From: LARRY KLAES (ljk4_at_msn.com)
Date: Thu May 06 2004 - 12:08:01 PDT
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From: physnews_at_aip.org<mailto:physnews_at_aip.org>
To: ljk4_at_MSN.COM<mailto:ljk4_at_MSN.COM>
Sent: Thursday, May 06, 2004 2:04 PM
Subject: Physics News Update 684
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 684 May 6, 2004 by Phillip F. Schewe, Ben Stein
NUCLEAR CAR WASH. To address the threat of smuggled nuclear
materials being brought into the U.S., a Lawrence Livermore National
Lab research program is developing a scanner which would examine
cargo shipping containers, which now carry up to 90% of the world's
trade. Six million such containers enter the U.S. each year, the
bulk arriving through 10 ports, the top three being Los Angeles,
Long Beach, and New York-New Jersey. A parcel of radioactive
material, intended as part of a terrorist bomb, would presumably be
shielded inside the cargo container, precluding passive detection.
The Livermore scanner would work in the following way: the
container, on a moving conveyor, would slide past and be exposed to
a neutron beam. The neutrons would irradiate all the contents of
the container, but would especially activate such dangerous
materials such as uranium-235 and plutonium-239. These radioactive
species, perturbed by the neutrons, would fission, resulting in the
emission of characteristic gamma rays detectable in arrays located
downstream of the neutron beam.
Speaking at this week's meeting of the American Physical Society
(APS) in Denver, Thomas Gosnell (gosnell1_at_llnl.gov<mailto:gosnell1_at_llnl.gov>) said that the
goal of the Livermore research is the development of a scanner
capable of locating 5 kg of highly enriched uranium or 1 kg of
plutonium with a false-positive or false-negative rate of 1% or
less. He expects a prototype "nuclear car wash" device would be
working within a year and be deployed on a trial basis in a port,
such as Oakland, California, a year after that.
THE CRYOGENIC DARK MATTER SEARCH (CDMS) collaboration reported their
PERSISTENT HOLES have been observed in a shaken fluid. Normally, a
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: Thu May 06 2004 - 12:20:12 PDT
first results at the APS meeting this week. They did not find any
specific evidence for weakly interacting massive particles (or
WIMPs), a finding which is at odds with positive results reported a
few years ago by the Dark Matter (DAMA) group in Italy. Both teams
maintain sensitive detectors far underground, the better to filter
out extraneous particles from entering their apparatus which
operate, in effect, as underground telescopes. As observatories,
they don't form images of celestial objects. Their mission is
rather more basic: they try to record the very existence of WIMPs
which may well be a component of the much sought dark matter, which
supposedly lurks unseen in and around and among galaxies. In CDMS,
located 2341 feet deep in the Soudan mine in Minnesota, a target of
germanium and silicon is maintained at temperatures close to
absolute zero. At masses as high as 100 times the mass of a proton,
an intruding WIMP, if it interacted inside the target at all, would
engender a characteristic pattern of crystalline vibrations and
secondary particles in the semiconductor target material. At the
meeting Harry Nelson (UC Santa Barbara) said that the CDMS null
measurement could be cast in the form of a cross section, which is
what particle physicists do when estimating the likelihood of
detecting certain kinds of interaction. In this case the CDMS
apparatus established a cross section of less than 4 x 10^-43 square
centimeters for a 60-GeV-mass WIMP particle to show up in their
detector. This level of sensitivity is the best yet for dark matter
searches, and is about four times better than another detector
group, the EDELWEISS experiment, located near Grenoble, France.
(http://cdms.berkeley.edu/index.html
fluid takes the shape of its container; any puncture of the surface
will quickly fill. However, in an experiment performed at the
University of Texas by Florian Merkt, Robert Deegan, and Erin
Rericha, a mixture of cornstarch and water is vertically vibrated at
frequencies as high as 120 Hz, with accelerations in the range
12g-25g, where g is the gravitational acceleration. If a stick or
puff of air is used to poke a hole in the fluid, the researchers
found that the hole can persist indefinitely, with a characteristic
diameter comparable to the depth of the fluid and extending to the
bottom of the container. This is quite surprising--a hole produced
in a similar way in ordinary fluids or in the cornstarch mixture at
rest quickly collapses. The holes in cornstarch can survive as
long as the shaking persists and can move around, coalesce,
annihilate, or even scatter. (Pictures and movie at
http://chaos.ph.utexas.edu/research/vibrated_cornstarch.htm
to watch to the end) As yet the physics behind the persistent holes
cannot be explained. (Merkt et al. Physical Review Letters, 7 May
2004; Contact Harry Swinney, swinney_at_chaos.ph.utexas.edu<mailto:swinney_at_chaos.ph.utexas.edu>,
512-471-4619.) The same research group had earlier reported the
existence of "oscillons," tiny long-lived spouts of sand grains that
developed when a shallow bed of sand was
shaken vertically (see
www.aip.org/enews/physnews/1996/split/pnu286-1.htm<http://www.aip.org/enews/physnews/1996/split/pnu286-1.htm> ).
PHYSICS NEWS UPDATE is a digest of physics news items arising
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