Number 127, May 5, 1993 by Phillip F. Schewe and Ben Stein
THE OPPOSITION EFFECT OF THE MOON , a phenomenon known for more than
a century in which the Moon's brightness goes up markedly as it becomes
full (goes into opposition), can now be attributed to coherent backscattering.
That is, incoming sunlight scatters repeatedly from lunar particles smaller
than the light's wavelength and builds up prominently in the backward direction.
The opposition effect, studied by scientists at the University of Pittsburgh
and JPL using lunar rock samples, applies also to other reflective objects
in the solar system such as Saturn's rings. (Science, 23 April.)
NIST-7, THE WORLD'S BEST CLOCK, , will drift by only 1 second every 3
million years. The new device essentially defines the second as being 9,192,631,770
cycles of microwave radiation from an atomic transition in cesium-133.
Built at the National Institute of Standards and Technology, the new clock
is 10 times better than the previous standard, a device called NBS-6. (Science
News, 1 May 1993.)
MARC. H. BRODSKY WILL BE THE NEXT DIRECTOR OF THE AMERICAN INSTITUTE
OF PHYSICS (AIP) . Brodsky, a noted condensed matter physicist, has been
associated with IBM since 1968 in a number of positions, including director
of technical planning for the IBM research division (1989-1991). From 1991
to 1992 he worked as an IEEE Fellow in the U.S. Dept. of Commerce. He will
replace Kenneth W. Ford who will retire on October 31, 1993 after serving
7 years as AIP's director. (For more information, contact Joan Wrather
at AIP, 212-661-9404.)
HELIUM ATOM SCATTERING (HAS) is developing into a high-resolution technique
for studying the structure and dynamical properties of crystal surfaces.
One reason for this is that a beam of low-energy (20 meV, equivalent to
a wavelength of 1 angstrom) helium atoms will bounce off the very surface
of the sample without sticking, making detection of the scattered atoms
and analysis of the sample properties relatively simple. By comparison,
in low energy electron diffraction (LEED) the probing electrons (with energies
of 100 eV) penetrate as far as several atomic layers into the solid, making
analysis somewhat more complicated. With HAS, Peter Toennies of the Max
Planck Institute for Fluid Dynamics in Gottingen, Germany has been able
to work out for the first time the surface structure for a close-packed
gold crystal. He discovered that the top layer of atoms is more densely
packed than the bulk layers beneath. According to Toennies, HAS is also
well suited to measuring vibrational modes (phonons) of the crystal surface
down to energies below 30 meV, with a resolution as small as 0.08 meV.
Toennies hopes to apply his techniques in making a helium microscope. (Physics
World, April 1993.)
A COMETARY BREAKUP into at least 18 fragments has been imaged by scientists
at the U.S. Geological Survey. The strung-out trail of objects, whose icy
facets brilliantly reflect the Sun's light, was photographed near Jupiter,
where gravitational forces may have ripped apart a passing comet that got
too close. (Science News, 10 April 1993.)
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