105 research outputs found
The Inclusive-Exclusive Connection and the Neutron Negative Central Charge Density
We find an interpretation of the recent finding that the central charge
density of the neutron is negative by using models of generalized parton
distributions at zero skewness to relate the behavior of deep inelastic
scattering quark distributions, evaluated at large values of Bjorken x, to the
transverse charge density evaluated at small distances. The key physical input
of these models is the Drell-Yan-West relation We find that the d quarks
dominate the neutron structure function for large values of Bjorken x, where
the large longitudinal momentum of the struck quark has a significant impact on
determining the center-of-momentum of the system, and thus the "center" of the
nucleon in the transverse position plane.Comment: 17 pages, 8 figures. Text of an invited talk presented by G. A.
Miller at the 2008 Division of Nuclear Physics Meeting in Oakland. Prepared
for Int. Journ. Mod. Phys.
Two-photon exchange measurements with positrons and electrons
Two-photon exchange contributions have potentially broad ranging impact on
several charged lepton scattering measurements. Previously believed to be
extremely small, based in part on comparisons of positron scattering and
electron scattering in the 1950s and 1960s, recent data suggest that the
corrections may be larger than expected, in particular in kinematic regions
that were inaccessible in these early positron scattering measurements.
Additional measurements using positron beams at Jefferson Lab would allow for a
detailed investigation of these contributions in a range of reactions and
observables.Comment: 6 pages, proceedings from the International Workshop on Positrons at
Jefferson Lab (JPOS09), Jefferson Lab, Newport News, VA, March 25-27, 200
The neutron negative central charge density: an inclusive-exclusive connection
Models of generalized parton distributions at zero skewness are used to
relate the behavior of deep inelastic scattering quark distributions, evaluated
at high x, to the transverse charge density evaluated at small distances. We
obtain an interpretation of the recently obtained negative central charge
density of the neutron. The d quarks dominate the neutron structure function
for large values of Bjorken x, where the large momentum of the struck quark has
a significant impact on determining the center of momentum, and thus the
``center'' of the nucleon in the transverse position plane.Comment: 5 pages, 4 figure
Geosmin Occurrence in Lake William C. Bowen and Municipal Reservoir #1, Spartanburg County, South Carolina, 2005 to 2006
2008 S.C. Water Resources Conference - Addressing Water Challenges Facing the State and Regio
Nucleon Form Factors - A Jefferson Lab Perspective
The charge and magnetization distributions of the proton and neutron are
encoded in their elastic electromagnetic form factors, which can be measured in
elastic electron--nucleon scattering. By measuring the form factors, we probe
the spatial distribution of the proton charge and magnetization, providing the
most direct connection to the spatial distribution of quarks inside the proton.
For decades, the form factors were probed through measurements of unpolarized
elastic electron scattering, but by the 1980s, progress slowed dramatically due
to the intrinsic limitations of the unpolarized measurements. Early
measurements at several laboratories demonstrated the feasibility and power of
measurements using polarization degrees of freedom to probe the spatial
structure of the nucleon. A program of polarization measurements at Jefferson
Lab led to a renaissance in the field of study, and significant new insight
into the structure of matter.Comment: 20 pages, 9 figures; Chapter in the book "A decade of Physics at
Jefferson Lab", to be published in Journal of Physics: Conference Serie
Harmful Algal Blooms: A Case Study in Two Mesotrophic Drinking Water Supply Reservoirs in South Carolina
2010 S.C. Water Resources Conferences - Science and Policy Challenges for a Sustainable Futur
Investigations of Aerobic Methane Oxidation in Two Marine Seep Environments: Part 1—Chemical Kinetics
