PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Publ

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.)