53,653 research outputs found
Planets as background noise sources in free space optical communications
Background noise generated by planets is the dominant noise source in most deep space direct detection optical communications systems. Earlier approximate analyses of this problem are based on simplified blackbody calculations and can yield results that may be inaccurate by up to an order of magnitude. Various other factors that need to be taken into consideration, such as the phase angle and the actual spectral dependence of the planet albedo, in order to obtain a more accurate estimate of the noise magnitude are examined
Two Populations and Models of Gamma Ray Bursts
Gamma-ray burst statistics are best explained by a source population at
cosmological distances, while spectroscopy and intensity histories of some
individual bursts imply an origin on Galactic neutron stars. To resolve this
inconsistency I suggest the presence of two populations, one at cosmological
distances and the other Galactic. I build on ideas of Shemi and Piran (1990)
and of M\'esz\'aros and Rees (1993) involving the interaction of fireball
debris with surrounding clouds to explain the observed intensity histories in
bursts at cosmological distances. The distances to the Galactic population are
undetermined because they are too few to affect the statistics of intensity and
direction; I explain them as resulting from magnetic reconnection in neutron
star magnetospheres.Comment: 25pp., WU-JIK-92-
Temperature Dependence of the Index of Refraction of Fused Silica
In order to explain the positive temperature coefficient of the visible index
of refraction of fused silica I develop a simple model which assumes a single
ultraviolet resonance. Assuming thermal broadening and using the Kramers-
Kronig relations leads to a temperature coefficient in qualitative, but not
quantitative, agreement with measurement.Comment: 3 pp, tex, Answer to Question # 50 in Am. J. Phy
Scanning of Vehicles for Nuclear Materials
Might a nuclear-armed terrorist group or state use ordinary commerce to
deliver a nuclear weapon by smuggling it in a cargo container or vehicle? This
delivery method would be the only one available to a sub-state actor, and it
might enable a state to make an unattributed attack. Detection of a weapon or
fissile material smuggled in this manner is difficult because of the large
volume and mass available for shielding. Here I review methods for screening
cargo containers to detect the possible presence of nuclear threats. Because of
the large volume of innocent international commerce, and the cost and
disruption of secondary screening by opening and inspection, it is essential
that the method be rapid and have a low false-positive rate. Shielding can
prevent the detection of neutrons emitted spontaneously or by induced fission.
The two promising methods are muon tomography and high energy X-radiography. If
they do not detect a shielded threat object they can detect the shield itself.Comment: 22 pp., 8 figs. APS Short Course on Nuclear Weapon Issues in the 21st
Centur
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