Microbial aerobic oxidation is known to be a significant sink of marine methane (CH4), contributing to the relatively minor atmospheric release of this greenhouse gas over vast stretches of the ocean. However, the chemical kinetics of aerobic CH4 oxidation are not well established, making it difficult to predict and assess the extent that CH4 is oxidized in seawater following seafloor release. Here we investigate the kinetics of aerobic CH4 oxidation using mesocosm incubations of fresh seawater samples collected from seep fields in Hudson Canyon, U.S. Atlantic Margin and MC118, Gulf of Mexico to gain a fundamental chemical understanding of this CH4 sink. The goals of this investigation were to determine the response or lag time following CH4 release until more rapid oxidation begins, the reaction order, and the stoichiometry of reactants utilized (i.e., CH4, oxygen, nitrate, phosphate, trace metals) during CH4 oxidation. The results for both Hudson Canyon and MC118 environments show that CH4 oxidation rates sharply increased within less than one month following the CH4 inoculation of seawater. However, the exact temporal characteristics of this more rapid CH4 oxidation varied based on location, possibly dependent on the local circulation and biogeochemical conditions at the point of seawater collection. The data further suggest that methane oxidation behaves as a first‐order kinetic process and that the reaction rate constant remains constant once rapid CH4 oxidation begins
Investigations of Aerobic Methane Oxidation in Two Marine Seep Environments: Part 2—Isotopic Kinetics
During aerobic oxidation of methane (CH4) in seawater, a process which mitigates atmospheric emissions, the 12C‐isotopologue reacts with a slightly greater rate constant than the 13C‐isotopologue, leaving the residual CH4 isotopically fractionated. Prior studies have attempted to exploit this systematic isotopic fractionation from methane oxidation to quantify the extent that a CH4 pool has been oxidized in seawater. However, cultivation‐based studies have suggested that isotopic fractionation fundamentally changes as a microbial population blooms in response to an influx of reactive substrates. Using a systematic mesocosm incubation study with recently collected seawater, here we investigate the fundamental isotopic kinetics of aerobic CH4 oxidation during a microbial bloom. As detailed in a companion paper, seawater samples were collected from seep fields in Hudson Canyon, U.S. Atlantic Margin, and atop Woolsey Mound (also known as Sleeping Dragon) which is part of lease block MC118 in the northern Gulf of Mexico, and used in these investigations. The results from both Hudson Canyon and MC118 show that in these natural environments isotopic fraction for CH4 oxidation follows a first‐order kinetic process. The results also show that the isotopic fractionation factor remains constant during this methanotrophic bloom once rapid CH4 oxidation begins and that the magnitude of the fractionation factor appears correlated with the first‐order reaction rate constant. These findings greatly simplify the use of natural stable isotope changes in CH4 to assess the extent that CH4 is oxidized in seawater following seafloor release
Overview of NASA GRCs Green Propellant Infusion Mission Thruster Testing and Plume Diagnostics
The Green Propellant Infusion Mission (GPIM) is sponsored by NASA's Space Technology Mission Directorate (STMD) Technology Demonstration Mission (TDM) office. The goal of GPIM is to advance the technology readiness level of a green propulsion system, specifically, one using the monopropellant, AF-M315E, by demonstrating ground handling, spacecraft processing, and on-orbit operations. One of the risks identified for GPIM is potential contamination of sensitive spacecraft surfaces from the effluents in the plumes of AF-M315E thrusters. NASA Glenn Research Center (GRC) is conducting activities to characterize the effects of AF-M315E plume impingement and deposition. GRC has established individual plume models of the 22-N and 1-N thrusters that will be used on the GPIM spacecraft. The models describe the pressure, temperature, density, Mach number, and species concentration of the AF-M315E thruster exhaust plumes. The models are being used to assess the impingement effects of the AF-M315E thrusters on the GPIM spacecraft. The model simulations will be correlated with plume measurement data from Laboratory and Engineering Model 22-N, AF-M315E thrusters. The thrusters will be tested in a small rocket, altitude facility at NASA GRC. The GRC thruster testing will be conducted at duty cycles representatives of the planned GPIM maneuvers. A suite of laser-based diagnostics, including Raman spectroscopy, Rayleigh spectroscopy, Schlieren imaging, and physical probes will be used to acquire plume measurements of AFM315E thrusters. Plume data will include temperature, velocity, relative density, and species concentration. The plume measurement data will be compared to the corresponding simulations of the plume model. The GRC effort will establish a data set of AF-M315E plume measurements and a plume model that can be used for future AF-M315E applications
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