Number 747, September 29, 2005
by Phil Schewe and Ben Stein
Nuclear Seismology
Physicists at the GSI lab in Darmstadt, Germany,
have discovered a new excited nuclear state, one in which a tide of
neutrons swells away from the rest of the nucleus. Ordinarily, in
its unexcited state, a typical atomic nucleus consists of a number
of constituent neutrons and protons (collectively known as nucleons)
bobbing around inside a roughly spherical shape. However, if struck
by a projectile from outside, such as a beam particle supplied by an
accelerator, the nucleus can be set to spinning, or it might
distend. In one kind of excited mode called a dipole resonance, the
protons can move slightly in one direction while the neutrons go the
other way. In another type of excitation, a nucleus might consist
of a stable core blob of nucleons surrounded by a surplus complement
of one or two neutrons, which constitute a sort of halo around the
core (see PNU 702).
In the new GSI experiment, yet another nuclear mode has been
observed. The nuclei used, two isotopes of tin, are the most
neutron-rich among the heavier nuclei that can be produced at this
time. Sn-130 and Sn-132 are so top-heavy with neutrons that they
are quite unstable and must be made artificially in the lab. At GSI
this is done by shooting a uranium beam at a beryllium target. The
U-238 nuclei, agitated by the collision, eventually fission in
flight, creating a swarm of more than 1,000 types of daughter nuclei,
from which the desired tin isotopes can be extracted for study. The
tin nuclei are excited when they pass through a secondary target,
made of lead. The excited tin states later disintegrate; the debris
coming out allows the researchers to reconstruct the turbulent
nature of the tin nuclei. The dipole resonance was seen, as
expected, but also a new resonance: an excess of neutrons pushing
off from the core nucleus. Furthermore, the neutron resonance
appears at a lower excitation energy than does the dipole
resonance. Team leader Hans Emling (h.emling@gsi.de) says that
there was some previous evidence for the existence for the neutron
mode in work with lighter nuclei, but not the actual oscillation
observed in the present work.
Hydrophobic water sounds like an impossibility. Nevertheless,
scientists at Pacific Northwest National Lab have produced and
studied monolayers of water molecules (resting on a platinum
substrate) which prove to be poor templates for subsequent ice
growth. Picture the following sequence: at temperatures below 60 K,
isolated water molecules will stay put when you place them on a
metallic substrate. At higher temperatures, the molecules become
mobile enough to begin forming into tiny islands of two-dimensional
ice. New molecules landing on the crystallites will fall off the
edges into the spaces between the islands. In this way the metal
surface becomes iced over completely with a monolayer. But because
the water molecules' four bonds are now spoken for (1 to the Pt
substrate and 3 to their neighboring water molecules), the addition
of more water does not result in layer-by-layer 3D ice growth. Only
when there is an amount of overlying water equivalent to about 40 or
50 layers does 3D crystalline ice completely cover the hydrophobic
monolayer. The PNL researchers (contact Greg Kimmel, 509-376-2501,
gregory.kimmel@pnl.gov) are the first to observe this effect. For
the novel hydrophobic property to show itself, the water-substrate
bond has to be strong enough to form a stable monolayer. Weaker
bonding results in a "classic" hydrophobic state, in which the water
merely balls up immediately; in other words, not even a first
monolayer of ice forms. This research should be of interest to
those who, for example, study the seeding of clouds, where ice is
nucleated on particles in the atmosphere.
Visa problems continue for foreign students attempting to enter
physics departments at U.S. universities. A new survey conducted by
AIP's Statistical Research Center shows that in 2004 half the
Ph.D.-granting physics departments reported that at least one admitted
student was either denied a visa or considerably delayed by visa
problems. About 60 percent of the departments also reported visa problems
for foreign students returning to the U.S. after trips abroad. The
AIP survey estimates that ultimately 12 percent of admitted foreign physics
graduate students in the Fall of 2004 were (at least initially)
denied entry. This actually represents an improvement; in 2002 the
same fraction was 20 percent. The falloff in foreign graduate physics
enrollment is matched by a substantial increase in U.S. students
admitted: a growth of 42 percent in four years.
More information and the full text of the report on the
AIP Statistical Research site
Contact Patrick Mulvey or Michael Neuschatz at stats@aip.org
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