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Articles: Pulsed natural reactor; radiological evidence of nearbysupernova
I thought these would be of interest. Physics is
phun.
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-----Original Message-----
From: physnews@aip.org [mailto:physnews@aip.org]
Sent: Wednesday, October 27, 2004 11:48 AM
To: Jacobus, John (NIH/OD/ORS)
Subject: Physics News Update 706
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)
. . .
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)
***********
PHYSICS NEWS UPDATE is a digest of physics news items
arising from physics meetings, physics journals,
newspapers and magazines, and other news sources. It
is provided free of charge as a way of broadly
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physicists. For that reason, you are free to post it,
if you like, where others can read it, providing only
that you credit AIP. Physics News Update appears
approximately once a week.
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"A devotee of Truth may not do anything in deference to convention. He must always hold himself open to correction, and whenever he discovers himself to be wrong he must confess it at all costs and atone for it."
Monhandas K. Gandhi, in "Autobiography"
-- John
John Jacobus, MS
Certified Health Physicist
e-mail: crispy_bird@yahoo.com
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