Physics News Update, Number 443

Number 443, August 16, 1999 by Phillip F. Schewe and Ben Stein

NUCLEAR THERMOMETER. How hot is it inside the nucleus of a dysprosium atom (element 66, abbreviated Dy)? Temperature is a statistical concept that normally applies to an ensemble of many particles, such as air molecules or a gas of atoms kept in a bottle. Inside a heavy nucleus, swarming with protons and neutrons (collectively called nucleons) it's not so easy to define temperature, owing to the many pairing and other inter-nucleon interactions that take place, but it can be done. The nuclear environment can be sampled by colliding nuclei together and then carefully measuring the photons that fly out: high energy gamma rays, in this case, rather than the visible and infrared photons that come out of heated-up atomic gases. In this way, physicists at the University of Oslo have deduced the temperature inside a Dy nucleus (in effect, a gas of 162 nucleons) to be 6 billion K. It can be said, therefore, that even in winter parts of Norway (very small parts) remain quite warm. This is the first time a nuclear temperature has been measured strictly on the basis of the spectrum of gammas emitted. (E. Melby et al., Physical Review Letters, 6 September 1999; contact Magne Guttormsen, magne.guttormsen@fys.uio.no, 011-47-2285-6460.)

GALAXY FORMATION IN AMOEBAS. Dictyostelium discoideum is the hydrogen atom of developmental biology. Depending on available nutrients the organism can exist in a uni-cellular or multi-cellular state (in which cells differentiate themselves as spore or stalk cells). Dictyostelium cells like to huddle together. A new experiment at UC San Diego shows, furthermore, that when constrained to two dimensions the ensemble will also start rotating and persist in this motion for tens of hours. Self-organized vortex states in biological systems (flocking birds, schools of fish, bacteria) have been seen before but not in deformable units as here. A chemical wave (of the organic molecule cyclic AMP) probably brings the cells together in the first place, but thereafter the vortex behavior seems to be guided by inter-cellular cohesion. There is so far no explanation why the cells proceed in this manner, but the vortex motion might aid in the process of sorting cell types following differentiation. (Rappel et al., Physical Review Letters, 9 August 1999; contact Herbert Levine, 858-534-7697, levine@herbie.ucsd.edu; movies and simulations at http://herbie.ucsd.edu/~levine/dicty.html.)

X-RAY CRYSTALLOGRAPHY OF NON-CRYSTALS has been carried out by a group at Stony Brook. X rays have long been used to determine the structure of crystalline objects: when the waves strike periodic arrays of atoms or molecules the waves diffract into patterns which, when analyzed by Fourier-transformation algorithms, provide a map of the sample's structure with approximately angstrom resolution. In the Stony Brook experiment x rays are shone onto a non-crystalline micron-sized specimen (a tiny array of letters spelled out with 100-nm gold nanoparticles). By pushing the algorithms a bit, images could be formed from the x rays scattered from this patently non-crystal object. The resolution, about 75 nm, is not nearly as good as for traditional x-ray crystallography, but still much better than could be achieved with visible light. The researchers believe their method can be applied to imaging biological specimens at the level of cells or even subcelluar objects. (Miao et al., Nature, 22 July 1999.)