PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 706 October 27, 2004
by Phillip F. Schewe, Ben Stein
                                                                        
A NATURAL NUCLEAR REACTOR IN GABON.   Since uranium-235 undergoes
self-sustaining fission in commercial reactors and since uranium lies in
the Earth in great quantities, Paul Kuroda predicted that naturally
operating reactors are possible under special conditions.   Not nowadays,
when the ratio of uranium-235 to
uranium-238 is only about 0.7%, but in the past, when the ratio was much
higher owing to the fact that U-235 has a shorter half life than U-238. The
conditions necessary for self-sustained fission would be as follows: a
uranium deposit where U-235 was present at the 3% level (the level at which
modern reactors operate); the presence of material (such as water, carbon,
and most organic compounds) that could moderate, or slow down, the neutrons
issuing from fission reactions; and the absence of material (such as Fe, K,
Be, Gd) that would absorb the neutrons outright.  In 1972, such a natural
reactor was found at the Oklo mine in Gabon, in West Africa.  There a
2-billion-year-old uranium deposit some 5-10 meters thick and 600-900
meters wide was bathed by an ancient river.  This "reactor" is
reckoned to have released 15 giga-watt-years of energy and operated at an
average power of 100 kilowatts.  Now physicists at Washington University in
St. Louis have defined a likely mode of operation for this ancient reactor
and confirmed one of the proposed mechanisms of its self regulation. 
According to Alex Meshik (am{at}wustl.edu), the reactor cycled on (producing
heat that boiled the nearby water) typically for 30 minutes and then off
(when the now-scarce water failed to moderate the nuclear fission process)
typically for 2.5 hours.  This cycling saga is deduced from microscopic
mass-spectrometric
examination of the rock samples from the area.  Meshik says that tiny
alumophosphate grains found in the material of ancient reactor preserve a
signature of the reactor's operational mode.  "It is fascinating that
xenon isotopic composition measured today provides us with such pristine
timing records for a natural reactor operated 2 billion years ago"
(Meshik et al., Physical Review Letters, 29 October 2004)
                        
SWIMMING IN NEWTONIAN SPACE.   Michael Longo, a physicist at the University
of Michigan, suggests that a satellite going around the Earth can change
its orbit by proper gymnastic alteration of its shape, without the need for
any external force other than gravity. This proposition has been previously
made by invoking the properties of curved spacetime at the heart of general
relativity which predicted a very tiny effect (Science, 28 Feb and 21
March, 2003), but Longo shows that Newtonian physics predicts a much larger
effect.  Consider, he says, a dumbbell in Earth orbit.  The outer mass
feels slightly less gravity than the inner mass.  If the masses are pulled
together the differential force on the separate masses will cause the
center-of-mass of the dumbbell to move inward slightly.  How slightly?  For
a 100-meter-sized dumbbell in a highly elliptical orbit, the object's
perigee (the point of closest approach to Earth) can move inward 1 mm on
each pass.  In effect, the satellite could use solar energy or other stored
energy to change its orbit, without the need for propellant.  (American
Journal of Physics, October 2004)

SUPERNOVA DEBRIS ON EARTH, in the form of deposits of iron-60, a
radioactive isotope of iron occurring on our planet at much smaller levels,
has been studied by German physicists.  The same team of scientists
reported first signs of the deposits five years ago
(http://www.aip.org/pnu/1999/split/pnu437-1.htm).  Back then they analyzed
three layers of South Pacific sediment, each over 2 million years thick in
geologic time.  The new measurements, acquired at a site some 3000 km away,
are much more robust: 28 layers (rather than 3), from deeper depths (4830 m
rather than 1300 m), with a better dating method (beryllium-10 dating) and
a more accurate estimate of the layers' age (in some cases to within a few
100,000 years).  On the basis of their measurement, the researchers deduce
that the samples represent the remains of a star that exploded 2.8 million
years ago (with an uncertainty of 0.3 million years) at a distance from
Earth of some tens of parsecs.  What, if any, were the implications of this
splash of foreign matter at the time?  Gunther Korschinek at the Technische
Universitat Muenchen (gunther.korschinek{at}ph.tum.de) says that depending on
exactly how far away the supernova was, it might have had caused an
increase in cosmic ray flux for about 300,000 years.
(Knie et al., Physical Review Letters, 22 October 2004;
accelerator-analysis website at
http://www.bl.physik.uni-muenchen.de/gams/index.html)

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