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Skriven 2005-09-30 07:06:48 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 747
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 747   September 28, 2005  by Phillip F. Schewe, 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
http://www.aip.org/pnu/2004/split/702-3.html ).
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 1000 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.  (Adrich et al., Physical Review
Letters, 23 September 2005.)
                                                
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.  (Kimmel et al., Physical Review Letters.)

VISA PROBLEMS CONTINUE FOR FOREIGN STUDENTS attempting to enter physics
departments at US universities.  A new survey conducted by AIP's Statistical
Research Center shows that in 2004 half the PhD-granting physics departments
reported that at least one admitted student was either denied a visa or
considerably delayed by visa problems.  About 60% of the departments also
reported visa problems for foreign students returning to the US after trips
abroad.  The AIP survey estimates that ultimately 12% 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%.  The falloff in foreign graduate physics enrollment is matched by a
substantial increase in US students admitted: a growth of 42% in four years. 
(Report text at http://www.aip.org/statistics/trends/intltrends.html; contact
Patrick Mulvey or Michael Neuschatz at stats@aip.org )

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 * Origin: Big Bang (1:106/2000.7)