Number 684, May 6, 2004
by Phil Schewe and 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@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
The Cryogenic Dark Matter Search (CDMS) collaboration reported their
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. (CDMS
Webpage)
Persistent Holes
Persistent holes have been observed in a shaken fluid. Normally, a
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 12 g-25 g, 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 UT
website; be sure to watch to the end.) As yet the physics behind
the persistent holes cannot be explained. (Florian
S. Merkt, Robert D. Deegan, Daniel I. Goldman, Erin C. Rericha, and
Harry L. Swinney, Physical Review Letters, 7 May 2004; contact
Harry Swinney, swinney@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 Update
286).
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