PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 640 June 5, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

FEMTOSECOND LASERS FOR CUTTING AND IMAGING BRAIN TISSUE have been
demonstrated in a research collaboration that includes physicists
and pharmacologists.  Speaking at this week's CLEO/QELS meeting in
Baltimore, Jeff Squier of the Colorado School of Mines
(squier{at}mines.edu) described an automated, all-optical technique for
performing histology, the study of biological tissue at the
microscopic level.  Used widely in clinical settings (e.g., to
examine biopsied tissue from a suspected breast tumor) and in
biological research (e.g, to study the anatomy of muscle), histology
is presently a manual process, requiring a skilled technician to
slice frozen tissue samples into thin pieces, and then view them
with an optical microscope.  Now, Squier and colleagues have
demonstrated a way to do histology by using femtosecond lasers,
which deliver light pulses that last just quadrillionths of a
second.  Compared to present methods, the femtosecond-laser
technique does not require the freezing of biological samples (which
can damage the specimen) and it can even slice and image very soft
tissue (which is a challenge with standard histological
techniques).  In the new technique, the researchers first stain a
tissue specimen with a layer of fluorescent dye to label desired
structures (such as nerve cells) in tissue.  Then, they use the
laser beam at relatively low power (about 100 gigawatts per square
centimeter) to obtain a picture (through various optical techniques)
of these structures in a tissue specimen's first layer.  The
resolution of the image can approach 30 microns.  After taking this
first picture, the researchers increase the laser power (to levels
of about 7000 terawatts per cm^2) so that the light ablates (wears
away) a 100-micron-deep layer of the tissue.  To this newly exposed
layer of tissue, the researchers add more fluorescent dye, and they
lower the laser intensity to take another image.  This process is
repeated until no tissue remains.  Stacking up the successive images
to create a 3D picture, Squier and colleagues have obtained
high-quality images of animal brain tissue, for example as blood
vessels in rat neocortex. Since the femtosecond technique completely
destroys its tissue samples, it may not be appropriate for certain
clinical applications such as biopsies of breast tissue, as
physicians may wish to preserve the tissue for future reference.
However, the technique may be especially suited for many other
applications, including studies in the burgeoning field of
transgenic animals, which include genetically altered mice.  For
example, researchers could inject a fluorescently labeled gene into
a mouse, and then obtain high-quality images showing how the gene
gets expressed in mouse tissue (Paper CMN3 at the meeting).

A PLASMA VALVE, a device that uses electrically charged particles to
act as a barrier between air and vacuum, has been invented by a
Brookhaven-Argonne collaboration.  These two DOE labs joined forces
to provide a needed component for Argonne's Advanced Photon Source
and similar facilities worldwide.   Inside the walls of
accelerators, synchrotrons and storage rings, a good vacuum--empty
space mostly devoid of matter--enables particle beams to travel
unimpeded for hours.  However, if a leak causes air to rush into the
vacuum, the particle beam spreads out and deposits its energy onto
surrounding walls, disrupting the beam and damaging valuable
equipment.  The faster the leak can be closed, the less
damage will be done to the walls.  The plasma valve, which has no
moving parts, can activate in a nanosecond, a million times faster
than mechanical valves.  To keep air from rushing in, the
Brookhaven-Argonne team create a dense, high-temperature plasma
(collection of charged particles) held together by electric and
magnetic fields. Housed inside a hollow copper cylinder, the plasma
reaches a temperature of 15,000 degrees Kelvin (about 50 times
greater than room temperature)--making the plasma particles bounce
around so vigorously that they collide with air molecules and
prevent them from passing into the vacuum.  Moreover, the valve's
confining electromagnetic fields prevent the plasma itself from
rushing into the vacuum.  (Brookhaven press release, May 28,
http://www.bnl.gov/bnlweb/pubaf/pr/2003/bnlpr052803.htm ).  A much
faster, more complex version of a previously introduced "plasma
window" (see New Scientist, 12 April 2003), the plasma valve is the
latest example of novel uses of plasma for particle-beam
applications; other recent ones include plasma acceleration of
antimatter (Update 634), a plasma lens (Update 508), and plasma
deflection of high-energy beams (Update 540).

***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
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