Text 225, 76 rader
Skriven 2004-10-27 17:41:02 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 706
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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@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@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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 * Origin: Big Bang (1:106/2000.7)