PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 807 December 29, 2006 by Phillip F. Schewe, Ben Stein, Turner Brinton, and Davide Castelvecchi www.aip.org/pnu
DIRECT FORCE SENSING AT THE PICONEWTON LEVEL. The measurement of mass can be carried out at the 10^-21 gram (zeptogram) level (www.aip.org/pnu/2005/split/725-1.html) and of force to the 10^-18 newton (attonewton) level (Arlett et al., in Nano Letters, 2006). But for many measurements in the cell biology world, this is too much sensitivity. Forces in this realm are typically at the piconewton (1 pN=10^-12 newton) level. Examples include the force applied by the kinesin molecular motor protein to transport vesicles (6 pN), the force needed to unzip a DNA molecule at room temperature (9-20 pN), or the force needed to pull a DNA apart by pulling on opposite ends (65 pN). Biophysicists need a cost-effective force sensor that works reliably in water at the pN level. Steven Koch and his colleagues at Sandia National Labs are well along on delivering the needed sensor. The core of the device is a spring one millimeter long but only a micron thick and is fabricated using a standard polysilicon micromachining process. This spring operates according to the classic experiment conducted by Robert Hooke in the 17th century: the force exerted on the spring equals the amount of the springs compression or extension multiplied by a spring constant, which in this case is about 1 piconewton per nanometer. The spring, mounted on a substrate, can be used in a number of ways: it can be entrained to move with the push or pull of a biological sample or it can be made sensitive to magnetic fields and so function as a field sensor. The displacement of the spring is currently viewed by a video camera with precision of 2 nm, but faster and more precise methods are possible. Koch (now at the University of New Mexico, [EMAIL PROTECTED]) says that the most likely applications of the new sensor will be in measuring forces on the kind of magnetic microspheres used in single-biomolecule experiments and to calibrate the electromagnets used in deploying microspheres in doing things such as stretch, twist, or unzip DNA. He also envisions direct mechanical force measurements, combined with other MEMS (microelectromechanical systems) implements, in biophysical experiments where optical tweezers (using laser beams to manipulate the microspheres attached to molecules) cannot be used. The Sandia sensor could be adapted to apply an adjustable tension to single DNA molecules in order to study protein binding or enzymatic processes. (Koch, Thayer, Corwin, de Boer., Applied Physics Letters, 23 Oct 2006) NEW CRANKED-UP NUCLEAR STATES. Some nuclear physicists seek to make new elements by fusing two nuclei and hope the amalgamated body will hold together at least for a while. Other researchers explore the nuclear world by creating new spin states. A highly spinning nucleus is not "excited" in the usual sense of possessing a lot of internal energy, but allows nevertheless the nucleus constituent protons and neutrons to deploy themselves in new ways. This high-spin universe is reached in off-center smashups of two nuclei. In new experimental work at the Lawrence Berkeley lab in California, erbium-158 nuclei were spun up to very high rates and then closely observed as they slowed down by offloading high energy photons. These gamma rays, each carrying off two units of angular momentum (each unit equals 2 times pi times Planck's constant), are observed in the Gammasphere detector surrounding the collision site; the number of the gamma rays provides information about nuclear spin. So, for example, a nucleus spun up to a level of 40 units would, by relaxing back to normal, throw off about 20 gammas; other forms of nuclear radioactive relaxation---throwing out electrons or alpha particles---take too long to come about. Theorists believe that above a spin value of about 46, the entire Er-158 nucleus cannot be spun up any further without a drastic rearrangement of the entire state of the nucleus. Instead a spherical core of nuclear particles (constituting a gadolinium-146 nucleus) rotates no more while a fleet of 12 "valence" particles (neutrons and protons) orbits the core at ever higher spin values (see this progression of spin states in the figure at http://www.aip.org/png/2006/274.htm). Eddie Paul of the University of Liverpool ([EMAIL PROTECTED]) and his colleagues have been able to discover new pathways to a higher spin regime by observing the pattern of gamma rays thrown out. They find evidence that the core observed in previous experiments can occasionally break up a bit, allowing collective rotation of all the nucleons to resume, permitting the total spin of the nucleus to attain higher values. The highest value observed in this way was a state with 65 units of spin. The researchers hope to explore even higher values of spin, maybe so high that the nucleus approaches the fission limit. At this point the nucleus does not de-excite by losing gammas, but by actually fissioning---that is by coming apart into large nuclear fragments. (Paul et al., Physical Review Letters, upcoming article) *********** 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. 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