231,337 research outputs found
Apollo 16 mission report. Supplement 2: Service Propulsion system final flight evaluation
The Apollo 16 Mission was the sixteenth in a series of flights using Apollo flight hardware and included the fifth lunar landing of the Apollo Program. The Apollo 16 Mission utilized CSM 113 which was equipped with SPS Engine S/N 66 (Injector S/N 137). The engine configuration and expected performance characteristics are presented. Since previous flight results of the SPS have consistently shown the existence of a negative mixture ratio shift, SPS Engine S/N 66 was reorificed to increase the mixture ratio for this mission. The propellant unbalance for the two major engine firings is compared with the predicted unbalance. Although the unbalance at the end of the TEI burn is significantly different than the predicted unbalance, the propellant mixture ratio was well within limits. The SPS performed six burns during the mission, with a total burn duration of 575.3 seconds. The ignition time, burn duration and velocity gain for each of the six SPS burns are reported
Internal photoemission from quantum well heterojunction superlattices by phononless free-carrier absorption
The possibility of phononless free-carrier absorption in quantum well heterojunction superlattices was investigated. Order of magnitude calculation showed that the absorption coefficient was significantly enhanced over the phonon-assisted process. Important aspects of the enhancement in the design of infrared photodetectors are discussed
Xenon in Mercury-Manganese Stars
Previous studies of elemental abundances in Mercury-Manganese (HgMn) stars
have occasionally reported the presence of lines of the ionized rare noble gas
Xe II, especially in a few of the hottest stars with Teff ~ 13000--15000 K. A
new study of this element has been undertaken using observations from Lick
Observatory's Hamilton Echelle Spectrograph. In this work, the spectrum
synthesis program UCLSYN has been used to undertake abundance analysis assuming
LTE. We find that in the Smith & Dworetsky sample of HgMn stars, Xe is vastly
over-abundant in 21 of 22 HgMn stars studied, by factors of 3.1--4.8 dex. There
does not appear to be a significant correlation of Xe abundance with Teff. A
comparison sample of normal late B stars shows no sign of Xe II lines that
could be detected, consistent with the expected weakness of lines at normal
abundance. The main reason for the previous lack of widespread detection in
HgMn stars is probably due to the strongest lines being at longer wavelengths
than the photographic blue. The lines used in this work were 4603.03A, 4844.33A
and 5292.22A.Comment: 8 pages, 4 figures. Accepted by Monthly Notices of the Royal
Astronomical Society, 8 January 200
Analytical Behaviour of Positronium Decay Amplitudes
Positronium annihilation amplitudes that are computed by assuming a
factorization approximation with on-shell intermediate leptons do not exhibit
good analytical behaviour. Using dispersion techniques, we find new
contributions that interfere with the known results to restore analytical
properties. Those new amplitudes which cannot be obtained using standard
factorized amplitude formalism, contribute at order alpha^2. Therefore they
have to be evaluated before any theoretical conclusion can be drawn upon the
orthopositronium lifetime puzzle.Comment: LaTeX, 22 pages, 3 eps figure
Constraints on turbulent velocity broadening for a sample of clusters, groups and elliptical galaxies using XMM-Newton
Using the width of emission lines in XMM-Newton Reflection Grating
Spectrometer spectra, we place direct constraints on the turbulent velocities
of the X-ray emitting medium in the cores of 62 galaxy clusters, groups and
elliptical galaxies. We find five objects where we can place an upper limit on
the line-of-sight broadening of 500 km/s (90 per cent confidence level), using
a single thermal component model. Two other objects are lower than this limit
when two thermal components are used. Half of the objects examined have an
upper limit on the velocity broadening of less than 700 km/s. To look for
objects which have significant turbulent broadening, we use Chandra spectral
maps to compute the expected broadening caused by the spatial extent of the
source. Comparing these with our observed results, we find that Klemola 44 has
extra broadening at the level of 1500 km/s. RX J1347.5-1145 shows weak evidence
for turbulent velocities at 800 km/s. In addition we obtain limits on
turbulence for Zw3146, Abell 496, Abell 1795, Abell 2204 and HCG 62 of less
than 200 km/s. After subtraction of the spatial contribution and including a 50
km/s systematic uncertainty, we find at least 15 sources with less than 20 per
cent of the thermal energy density in turbulence.Comment: 17 pages, 17 figures, accepted by MNRAS. Includes minor edits to
proo
The environmental analysis of helicopter operations by Federal agencies: Current procedures and research needs
The technical, economic, and environmental problems restricting commercial helicopter passenger operations are reviewed. The key considerations for effective assessment procedures are outlined and a preliminary model for the environmental analysis of helicopters is developed. It is recommended that this model, or some similar approach, be used as a common base for the development of comprehensive environmental assessment methods for each of the federal agencies concerned with helicopters. A description of the critical environmental research issues applicable to helicopters is also presented
Parapositronium Decay and Dispersion Relations
Positronium decay rates are computed at the one-loop level, using
convolution-type factorized amplitudes. The dynamics of this factorization is
probed with dispersion relations, showing that unallowed approximations are
usually made, and some ordre alpha^2 corrections missed. Further, we discuss
the relevance of the Schrodinger wavefunction as the basis for perturbative
calculations. Finally, we apply our formalism to the parapositronium two-photon
decay.Comment: LaTeX, 13 pages, 1 eps figur
Persistence of Tripartite Nonlocality for Non-inertial Observers
We consider the behaviour of bipartite and tripartite non-locality between
fermionic entangled states shared by observers, one of whom uniformly
accelerates. We find that while fermionic entanglement persists for arbitrarily
large acceleration, the Bell/CHSH inequalities cannot be violated for
sufficiently large but finite acceleration. However the Svetlichny inequality,
which is a measure of genuine tripartite non-locality, can be violated for any
finite value of the acceleration.Comment: 4 pages, pdflatex, 2 figure
Currents, Torques, and Polarization Factors in Magnetic Tunnel Junctions
Application of Bardeen's tunneling theory to magnetic tunnel junctions having
a general degree of atomic disorder reveals the close relationship between
magneto-conduction and voltage-driven pseudo-torque, as well as the thickness
dependence of tunnel-polarization factors. Among the results: 1) The torque
generally varies as sin theta at constant applied voltage. 2) Whenever
polarization factors are well defined, the voltage-driven torque on each moment
is uniquely proportional to the polarization factor of the other magnet. 3) At
finite applied voltage, this relation predicts significant voltage-asymmetry in
the torque. For one sign of voltage the torque remains substantial even when
the magnetoconductance is greatly diminished. 4) A broadly defined junction
model, called ideal middle, allows for atomic disorder within the magnets and
F/I interface regions. In this model, the spin dependence of a state-weighting
factor proportional to the sum over general state index of evaluated within the
(e.g. vacuum) barrier generalizes the local state density in previous theories
of the tunnel-polarization factor. 5) For small applied voltage,
tunnel-polarization factors remain legitimate up to first order in the inverse
thickness of the ideal middle. An algebraic formula describes the first-order
corrections to polarization factors in terms of newly defined lateral
auto-correllation scales.Comment: This version no. 3 is thoroughly revised for clarity. Just a few
notations and equations are changed, and references completed. No change in
results. 17 pages including 4 figure
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