Number 854 , January 23, 2008 by Phillip F. Schewe and Jason S. Bardi
A New Calculation Explains the Mechanism Behind Carbon Dating
The new approach looks at the way the mass of mesons changes as they travel through an atomic nucleus. Mesons (particles such as pions, containing a quark and an antiquark) are thought to mediate the nuclear force between two nuclei. Radiocarbon dating began in 1949 when Willard Libby said that the amount of carbon-14 (the radioactive cousin of carbon-12) left in an object (such as a fossil tree) could provide an estimate of how old the object was.
The thinking was that the organism, while it was alive, would constantly ingest enough of the rare C-14 to replace those nuclei that were decaying into N-14 (the other products being an electron and a neutrino). But as soon as the organism died, the ratio of C-14/C-12 would begin to drop exponentially since the C-14 was no longer being replaced. Measuring the ratio in terms of radioactive half-lifes would provide a good estimate of the fossil. This method has been used by archeologists ever since to measure the age of things, at least those things that had been alive at some point during the past thousands of years.
A big questions presented itself: if the radioactive half-life of C-11 is 20 minutes, and that of O-14 is 1 minute, and that of O-15 is 2 minutes, and that of N-13 is 10 minutes, why is the life-time of C-14 some 3 billion minutes (5730 years)? This is what Jeremy Holt and his colleagues at Stony Brook, TRIUMF (the accelerator facility in Vancouver), and the University of Idaho have set out to determine. Holt says that the anomalously long C-14 half-life has been a mystery to theorists for half a century.
An earlier theory, called Brown-Rho scaling (named for Gerry Brown and Mannque Rho, advanced in 1991), suggested that the masses of most mesons decrease uniformly when (insofar as they carry the nuclear force operating inside nuclei) they travel through dense nuclear material (see figure at http://www.aip.org/png/2008/294.htm ). Holt (jeholt@tonic.physics.sunysb.edu, 631-632-9843) and his fellow authors bring things up to date by accounting, with fair accuracy, for the observed long C-14 lifetime. (Holt et al., Physical Review Letters upcoming article)
Graphene Speed Record
Andre Geim and his colleagues at the University of Manchester have observed the highest electron mobility for an electron in any electronic material. In this case the electrons were moving through graphene, single-atom-thick sheets of carbon, with an electron mobility of 200,000 cm^2/volt-second. Graphene was only discovered a few years ago (by Geim: see http://www.aip.org/pnu/2006/split/769-2.html ). A true two-dimensional material is striking enough, but even more unusual was the observed ease with which electrons moved in graphene.
Electrons moving through any crystal lattice are constantly interacting with the atoms in that lattice, especially if there are irregularities present. This causes the electrons to slow. Their effective mass will be different for each type of crystal. In graphene, the effective mass of electrons is zero. Still another way of quantitatively describing an electron’s journey through the alleyways of a crystal is in terms of its “mobility,” in units of square centimeters per volt/sec.
The charge-carrier mobility is perhaps the most important figure of merit for an electronic material, so researchers have sought a larger mobility. To take some examples: the mobility in silicon is 1500, while in GaAs it is 8500. That’s why the circuitry in cell phones is based on GaAs. For InSb, the mobility is even higher: 80,000. Geim’s new mobility record of 200,000 won’t cause the electronics industry to ditch Si or GaAs any time soon.
The problems with early graphene circuits right now, says Geim, are, first, that graphene can’t yet be made into uniform high-quality wafers; and second that prototype graphene transistor switching (going from Off to On) is too slow. However, Geim predicts that over the short run (3-5 years) graphene might emerge as a basis for chemical sensors and for generators of terahertz-range light-a frequency span (and not yet achieved in any practical way) where human bodies are transparent-making possible security or medical scanning machines. (Morozov et al., Physical Review Letters)
U.S. Presidential Politics
Preoccupied with issues like war, immigration, and the economy, also has a science-and-technology side.
To have a look at what the candidates are saying about science topics, visit these Physics Today websites:
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