Number 661, November 11, 2003
by Phillip F. Schewe, Ben Stein, and James Riordon
The Furthest Manmade Thing
The furthest manmade thing, the Voyager 1 spacecraft, has recently
detected a change in its local environment in the form of a greatly
enhanced density of energetic particles. Two papers published last week
in Nature give different interpretations of the change. Krimigis et
al. believe that Voyager has finally begun to encounter (at a distance
of 85 astronomical units or 85 times the Earth-Sun distance) our solar
system's "termination shock," the region of space where the
outward going river of solar particles flags from supersonic to subsonic
speeds in its confrontation with the interstellar medium. One would
expect the shock front to be a good accelerator of particles, and the
observed upswing in fast particles is suggestive. McDonald et al., however,
argue that Voyager has not yet reached the termination shock, citing
the relatively unimpressive presence of one species of energetic particles,
namely so called anomalous cosmic rays. (Nature
6 November 2003.) Voyager 1 and its twin, Voyager 2 (some 20 AU behind
in the effort to leave the solar system) were launched way back in 1977.
ITER Tops the DOE List of Facility Priorities
Yesterday the US Department of Energy released a list specifying priorities
for construction or upgrading of science facilities over the next 20
years. The International Thermonuclear Experimental Reactor (ITER),
which would represent the next big generation in fusion reactor design,
occupied the number one slot on the list, followed by the Ultrascale
Scientific Computing Capability, a network which aims at greatly enhancing
high-end scientific computing. Four facilities were grouped together
as the number three priorities among near-term projects. They are the
Joint Dark Energy Mission, the Linac Coherent Light Source, the Protein
Production and Tags Facility, and the Rare Isotope Accelerator. (The
full report and list can be see at the US Dept. of Energy website)
A Better Look at Atomic Vibrations
The atoms that make up liquids, gases, and even solids are constantly
in motion. And in many substances, slight differences in the vibrations
of the constituent atoms may have important effects on macroscopic material
properties. For example, the motions of impurity atoms can determine
whether a material is a useful semiconductor, and measuring the motions
of atoms is critical to understanding high-temperature superconductivity,
colossal magnetoresistance (which has led to new, high capacity hard
drives), and numerous other important effects. Recently, a group of
researchers from Kyoto University, the Japan Synchrotron Radiation Research
Institute, the Japan Atomic Energy Research Institute, and Osaka University
of Education developed a new method that reveals differences in the
quantum oscillations of atoms that have, until now, been beyond detection
by conventional measurement techniques. The new approach is a refinement
of nuclear resonant inelastic scattering, which relies on x-ray radiation
from particle beam machines known as synchrotrons to excite atoms, which
in turn emit characteristic gamma radiation. Although previous techniques
could identify various elements in a material, they were unable to distinguish
between identical atoms bound in different atomic configurations.
The researchers have found that by exciting atoms with a pulse of synchrotron
radiation and observing oscillations, or quantum beats, in the time
spectrum of radiation that the atoms emitted, they could measure the
ratio of atoms in various environments in a material. Specifically,
the group studied iron atoms in a common iron oxide known as magnetite.
Two thirds of the atoms in the magnetite sample are surrounded by six
oxygen atoms, and the remaining third are surrounded by only four oxygen
atoms. The quantum beats in the gamma radiation signal, which was emitted
as a result of the nuclear resonant inelastic scattering, clearly revealed
the ratio of iron atoms in the two different atomic environments. The
researchers (Makoto Seto,
+81-724-51-2445) explain that the new method can be extensively applied
to studying the differences in the dynamical properties of atoms in
complex condensed matter and large biological molecules, among other
substances, leading to a better understanding of the characteristics
of such materials. (M.
Seto et al., Physical Review Letters, 31 October 2003)
Gallium Clusters are Too Small to Melt
Nanoscopic clusters of gallium atoms, consisting of as few as 17 atoms,
melt at much higher temperatures than bulk gallium, according to recent
research at the Indiana University. The observation runs counter to
theoretical expectations of melting points for small clusters. In fact,
current theory suggests that the melting point should fall as a cluster
size is reduced, and that nanoscopic lumps of many materials should
be liquid at room temperature. In previous work, the researchers (Martin
Jarrold, 812-856-1182) discovered similar trends in the melting
of tin clusters, but did not observe melting transitions directly. Instead
they monitored the shapes of small clusters to determine their state.
In the recent experiment, the researchers launched the gallium clusters
through a high pressure collision cell where they were heated in collisions
with a helium buffer gas. By monitoring the portion of dissociated clusters
that exited the collision cell, the researchers could directly determine
the clusters' melting temperatures. While bulk gallium melts at 303
K, thirty-nine and forty atom gallium clusters melt at about 550 K,
and seventeen atom clusters show no sign of melting at temperatures
as high as 800 K. No theoretical framework currently exists to explain
the high melting temperatures of gallium clusters. The researchers explain
that their measurements may have important implications for nanotechnology
and material science. In particular, nanoscopic clusters may not sinter
at low temperatures if they don't melt as predicted by established theory.
(G. A. Breaux et al., Physical
Review Letters, upcoming article)
CLARIFICATION: Last week's update, number 660, was incorrectly
labeled as number 661 in some mailings. This Update is number 661.
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