A SONOLUMINESCING AIR BUBBLE MAY CONSIST CHIEFLY OF ARGON, William Moss of Livermore (wmoss@llnl.gov) reported at last week's Acoustical Society of America meeting. In sonoluminescence (SL), an air bubble collapses to produce ultrashort flashes of light. Recently, a German-US team proposed that the nitrogen and oxygen molecules in the hot collapsing air bubble dissociate into chemically reactive atoms which perform a mass exodus prior to the light flash; the atoms form products in the fluid surrounding the bubble, leaving behind only argon and water vapor to produce the light. (D. Lohse, Phys. Rev. Lett., 17 Feb.) Employing a rigorous fluid dynamics simulation originally developed for imploding fusion pellets, the Livermore model calculates a light spectrum for a sonoluminescing bubble of pure argon which closely matches the experimental spectrum for a collapsing air bubble, confirming and clarifying the results of earlier experiments which produced SL in air and argon. (Science, 30 May; lay-language paper at the ASA World Wide Press Room.)

NON-EXPONENTIAL DECAY of a quantum system has been observed for the first time. Many unstable systems such as a mass of radioactive nuclei will characteristically undergo a process (quantum tunneling) whereby the number of nuclei remaining in an initial state after a time t will be proportional to e^-at, where a is a constant. Quantum mechanics does not expressly forbid non-exponential decay, and physicists at the University of Texas have now devised a scheme---sodium atoms trapped in a web of laser light---in which the rate of atoms escaping (under the auspices of quantum tunneling) is non-exponential, at least over intervals on the order of 10 microseconds. (Nature, 5 June 1997.)

A SECOND ASTEROID BELT, possibly as big as the one between Mars and Jupiter, might exist between the orbits of Uranus and Neptune. Matthew Holman's (University of Toronto) computer simulation places collections of test particles in various graviational niches throughout the solar system and then tracks the particles over billions of years. Many of these pretend asteroids are captured by planets or ejected from the solar system. Holman finds that the region between 24 and 27 astronomical units undergoes major asteroid attrition but could harbor a large enough population of survivors to make an observational search worthwhile. (Nature, 19 June 1997.)

SILICON CARBIDE chips will go where no chips have gone before---onto automobile engine blocks, down bore holes, and into the inferno of exhaust outlets. SiC semiconductors have a large energy gap: that is, it takes 2.9 eV (at room temperature) to promote the material from an insulator into a conductor of electrons. Silicon (energy gap of 1.1 eV) and gallium arsenide (1.43 eV) are much better suited to ordinary circuitry but fail at higher temperatures and voltages, which is precisely where SiC circuits will come into their own. For example, SiC components will appear later this year in high-power microwave transmitters, the kind that will be needed for high-definition television. (New Scientist, 14 June 1997.)

THE LIGHTEST STRONGLY INTERACTING PARTICLE, THE PION, was discovered 50 years ago by Cecil Powell of Bristol University in emulsions exposed to cosmic rays. Pions had been postulated by Hideki Yukawa in 1935 as the hypothetical carrier of the strong force operating inside the nucleus. (CERN Courier, June 1997.)