PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 762 January 19, 2006 by Phillip F. Schewe, Ben Stein
FISSION FRAGMENTS WEIGHED. The fissioning of uranium results in a variety of unstable neutron-rich nuclei. A team of scientists from the University of Jyväskylä in Finland has for the first time made high-precision mass measurements of a number of isotopes produced in proton-induced fission reactions of uranium, including strontium, zirconium, and molybdenum. These so called refractory elements are hard to study as ionized beams because of their high boiling points. Instead the researchers reach a high level of precision by coaxing the nuclei into a Penning trap, which employs a combination of a strong magnetic field and a static quadrupole fields to trap ions. In this kind of device the particle's mass can be deduced from the observed cyclotron motion---that is, from the particle's looping orbit in a strong magnetic field. The reason for wanting better isotope masses is that they provide information about nuclear binding energies. The mass of the simplest compound nucleus, the deuteron, for instance, is several million electron volts less than the sum of the masses of its constituent proton and neutron. The difference is the net binding energy. In the case of the new studies, the isotope masses are determined with a precision of thousands of electron volts. By measuring the mass of several Zr isotopes of increasing neutron numbers, one can see subtle effects in the complex structures of these nuclei. Astrophysicists, who consider how larger nuclei are built inside stars or novas also will be interested in knowing how nuclear mass increases with neutron number. (Hager et al., Physical Review Letters, upcoming article; contact Ari Jokinen, [EMAIL PROTECTED]) NUCLEAR MOLECULE: NATURE'S SMALLEST DUMBBELL. An oxygen molecule is a small dumbbell less than a nanometer across: two oxygen atoms with two electrons flying between acting as the bonding agent. Now, an international consortium has succeeded in making a dumbbell far smaller: a beryllium-10 nucleus consisting of two alpha particles (nuclear fragments containing two protons and two neutrons) with two neutrons flying between acting as a sort of nuclear bonding agency. This nuclear dumbbell is only a few fermis (10^-15 m) across (see figure at http://www.aip.org/png/2006/246.htm ). These tiny oblong nuclei are made by colliding a beam of helium-6 nuclei into a gas of helium-4 atoms. (The He-6 nuclei, which are themselves a novelty, were made by shooting protons at lithium.) The Be-10 nuclei created in this way don't live very long. With a lifetime of about 10^-21 seconds, they fly apart, usually back into He-4 and He-6 fragments. Martin Freer ([EMAIL PROTECTED]) says that the beryllium results support the idea that nuclei sometimes behave like atomic systems in that they can be thought of as a core of particles with extra "valence" particles (electrons/neutrons) exchanged between cores. Several exotic shapes are thought to be possible among the light nuclei. Carbon-12, for instance, can exist as a triangular arrangement of three alpha particles and oxygen-16 as a tetrahedron of alphas. But these nuclei are tightly bound, so their exotic geometry cannot be discerned. But Be-10's prolate shape can be seen clearly through the rotational behavior of the decaying system. Freer is part of a team from the Universities of Birmingham and Surrey (UK), Universite catholique de Louvain and University of Leuven (Belgium), Universite de Caen (France), and the Rudjer Boskovic Institute (Croatia). (Freer et al., Physical Review Letters, upcoming article) MAGNETIC MICRO-COILS for moving beads along a microfluidic track have been devised by scientists in Singapore. Microfluidics---the transport of small objects or fluids around a microchip often fabricated using the same lithographic techniques used for photonics or electronics---has many current and potential applications in materials science and in bio/medical studies. Researchers at three Singapore organizations, the Institute of Microelectronics, the Institute of Bioengineering and NanoTechnology, and the Nanyang Technological University, have actually constructed several different types of magnetized pillar structures which, when energized, and move beads tens of microns around a microfluidic chip. (Ramadan et al., Applied Physics Letters, 16 January 2006; contact Qasem Ramadan, [EMAIL PROTECTED]) *********** 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. 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