PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 649 August 13, 2003   by Phillip F. Schewe, Ben Stein, and
James Riordon

DETECTING PLASTIC EXPLOSIVES IN AIR at the parts-per-trillion level
has been achieved by researchers at Oak Ridge National Laboratory
and the University of Tennessee (Thomas Thundat, 865-574-6201,
thundattg{at}ornl.gov), potentially leading to a fast, portable, and
ultrasensitive plastic-bomb "sniffer."  Plastic explosives such as
pentaerythritol tetranitrate (PETN) and hexahydro-1,3,5-triazine
(RDX) pose serious threats because (1) they are easily to mold into
desired shapes, (2) they remain highly stable until detonated, and
(3) they can inflict significant damage even in small amounts. In
fact, the infamous shoe bomber had PETN in his footwear.  Most
current plastic-bomb sensors are bulky and expensive.  In contrast,
the new sensor is a microelectromechanical system (MEMS), or a tiny
mechanical device with microscopic dimensions.  Potentially cheap
and easy to mass-produce, the bomb-sniffing MEMS device is a
microcantilever, a 180-by-25-micron slab of silicon attached to a
spring-loaded wire.  Similar in structure to a diving board attached
to a pool, the microcantilever is coated on one side with gold.  On
one end of the gold-coated surface is a single layer of 4-MBA
(4-mercaptobenzoic acid), to which PETN and RDX both attach.    Like
hair that curls up on a humid day as water molecules adsorb to it,
this specially coated cantilever will bend by significant amounts
when PETN and RDX molecules attach to it.  A laser-microscope system
can detect the degree of bending to nanometer precision. Placed in a
vacuum-tight glass cell, the cantilever was exposed to a stream of
ambient air with tiny traces of plastic explosive.  Using a modified
atomic force microscope to measure the deflections of the
cantilever, the researchers determined that their MEMS device could
detect the explosives at a level of 14 parts per trillion, after
only 20 seconds of operation. By another measure, the device becomes
sensitive to plastic explosives even if only a few femtograms (1
fg=10^-15 g) impinges upon it.  A future step is to take the device
out of the laboratory and develop it into a portable sensor. While
much activity has centered on the development of sensors for
detecting vapors from all kinds of explosives, this is, to the
authors' knowledge, only the third device of its kind that uses
MEMs.  (Pinnaduwage et al., Applied Physics Letters, 18 August 2003)

BARIUM SHIELD TO PROTECT THE FETUS DURING CT SCANS.  Computed
tomography (CT) on a pregnant woman's chest puts the fetus at risk
owing to the adverse effects of radiation. However, researchers from
the University of Chicago propose that it may be possible to protect
the fetus if the mother ingests barium sulfate before CT radiation
exposure. Because the fetal dose during chest scans is mainly due to
internal scatter of incident radiation, the barium compound acts as
an internal shield that absorbs errant radiation. A study that
simulated a CT scan of a pregnant woman showed that ingesting a 40
percent solution of  barium sulfate would decrease the fetal dose to
a negligible level, so that even high-quality CT imaging could be
performed with minimal risk. Chester Reft presented data from the
study and discussed the potential for barium sulfate internal
shields at this week's meeting of the American Association of
Physicists in Medicine in San Diego
(http://www.aapm.org/meetings/03AM/, Paper WE-C23A-4:
creft{at}radonc.bsd.uchicago.edu, 773-702-6873)

CELLOPHANE AND 3D DISPLAYS.   New research on ordinary cellophane
shows that it can be used to convert a laptop screen image into a
seemingly three-dimensional display.  Cellophane is birefringent:
its index of refraction is not the same in all directions in the
material.  This means that the polarization of an entering light
wave can be rotated.  Keigo Iizuka's lab at the University of
Toronto verified that a cellophane sample 25 microns thick was
better at rotating the polarization direction of white light than a
commercially available device (called a half-waveplate) designed for
a specific wavelength. Taking
advantage of the fact that light emitted from a laptop display is
naturally polarized to begin with, a 3D stereoscopic effect can be
achieved by covering half the screen with a cellophane sheet in
order to construct orthogonally polarized left and right scenes
while the viewer wears eyeglasses holding two polarizers oriented 90
degrees apart (see series of figures at
http://www.keigo-iizuka.com/research/cellophane.htm ).  Actually,
the crossed polarizers could be suspended between the screen and the
observer, obviating the need for the viewer to wear the glasses.
According to Iizuka (keigo.iizuka{at}utoronto.ca, 416-978-8657), this
"cellography" method for producing 3D effects will be
far cheaper than those using commercially available half-waveplates,
and should be amenable to arcade gaming applications and for medical
and scientific imaging applications.  Iizuka is now at work on
converting liquid crystal displays on cellular phones to 3D.
(Review of Scientific Instruments, August 2003)

***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources.  It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
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