---------- Forwarded message ---------- Date: Wed, 7 May 2003 11:48:12 -0400 From: physnews@aip.org To: ravage@SSZ.COM Subject: Physics News Update 636 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 electron volts. Many of these secondary particles quickly lose their energy and become attached (solvated) to water molecules in the cell. What is the general effect of electron energies below 20 eV? A report from three years ago (Boudaiffa et al., Science 287, 1658, 2000) showed that electrons in the 3-20 eV range are able to produce substantial genotoxic damage, including breaking 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 the 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 efficiently 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@uibk.ac.at, 43-512-507-6240) says that this low-energy damage seems to be a general result since his group has since performed similar work with thymine (a DNA base) and have seen similar fragmentation. (Hanel et al., Physical Review Letters, 9 May 2003; Innsbruck website, http://info.uibk.ac.at/c/c7/c722/e-index.html ) PERFECT INSULIN CRYSTALS. {SSZ: Text deleted] THE TINIEST SOLID-STATE LIGHT EMITTER, produced by Phaedon Avouris and his colleagues at IBM, consists of a single-walled carbon 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 positively 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 photonic applications. Why use a NT when a larger piece of bulk semiconductor could also produce light? Because of the potentially much greater energy efficiency and compactness of the light emitting region. Single-molecule light emission has been instigated before, 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 current, for a current density of more than 100 million amps per square cm. (Misewich et al., Science 2 May 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. Physics News Update appears approximately once a week. AUTO-SUBSCRIPTION OR DELETION: By using the expression "subscribe physnews" in your e-mail message, you will have automatically added the address from which your message was sent to the distribution list for Physics News Update. If you use the "signoff physnews" expression in your e-mail message, the address in your message header will be deleted from the distribution list. Please send your message to: listserv@listserv.aip.org (Leave the "Subject:" line blank.)