PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Publ

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PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Public Information Number 152 November 19, 1993 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 in 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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