PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 636 May 7, 2003   by Phillip F. Schewe, Ben Stein, and James Riordon

ULTRA-LOW-ENERGY ELECTRONS CAN BREAK UP URACIL, a new study shows.  How
injurious is radiation (alpha, beta, and gamma rays or heavy ions) to
living cells?  This important question has been addressed
 in many ways.  Much attention has centered on the secondary particles
produced in the wake of the intruding primary radiation,
 especially electrons (about 40,000 electrons are produced for each MeV of
energy deposited) with typical energies of tens of e
lectron volts.  Many of these secondary particles quickly lose their energy
and become attached (solvated) to water molecules i
n the cell.   What is the general effect of electron energies below 20 eV? 
A report from three years ago (Boudaiffa et al., Sc
ience 287, 1658, 2000) showed that electrons in the 3-20 eV range are able
to produce substantial genotoxic damage, including b
reakin
g single- and double-stranded DNA?  What about secondary electrons with
even smaller energies?
To look at this energy range for the first time, Tilmann Maerk and his
colleagues at the Universitat Innsbruck (Austria) and th
e University Claude Bernard Lyon (France) scattered a beam of sub-eV
electrons from a beam of gaseous uracil molecules.  Uracil
 is one of the base units of RNA molecules, and is thus a crucial component
in cells.  These scientists found that uracil is ef
ficiently fragmented by electrons with energies as small as
milli-electron-volts.  It's not the electron's kinetic energy that
causes the disruption, but the electron's charge, which changes the
uracil's internal potential energy environment. Furthermore
, in the process a very mobile  atomic hydrogen can be freed, which on its
own, as a radical (a free chemical unit by itself),
can do
 damage to biomolecules (see a movie of this process at
http://info.uibk.ac.at/ionenphysik/ClusterGroup/Uracil.html; schematic
at http://www.aip.org/mgr/png/2003/187.htm ).  Maerk
(tilmann.maerk{at}uibk.ac.at, 43-512-507-6240) says that this low-energy dama
ge seems to be a general result since his group has since performed similar
work with thymine (a DNA base) and have seen simila
r fragmentation.  (Hanel et al., Physical Review Letters, 9 May 2003;
Innsbruck website, http://info.uibk.ac.at/c/c7/c722/e-ind
ex.html )

PERFECT INSULIN CRYSTALS.   Perfection is elusive both in nature and in the
laboratory, but researchers at the University of Ho
uston have found that crystals of insulin often grow in a perfect fashion.
It is a discovery that may lead to improvements in f
uture microelectronics, as well as higher quality medicines, chemicals, or
devices that can benefit from improved crystal-growi
ng methods. The researchers (Peter Vekilov, 713-743-4315, vekilov{at}uh.edu)
found that as insulin proteins crystallize around a s
crew dislocation defect in an existing insulin crystal, they form spiraling
hillocks of perfect crystalline insulin (see image
at www.aip.org/mgr/png ). (Screw dislocations are a common type of crystal
defect that results when there is a slight angular m
isalig
nment between crystal layers.) In most crystals, interactions between
stepped layers that make up the edges of a growing crysta
l cause the steps to bunch up, which in turn leads to striated crystals. In
addition, competition for dissolved material carrie
d in the surrounding solution can also cause step bunching. Insulin,
however, is unusual in that there is there is little inter
action between steps. Although the researchers say that it is not clear
whether such perfection is possible in many other subst
ances, by coming to understand the factors that lead to perfect growth of
insulin crystals we may soon learn how to tweak growi
ng conditions to improve dramatically other crystals. For example, by
properly stirring a solution, it may be possible to reduc
e step
 bunching that results from competition for dissolved material between
different crystal regions. Alternatively, manufacturers
may choose to introduce screw dislocations to induce crystal growth, rather
than allowing crystals to form around other types o
f defects that tend to generate imperfect structures. Microelectronics is
one field that could benefit from better crystal grow
ing techniques. In particular, microchips built of gallium arsenide are
frequently much faster that ones built of silicon, but
it is currently very difficult to grow the perfect gallium arsenide
crystals necessary for chip manufacturing.. Lessons learned
 from studying factors that lead to perfect insulin crystals may help solve
the problem. (O. Gliko et al., Physical Review Lett
ers, u
pcoming article)

THE TINIEST SOLID-STATE LIGHT EMITTER, produced by Phaedon Avouris and his
colleagues at IBM, consists of a single-walled carbo
n nanotube (NT) strung between two electrodes, and controlled by a third. 
The business part of this minuscule transistor is a
nanotube only 1.4 nm wide and tailored to be semiconducting.  In this arena
electrons coming from one electrode meet with posit
ively charged "holes" coming from the other electrode.  When the
two species meet they combine and emit a tiny burst of light.
 This light is conveniently engineered to be at a wavelength of 1.5
microns, invisible to the human eye but perfect for photoni
c applications.  Why use a NT when a larger piece of bulk semiconductor
could also produce light?  Because of the potentially m
uch gr
eater energy efficiency and compactness of the light emitting region. 
Single-molecule light emission has been instigated befor
e, but not under the auspices of solid state wiring. The NT wire also seems
to be robust: it is able to carry 6 micro-amps of c
urrent, for a current density of more than 100 million amps per square cm. 
(Misewich et al., Science 2 May 2003.)

***********
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