Number 168, March 10, 1994 by Phillip F. Schewe and Ben Stein HIGH ENERGY SULPHUR-SULPHUR COLLISIONS at the CERN SPS accelerator exhibit a greater "stopping power" or "stickiness" than proton-proton collisions. That is, when two heavy nuclei slam into each other the subsequent residual particles have a much greater spread of energies than is the case with mere protons. This suggests that some collective effect is at work. The sulphur beam at CERN has an energy of 200 GeV per nucleon or a whopping 6.4 TeV per sulphur atom. Using like-mass nuclei insures that many of the nucleons in each nucleus will take part in the collision process and not stand idly by as "spectators," at least for "central" events in which the nuclei hit nearly dead on. In many experiments light projectiles are aimed at heavy- nuclei targets, with the result that the projectile passes through the target like a bullet, interacting with only a few nucleons along the way. Presently experiments at Brookhaven and CERN will use even heavier nuclei, such as gold and lead. (J. Bachler et al., Physical Review Letters, 7 Mar. 1994.) AEROCRYSTAL NETWORKS combine the preparation techniques used for aerogels---gel materials that are more than 90% air---with the technical promise of porous silicon---silicon that has been etched by acid into a honeycomb of light-emitting filaments. Scientists at the DRA Malvern lab in Britain use a "supercritical drying" technique to make a crystalline, columnar silicon network with a porosity in excess of 95%. The material is strongly photoluminescent, which will make it useful for optoelectronics applications. (L.T. Canham et al., Nature, 10 March 1994.) SONOLUMINESCENCE CAN BE CHAOTIC . Previously, researchers have observed sonoluminescence to be remarkably stable: when applying sound waves to a liquid and thereby creating light flashes from collapsing bubbles, they observed that the time between successive flashes remained constant. However, new experiments, performed by R. Glynn Holt of JPL (818-393-6946) show that slight adjustments in experimental parameters (such as sound wave frequency and intensity) away from these stable conditions can lead to variations in the time between successive flashes. Taken as a sequence, the variations in successive flashes exhibit chaotic or other non-periodic characteristics. For example, the experimenters observed quasi- periodic behavior in which the timing between flashes could be broken down into two frequencies. Such behavior suggests, according to the experimenters, that the bubbles in sonoluminescence may change shape as well as volume, complicating the current theoretical picture whereby the bubbles are assumed always to have a spherical geometry. (R. Glynn Holt et al., Phys. Rev. Lett, 28 February 1994.)