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Number 397, October 16, 1998 by Phillip F. Schewe and Ben Stein
NOBELIUM-254 IS THE HEAVIEST ELEMENT TO BE STUDIED IN DETAIL. The natural elements run out at uranium (92 in the Periodic Table), but physicists can go further by artificial means. At Argonne's ATLAS accelerator, projectile calcium nuclei plow into target lead nuclei. Most of the time the product nucleus fissions immediately, but in a small fraction of the collisions, a nucleus of nobelium (element 102) is formed. The very heavy isotope No-254, with 102 protons and 152 neutrons, has a relatively long lifetime of 55 seconds, but it is difficult to study since it is produced amidst a welter of fission products from other nuclei. At Argonne the detection process is twofold. First, the No-254 is created within a device called the Gammasphere, a 10-foot-across spherical ball lined with gamma-ray detectors, which sample the high-energy photons cast off by the newly created, highly excited (and often rapidly spinning) nuclei. Second, the forward-going No-254 specimen is steered through a fragment mass analyzer where it follows a course favoring only such heavy nuclei. Eventually it buries itself in a silicon detector where it is identified by its characteristic decays into lighter nuclei. The signals from the silicon detector are coordinated with those from the upstream Gammasphere, so that the gammas which came from the nobelium can be attributed to that nucleus alone. Among nuclei for which gamma specta exist, Fermium-256 (element 100) is the heaviest isotope, but Nobelium-254 is the heaviest element (102). The gamma information reveals that (1) the nobelium is born prolate (squashed from the spherical by about 20%) and (2) that a nucleus this big can sustain a spin as large as 14 quantum units. These findings will be reported (contact Robert Janssens, 630-252-8426) at the meeting of the APS Division of Nuclear Physics in Santa Fe, October 28-31. (see figure at Physics News Graphics.)
THE 1998 NOBEL PRIZE FOR CHEMISTRY goes to Walter Kohn of the University of California at Santa Barbara and John A. Pople of Northwestern University for their contributions toward establishing computational chemistry. The Nobel citation quoted P.A.M. Dirac to the effect that although the basic quantum laws governing large parts of physics and chemistry are known, progress will still be obstructed by the fact that the pertinent equations are too difficult to solve. Kohn's solution was "density-functional theory," which describes atomic and molecular bonding not by accounting for the motions of all the participating electrons, but rather by specifying the effective density of electrons, making the whole problem much more computationally tractable. Pople wrote a number of computer programs over the years combining new quantum chemistry insights with the increasing power of computers. Both Kohn and Pople are as much physicist as chemist. Kohn was head of the Institute of Theoretical Physics in Santa Barbara for 1979-1984.
NULLED STARLIGHT. Under the right circumstances light from two separated telescopes can be combined to create a signal whose spatial resolution is better than that for either of the single telescopes. This interferometry technique can also be used in reverse: the light paths for the two beams can be adjusted to create not a maximum but a minimum. Thus the star's light can be nulled out. Astronomers have demonstrated this principle by canceling the image of the star Betelgeuse, leaving behind the faint glow of a surrounding dust nebula. The researchers expect that with adaptive optics, a ground-based nulling system could be used to image Jupiter-sized planets around nearby stars beyond the sun by subtracting the distracting stellar glare. (Hinz et al., Nature, 17 September 1998.)
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