Number 152, November 19, 1993 by Phillip F. Schewe and Ben Stein
THE NATURE AND ORIGIN OF COSMIC RAYS seem to change above an energy
of about 10**18.5 eV. The Fly's Eye collaboration---Utah, Illinois, Adelaide,
Bartol---which operates in Dugway, Utah an array of photodetectors aimed
at the sky, has measured the energy and composition of cosmic rays since
1981 (contact Pierre Sokolosky, 801-581-5398). The apparatus looks for
the fluorescence of atmospheric nitrogen caused by airshowers set up by
the incoming cosmic rays. The energy spectrum of the rays falls off steeply
at a certain rate up to an energy of about 10**17.5 eV. Until that point,
the flux is dominated by heavy nuclei. At higher energies, above about
10**18.5 eV, the spectrum, now dominated by protons, flattens, indicating
a different sort of cosmic ray at these higher energies. (The data sample
in this energy range consists of several thousand events.) Furthermore,
no anisotropy in the directionality of the highest-energy events can be
detected, suggesting to the Fly's Eye scientists that these particles do
not originate in the galactic disk. (D.J. Bird et al., Physical Review
Letters, 22 Nov. 1993.)
THE DIFFUSION-CONTROLLED AGGREGATION of nanostructures on a substrate
is directed by a hierarchy of energy barriers. For example, adsorbed atoms
on a surface need more energy to climb from one terrace of atoms to another
than to simply move across the a terrace surface. Moving along the edge
of a terrace requires even more energy; departing from an island of terraces
altogether requires more energy still. Klaus Kern at the Institute for
Experimental Physics in Lausanne, Switzerland has studied the effect of
atom mobility on the nucleation and growth of several types of atom clusters.
He finds that in this world of atom archipelagos temperature (at least
within a certain range) is destiny: different patterns of self-organization
emerge depending on such environmental factors as temperature or the flux
of new atoms onto the substrate. Horia Metiu of UC Santa Barbara compares
this aggregation process to the activity at a construction site: the seemingly
random motions of atoms, subject to a number of "building codes,"
do eventually arrive at a condition of geometrical order, whether in the
form of one- dimensional strands or two-dimensional islands. (Holger Roder
et al., Nature, 11 Nov. 1993.)
OPTICAL PARAMETRIC OSCILLATION (OPO) is a nonlinear optical phenomenon
(the light output is not proportional to the input light) in which a light
beam at one frequency can, by passing through a special crystal, be split
into two beams at lower frequencies. The whole process can be tuned---creating
in effect a multicolor laser system---by changing the refractive index
of the nonlinear crystal, which in turn can be accomplished by rotating
the crystal relative to the incident light beam. The OPO technique may
prove to have applications in the study of quantum optics, in the activation
(at specific wavelengths) of photosensitive drugs, in the monitoring of
tiny amounts of environmental pollutants, in full-color compact television,
and in sampling a system's spectrum over very short time intervals (time-resolved
spectroscopy), an application in which the high peak power of pulsed lasers
can only make the OPO process more efficient. (Physics World, Oct. 1993.)
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