19,608 research outputs found
Multi-photon effects in energy losses spectra
Effect of radiation of many photons by a single electron traversing a target
is discussed. When the summary energy of emitted photons (the energy losses
spectrum) is measured only, the photon spectrum is distorted comparing with the
photon spectrum in one interaction. Influence of this effect is discussed for
the cases (1) bremsstrahlung (described by Bethe-Heitler formula), (2) the
strong Landau-Pomeranchuk-Migdal effect and (3) transition radiation.
Qualitative picture of the phenomenon is discussed in detail. Comparison with
the recent SLAC experiment in relatively thick target (2.7% of the radiation
length), where the effect of emission of many photons by a projectile is very
essential, shows perfect agreement of the theory and data.Comment: LaTeX2.09, 19 pages, 5 PostScript figure
Photo-response of the conductivity in functionalized pentacene compounds
We report the first investigation of the photo-response of the conductivity
of a new class of organic semiconductors based on functionalized pentacene.
These materials form high quality single crystals that exhibit a thermally
activated resistivity. Unlike pure pentacene, the functionalized derivatives
are readily soluble in acetone, and can be evaporated or spin-cast as thin
films for potential device applications. The electrical conductivity of the
single crystal materials is noticeably sensitive to ambient light changes. The
purpose, therefore, of the present study, is to determine the nature of the
photo-response in terms of carrier activation vs. heating effects, and also to
measure the dependence of the photo-response on photon energy. We describe a
new method, involving the temperature dependent photo-response, which allows an
unambiguous identification of the signature of heating effects in materials
with a thermally activated conductivity. We find strong evidence that the
photo-response in the materials investigated is predominantly a highly
localized heating mechanism. Wavelength dependent studies of the photo-response
reveal resonant features and cut-offs that indicate the photon energy
absorption is related to the electronic structure of the material.Comment: Preprint: 18 pages total,7 figure
Atmospheric circulation patterns, cloud-to-ground lightning, and locally intense convective rainfall associated with debris flow initiation in the Dolomite Alps of northeastern Italy
The Dolomite Alps of northeastern Italy experience debris flows with great
frequency during the summer months. An ample supply of unconsolidated
material on steep slopes and a summer season climate regime characterized by
recurrent thunderstorms combine to produce an abundance of these destructive
hydro-geologic events. In the past, debris flow events have been studied
primarily in the context of their geologic and geomorphic characteristics.
The atmospheric contribution to these mass-wasting events has been limited
to recording rainfall and developing intensity thresholds for debris
mobilization. This study aims to expand the examination of atmospheric
processes that preceded both locally intense convective rainfall (LICR) and
debris flows in the Dolomite region. 500 hPa pressure level plots of
geopotential heights were constructed for a period of 3 days prior to
debris flow events to gain insight into the synoptic-scale processes which
provide an environment conducive to LICR in the Dolomites. Cloud-to-ground (CG)
lightning flash data recorded at the meso-scale were incorporated to
assess the convective environment proximal to debris flow source regions.
