700 tons.yrâ1 of uMMs and CSs, respectively). We indicate the statistical uncertainties expected for collections with exposure parameters in the range of 0.1 up to 105 m2.yr. In addition, we estimated the flux of altered and unaltered carbon carried by heated and un-heated particles at Earthâs surface. The mass distributions of CSs and uMMs larger than 100 ÎŒm are fairly well reproduced by the CABMOD-ZoDy model that includes melting and evaporation during atmospheric entry of the interplanetary dust flux. These numerical simulations suggest that most of the uMMs and CSs originate from Jupiter family comets and a minor part from the main asteroid belt. The total dust mass input before atmospheric entry is estimated at 15,000 tons.yrâ1. The existing discrepancy between the flux data and the model for uMMs below 100 ÎŒm suggests that small fragile uMMs may evade present day collections, and/or that the amount of small interplanetary particles at 1 AU may be smaller than expected
C/N and other Elemental Ratios of Chondritic Porous IDPS and a Fluffy Concordia Micrometeorite
Chondritic porous interplanetary dust particles (CP-IDPs) may be cometary in origin [1], as may ultracarbona-ceous (UCAMMs) [2] and 'fluffy' [3] micrometeorites from the Concordia collection. They are all rich in organics, which can rim grains and may have helped glue grains together during accretion [4]. The organics also contain nitrogen the input of which to Earth has potential biological importance. We report C/N ratios, and other properties of CP-IDPs and a Concordia fluffy microme-teorite
Thermal Processing of Silicate Dust in the Solar Nebula: Clues from Primitive Chondrite Matrices
The most abundant matrix minerals in chondritic meteorites, hydrated
phyllosilicates and ferrous olivine crystals, formed predominantly in asteroids
during fluid-assisted metamorphism. We infer that they formed from minerals
present in three less altered carbonaceous chondrites that have silicate
matrices composed largely of micrometer- and nanometer-sized grains of
crystalline forsterite, Mg2SiO4, and enstatite MgSiO3, and amorphous,
ferromagnesian silicate. Compositional and structural features of enstatite and
forsterite suggest that they formed as condensates that cooled below 1300 K at
\~1000 K/h. Most amorphous silicates are likely to be solar nebula condensates
also, as matrix, which is approximately solar in composition, is unlikely to be
a mixture of genetically unrelated materials with different compositions. Since
chondrules cooled at 10-1000 K/h, and matrix and chondrules are chemically
complementary, most matrix silicates probably formed close to chondrules in
transient heating events. Shock heating is favored as nebular shocks capable of
melting millimeter-sized aggregates vaporize dust. The crystalline and
amorphous silicates in the primitive chondrite matrices share many
characteristic features with silicates in chondritic interplanetary dust
particles suggesting that most of the crystalline silicates and possibly some
amorphous silicates in the interplanetary dust particles are also nebular
condensates. Except for small amounts of refractory oxides that formed with
Ca-Al-rich inclusions at the inner edge of the disk and presolar dust, most of
the crystalline silicate dust that accreted into chondritic asteroids and
long-period comets appears to have formed from shock heating at ~2-10 AU.
Forsterite crystals around young stars may have a similar origin.Comment: 16 page
The parent body controls on cosmic spherule texture: Evidence from the oxygen isotopic compositions of large micrometeorites
High-precision oxygen isotopic compositions of eighteen large cosmic spherules (>500 ”m diameter) from the Atacama Desert, Chile, were determined using IR-laser fluorination â Isotope Ratio Mass spectrometry. The four discrete isotopic groups defined in a previous study on cosmic spherules from the Transantarctic Mountains (Suavet et al., 2010) were identified, confirming their global distribution. Approximately 50% of the studied cosmic spherules are related to carbonaceous chondrites, 38% to ordinary chondrites and 12% to unknown parent bodies. Approximately 90% of barred olivine (BO) cosmic spherules show oxygen isotopic compositions suggesting they are related to carbonaceous chondrites. Similarly, âŒ90% porphyritic olivine (Po) cosmic spherules are related to ordinary chondrites and none can be unambiguously related to carbonaceous chondrites. Other textures are related to all potential parent bodies. The data suggests that the textures of cosmic spherules are mainly controlled by the nature of the precursor rather than by the atmospheric entry parameters. We propose that the Po texture may essentially be formed from a coarse-grained precursor having an ordinary chondritic mineralogy and chemistry. Coarse-grained precursors related to carbonaceous chondrites (i.e. chondrules) are likely to either survive atmospheric entry heating or form V-type cosmic spherules. Due to the limited number of submicron nucleation sites after total melting, ordinary chondrite-related coarse-grained precursors that suffer higher peak temperatures will preferentially form cryptocrystalline (Cc) textures instead of BO textures. Conversely, the BO textures would be mostly related to the fine-grained matrices of carbonaceous chondrites due to the wide range of melting temperatures of their constituent mineral phases, allowing the preservation of submicron nucleation sites. Independently of the nature of the precursors, increasing peak temperatures form glassy textures
A far-ultraviolet-driven photoevaporation flow observed in a protoplanetary disk.
Most low-mass stars form in stellar clusters that also contain massive stars, which are sources of far-ultraviolet (FUV) radiation. Theoretical models predict that this FUV radiation produces photodissociation regions (PDRs) on the surfaces of protoplanetary disks around low-mass stars, which affects planet formation within the disks. We report James Webb Space Telescope and Atacama Large Millimeter Array observations of a FUV-irradiated protoplanetary disk in the Orion Nebula. Emission lines are detected from the PDR; modeling their kinematics and excitation allowed us to constrain the physical conditions within the gas. We quantified the mass-loss rate induced by the FUV irradiation and found that it is sufficient to remove gas from the disk in less than a million years. This is rapid enough to affect giant planet formation in the disk
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