Number 169, March 17, 1994 by Phillip F. Schewe and Ben Stein
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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