Twelve events were analyzed and from this analysis three common synoptic-scale circulation patterns were identified. Evaluation of CG flashes at
smaller spatial and temporal scales illustrated that convective processes
vary in their production of CF flashes (total number) and the spatial
distribution of flashes can also be quite different between events over
longer periods. During the 60 min interval immediately preceding debris
flow a majority of cases exhibited spatial and temporal colocation of LICR
and CG flashes. Also a number of CG flash parameters were found to be
significantly correlated to rainfall intensity prior to debris flow initiation
The Apparent Anomalous, Weak, Long-Range Acceleration of Pioneer 10 and 11
Recently we reported that radio Doppler data generated by NASA's Deep Space
Network (DSN) from the Pioneer 10 and 11 spacecraft indicate an apparent
anomalous, constant, spacecraft acceleration with a magnitude cm s, directed towards the Sun (gr-qc/9808081). Analysis of
similar Doppler and ranging data from the Galileo and Ulysses spacecraft
yielded ambiguous results for the anomalous acceleration, but it was useful in
that it ruled out the possibility of a systematic error in the DSN Doppler
system that could easily have been mistaken as a spacecraft acceleration. Here
we present some new results, including a critique suggestions that the
anomalous acceleration could be caused by collimated thermal emission. Based
partially on a further data for the Pioneer 10 orbit determination, the data
now spans January 1987 to July 1998, our best estimate of the average Pioneer
10 acceleration directed towards the Sun is cm
s.Comment: Latex, 7 pages and 2 figures. Invited talk at the XXXIV-th Rencontres
de Moriond Meeting on Gravitational Waves and Experimental Gravity. Les Arcs,
Savoi, France (January 23-30,1999). Corrected typo
Measurement of Dielectric Suppression of Bremsstrahlung
In 1953, Ter-Mikaelian predicted that the bremsstrahlung of low energy
photons in a medium is suppressed because of interactions between the produced
photon and the electrons in the medium. This suppression occurs because the
emission takes place over on a long distance scale, allowing for destructive
interference between different instantaneous photon emission amplitudes. We
present here measurements of bremsstrahlung cross sections of 200 keV to 20 MeV
photons produced by 8 and 25 GeV electrons in carbon and gold targets. Our data
shows that dielectric suppression occurs at the predicted level, reducing the
cross section up to 75 percent in our data.Comment: 11 pages, format is postscript file, gzip-ed, uuencode-e
Unstable Nonradial Oscillations on Helium Burning Neutron Stars
Material accreted onto a neutron star can stably burn in steady state only
when the accretion rate is high (typically super-Eddington) or if a large flux
from the neutron star crust permeates the outer atmosphere. For such situations
we have analyzed the stability of nonradial oscillations, finding one unstable
mode for pure helium accretion. This is a shallow surface wave which resides in
the helium atmosphere above the heavier ashes of the ocean. It is excited by
the increase in the nuclear reaction rate during the oscillations, and it grows
on the timescale of a second. For a slowly rotating star, this mode has a
frequency of approximately 20-30 Hz (for l=1), and we calculate the full
spectrum that a rapidly rotating (>>30 Hz) neutron star would support. The
short period X-ray binary 4U 1820--30 is accreting helium rich material and is
the system most likely to show this unstable mode,especially when it is not
exhibiting X-ray bursts. Our discovery of an unstable mode in a thermally
stable atmosphere shows that nonradial perturbations have a different stability
criterion than the spherically symmetric thermal perturbations that generate
type I X-ray bursts.Comment: Accepted for publication in Astrophysical Journal, 22 pages, 14
figure
Comprehensive Income Options: A Detriment To Transparency
The Financial Accounting Standards Board (FASB) in 1997 compromised its belief that comprehensive income (CI) should be listed either in a combined statement of net income and CI or in a separate statement of CI and allowed corporations to choose using the statement of changes in stockholders’ equity (SCSE). Of course, the latter option implies just as Jordan and Clark (2002) suggest, that CI is not a measure of financial performance. Studies incorporating professional analysts by Hirst and Hopkins (1998) and a study of nonprofessional investors by Maines and McDaniel (2000) both conclude that format presentation matters and behaviors can be affected. We believe that FASB should revisit the format structure of CI and eliminate the SCSE option, which was their initial intent before they compromised with corporate managers in 1997. In addition, we believe that all items of other comprehensive income (OCI) – foreign currency translation adjustment, pension value adjustments and adjustment to securities-for-sale should be presented on an after-tax basis only in order to prevent investors from being forced to comb through the footnotes
Anderson et al. Reply (to the Comment by Murphy on Pioneer 10/11)
We conclude that Murphy's proposal (radiation of the power of the main-bus
electrical systems from the rear of the craft) can not explain the anomalous
Pioneer acceleration.Comment: LaTex, 3 pages, Phys. Rev. Lett. (to be published
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