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 169 March 17, 1994 LIMITS ON THE FLUX OF MAGNETIC MONOPOLES streaming through the solar system can be determined from the apparent lack of proton decays. Magnetic monopoles have never been unambiguously observed, but some theories suggest that if they exist they might catalyze proton decays. Working backwards, scientists at the IBM proton-decay detector in Ohio use their measured lack of proton-decay events to calculate monopole flux limits for a variety of hypothetical monopole velocities. For a velocity of 0.001 c and a proton-monopole cross section of 10**-24 cm**2, they derive a monopole flux limit of 2.7x10**-15 cm**-2 sr**-1 sec**-1. (R. Becker-Szendy et al., Physical Review D, 1 Mar.) PHONONIC CRYSTALS would be to sound waves what photonic crystals are to light waves or what semiconductors are to electrons: they would exclude sound waves with a certain range (band gap) of acoustic energies. Scientists at Puebla University in Mexico have proposed ideas on how to engineer such a material from long elastic metal (nickel, say) cylinders arranged in a hexagonal lattice and embedded in a second material (aluminum, say) with different elastic properties. Phononic crystals would be of use in sound transducers and in creating vibrationless environments. (M.S. Kushwaha and P. Halevi, Applied Physics Letters, 28 Feb. 1994.) ASTEROID IDA SEEMS TO HAVE A MOON. Images taken last August by the Galileo spacecraft on its way to Jupiter reveal a bright area near the asteroid which mission scientists believe is sunlight reflected from a subsidiary body. (Science News, 12 Mar.) GERMANIUM-SILICON TRANSISTORS are more than twice as fast as silicon transistors and this year IBM, in collaboration with the company Analog Devices, will market GeSi products, such as analog-to-digital converters. Until recently it had been difficult to make integrated circuits with germanium because of the 4% lattice mismatch between germanium and silicon; this resulted in a disruptive strain in the interfaces present in all semiconductor devices. This problem was solved by using not pure Ge on silicon but a GeSi alloy. Still, the use of GeSi in commercial products was delayed because the fabrication process---whether molecular beam epitaxy or high-temperature chemical vapor deposition (CVD)---was so arduous. More recently IBM researchers have devised a lower-temperature CVD process and circuits can now be manufactured economically. GeSi devices have operated at switching speeds as great as 117 GHz. (Scientific American, Jan. and Mar. 1994; Physics World, Feb. 1994.) A HYPERVELOCITY LAUNCHER has accelerated a quarter-inch disk of metal to a velocity of 15.8 km/sec, or about 36,000 miles per hour, a record for a macroscopic object. For comparison, the Space Shuttle's orbit velocity is 17,500 mph, while the velocity for total escape from the Earth is 25,000 mph. The tremendous acceleration ensues from the following sequence: a gun fires a piston, which compresses a column of hydrogen gas, which moves a specially-sculpted impactor down a barrel where it strikes the projectile. The launcher, developed at Sandia (Philip Stanton, 505-845-8439), is currently used for studying the effect of space debris collisions with the prospective orbiting space station. (Sandia news release, 9 Mar.)


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