Great idea and great job with these posts. Now that you have so eloquently pointed out one reason why nuclear decay rates cannot vary, this might be a good place to discuss the Oklo reactor, proof of an old earth.
quote:For those who need a refresher on how a nuclear reactor works, and why U-235 is necessary and why U-238 by itself isn't sufficient see this link, 2nd page.
Natural Uranium is almost all the 238 isotope, with small amounts of the 235 isotope (and trace amounts of U-234 which is unimportant to this discussion). You need a 3% concentration U-235/U-238 to initiate chain reactions. Weapons grade Uranium contains 90% U-235.
Dr. Bouzigues did a routine measurement on the composition of ores for a reprocessing plant in France, June 2, 1972. He noticed that some samples displayed a 235 to 238 ratio of 0.717% instead of the 0.720% usually found in all terrestrial samples - and even in meteorites and moon rocks.
The discrepancy was traced back to the mine site seams. Chemical reactions cannot account for isotopic variations, only nuclear reactions and decays can do that. The depleated seams of U-235 contained the distinctive pattern of 30 or more other atomic elements that are formed as by-products of nuclear fission reactions.
The tell-tale signature of nuclear fission products is known from man-made reactor experiments. Some of the elements present, like neodymium, have many isotopes but not all are fission products. The non-fission products provide a gauge of the abundance of all the isotopes before the natural reactions began and so enable the determination of the effects and running time of those reactions.
A detailed geochemical survey of the site was carried out and 15 fossilized ancient reactor sites were found, 14 at Oklo and 1 at Bangombe 35km to the south. Reactors are controlled by introducing a moderator like graphite or water. A moderator is necessary to slow down the fast neutrons created during fission to the thermal energy range so as to increase their efficiency in causing further fissions in U-235.
Nuetrons are emitted with high speeds and are readily absorbed by U-238 nuclei. They need to be slowed down in order to be absorbed by U-235 in order to sustain a chain reaction. Although today the natural abundance of U-235 relative to U-238 is about 0.7%, the ratio of the two isotopes has not been constant throughout the past.
They both decay slowly but at different rates. The half-life of U-235 is about 700 million years while that of U-238 is about 4.5 billion years. The faster decay of U-235 means that there was more U-235 relative to U-238 in the past than there is today.
So to have the 3% ratio of U-235 to U-238 necessary to carry on a reaction moderated by water the natural reactor had to be 2 billion years old. The original source of U in the earth was quite small, just a few parts per million in the earth's make-up. The Oklo natural reactors were made possible by the deposition of a Uranium rich seam inside a layer of sandstone lying on top of sheets of granite.
The granite layers are tilted at about 45 degrees and this led to a build up of rainwater and soluble uranium oxide deep underground at the bottom of the slope. The oxidizing environment needed to create the water required to concentrate the Uranium was brought about by a significant change in the earth's biosphere.
About two billion years ago a change of atmosphere occured, brought about by the evolution of blue-green algae, the first organisms able to carry out photosynthesis. Their activity increased the oxygen content of the water and allowed some of the Uranium to change into soluble oxides.
The reactor heated the water turning it to steam at which the nuetrons, no longer moderated (slowed down), were no longer absorbed by the U-235, only by the U-238 and so the chain reaction shut off. This allowed things to cool down whereupon the water condensed again and the reaction switched back on.
By the amount of fission by products it is possible to determine that this process of stop go activity seems to have been repeated intermittently over nearly a million years, with episodes of chain reactions lasting for periods varying from just a few years to thousands of years before the reactor finally switched itself off.
So to summarize, there is no way for the current natural concentration ratio between U-235/U-238 of 0.7% to carry out a chain reaction. There is no chemical means to selectively concentrate one isotopic form of an element relative to another isotope of that element.
This means that the natural reactor has to be 2 billion years old in order for an isotopic ratio to exist to allow a chain reaction to occur. The by products of the chain reaction alone required this reactor to be in operation for nearly a million years to accumulate.
So again the earth is old.
Edited by shalamabobbi, : formatting
Edited by Admin, : Replace hash lines with horizontal rule.