Number 209, January 8, 1995 by Phillip F. Schewe and Ben Stein
THE INDEX OF REFRACTION FOR SODIUM ATOM WAVES through various gases
has been measured, providing new details about interatomic forces between
sodium and the individual gases. Under certain experimental conditions,
whole atoms, like light, can behave as rippling waves rather than pointlike
particles. In the past few years, researchers have successfully demonstrated
some of atoms' hard-to-detect wavelike properties using atom interferometers,
devices that split up atom waves and recombine them to form interference
patterns. Now, David Pritchard of MIT (617-253-6812) and his co-workers
have passed one portion of a sodium atom wave through one of numerous gas
samples (such as helium, neon, argon, ammonia, and water) and then recombined
it with another portion of the wave that did not pass through the gas.
From the resulting interference pattern the researchers derived the index
of refraction, basically a measure of the "bending" angle of
the sodium atom ray as it traverses the gas medium. From the index of refraction
researchers extracted new details about the long-range forces between the
sodium atom and the gases. Of the gases studied, they have found that helium
behaves most like a hard sphere in its interactions with sodium, exerting
the weakest long-range attraction, while xenon has the most dominant long-range
attraction. (J. Schmiedmayer et al., 13 Feb. in Physical Review Letters.)
CAN FUSION BE INITIATED BY SONOLUMINESCENCE? In sonoluminescence carefully
tuned sound waves cause bubbles in a fluid to oscillate; in the collapsing
part of this motion the bubbles emit short (50 psec) bursts of light, most
likely by some implosion effect. At a recent meeting of the Acoustical
Society of America, Livermore physicist William Moss presented computer
simulations which show that peak temperatures (as high as 1 million K)
and pressures inside the bubbles could, with further experimental refinements,
be sufficient to support nuclear fusion reactions. Experiments at several
labs have not been able to measure such high temperatures; nor have they
observed neutrons, an important product of nuclear fusion. One current
experiment, at Livermore, is using bubbles filled with deuterium rather
than air. (Science, 16 December 1994.)
HELIUM DOES NOT FLOW WELL ON CESIUM . Above a temperature of 2 K, thin
films of helium-4 on a cesium surface can become thick films in a process
called wetting. Below 2 K, however, the cesium only allows thin films (only
a few atomic layers at most) of He-4 to form, and this, according to James
Rutledge of the University of California at Irvine, is not enough for the
helium to flow as a superfluid (Physics World, December 1994). In a recent
experiment researchers at the University of Exeter in Britain have measured
the flow of helium across cesium to be a factor of 10**9 less than the
flow of helium across glass. Furthermore, at a temperature of 0.12 K, there
was an average of less than 4 x 10**-6 monolayers of helium on the Cs surface.
This property may make cesium a good coating for surfaces where the presence
of superfluid helium is undesirable. (P. Stefanyi et al., Physical Review
Letters, 1 August 1994.)
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