Number 131, June 3, 1993 by Phillip F. Schewe and Ben Stein RADIO SIGNALS FROM THE HELIOPAUSE have been detected by the two Voyager spacecraft. The heliopause, where the solar wind particles streaming out from the sun meet the directional flow of interstellar-medium particles, essentially constitutes the edge of the solar system. Speaking at last week's meeting of the American Geophysical Union, Donald Gurnett of the University of Iowa provided this explanation: a powerful solar flare event in May-June 1991 caused a surge of solar-wind particles which subsequently interacted with the heliopause, setting up huge radio bursts (at more than 10**13 Watts, the most powerful radio source in the solar system) detected by the Voyagers beginning in July 1992. High in power but low in frequency (2-3 kHz), the radio signals could not be detected in the inner solar system. However, Voyager 1, at a distance of 52 AU (an astronomical unit is the distance between the Earth and sun), and Voyager 2, at 40 AU, were well placed to made a measurement. Ralph McNutt of Johns Hopkins said that from the timing of the signals the distance to the heliopause could be estimated to be between 80 and 130 AU. PURIFIED DIAMOND HAS THE HIGHEST THERMAL CONDUCTIVITY. Natural diamond is 98.9% carbon-12 and 1.1% carbon-13. A team of scientists from Wayne State University and General Electric showed in 1990 that purifying the C-12 component of diamond to 99.9% increased its room-temperature thermal conductivity by 50%, a much bigger isotope effect than in other materials. The same scientists have now recorded the highest thermal conductivity ever observed for a solid above liquid nitrogen temperatures, 410 W/cm-K, in 99.9%-pure C-12 at 104 K. The researchers predict that the thermal conductivity for 99.999%-pure C-12 diamonds would exceed 2000 W/cm-K and that integrated circuits mounted on such diamonds (cooled in liquid nitrogen) could operate at 500 times the power density of circuits mounted on copper substrates at room temperature. (Lanhua Wei et al., Physical Review Letters, 14 June 1993.) LUMINESCENCE IN POROUS SILICON is probably due to quantum-confinement effects. Although silicon does not readily emit light---certainly a drawback when it comes to optoelectronic applications---several years ago scientists noticed that silicon that had been bathed in acid, forming a sort of mangrove swamp of silicon filaments, did emit light, even into the visible range. Several explanations of this phenomenon, including the idea that the light is emitted by a residue (siloxene) left on the silicon surface, now seem to be ruled out by new experiments conducted by a Western Ontario-Wisconsin-Argonne-NRC (Ottawa) collaboration. The observed dependence of the luminescence wavelengths on porosity supports the notion that the narrowness of the filaments (some only a few nm wide) changes the band structure of silicon enough to permit light emission. (T.K. Sham et al., Nature, 27 May 1993.)