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