The Oklo reactor and a varying fine structure constant April 18, 2007Posted by dorigo in Blogroll, books, physics, science.
In the process of slow absorption of Ray Kurzweil’s enlightening book The Singularity is Near, I learned about the existence of a natural nuclear reactor in Oklo (Gabon – see aerial picture above), which was active until about 1.5 billion years ago, and was discovered in 1972 by Francis Perrin.
A natural nuclear reactor! I remember thinking about the possibility that something like that could occur in a uranium ore while reading The Making of the Atomic Bomb, an entertaining account of the history of the first years of nuclear physics by Richard Rhodes.
The physics of nuclear reactions is complex, but a few basic facts are sufficient to understand what happens with uranium, which in nature is a mixture of relatively stable U-238 isotopes and a small percentage (0.72%) of more reactive U-235 isotopes. U-235 undergoes spontaneous fission into two nuclear fragments by emitting a few energetic neutrons, a process that can also be triggered by the collision with a fast neutron. A nuclear chain reaction occurs spontaneously in uranium if the neutrons emitted in the fission of a U-235 nucleus have a sizable probability of producing another fission before being captured by a U-238 nucleus or escaping the fissile material. The probability that a neutron initiates another fission of course grows with the fraction of U-235 in the uranium mass and with the amount of substance, but it also critically depends on other subtle conditions.
I am not a nuclear physicist so I will abstain from attempting a thorough explanation of the many details here. One thing suffices: water is a substance which has the potential of slowing down the neutrons emitted by U-235 to a speed which is just about right for maximum fission probability when they hit another U-235 atom.
What is thought to have happened two billion years ago in Oklo and in a few other sites nearby is that water infiltrated into the uranium ore, effectively moderating the neutrons continuously produced by the sub-critic mass of U-235, and turning on a chain reaction. The generated heat would boil the water away, turning the reaction off until new water seeped through the vein.
Physics is fascinating… And the dynamical equilibrium that prevented a meltdown of the Oklo reactor, generating periodic amounts of heat, is intriguing indeed. One thing to note: such things cannot happen nowadays, when the higher decay rate of U-235 has made it four times less abundant in uranium ore than it was two billion years ago… Conditions were just about right back then, with a 3.5% fraction of U-235 which closely matches that of fission reactors running on enriched uranium. Also, the pressure deep underground certainly helped.
Physicists who analyzed the substances collected on the Oklo site believe that the reaction went on for several hundred thousand years, and they actually are able to measure many details of those ancient events – the duration of each reaction, the generated temperature, and more. They do so by looking at the relative abundance of several tale-telling isotopes, ones which are produced by secondary products of the reaction (such as 142-Neodymium or 99-Ruthenium) and ones which disappear from the ore because of that (such as, of course, U-235, whose concentration in uranium ore was found to be as small as 0.4% with respect to the usual 0.72% of present-day uranium ore).
What is interesting is that the amount of specific isotopes (such as Samarium-149)appears to allow a measurement of the value that the fine structure constant, labeled by the greek letter alpha, had two billion years ago. This is possible because the cross section of neutron capture by that substance changes with the fine structure constant, and the concentration of Sm-149 becomes a yardstick for the latter.
What if alpha was different from now ? Well, since alpha is the square of the electric charge of electrons divided by the product of Planck’s constant by the speed of light, finding a smaller alpha would imply that the speed of light was larger, unless you were to buy the even steeper hypothesis of a changing electron charge or a changing Planck’s constant.
Measurements in Oklo ore were actually used for a while to demonstrate that the speed of light was constant in the last two billion years, but a 2004 paper by Steve Lamoreaux and Justin Thorgerson of Los Alamos National Laboratories [PRD 69 (2004), 121701] appeared to show a decrease of alpha by 45 parts in a billion. They used a more realistic spectrum of the energy of neutrons in the reactor than what had been done in previous studies, and claimed their result was thus more precise.
The whole thing is certainly fascinating! For a longer and better account of the story and some additional insight in other measurements of alpha in the far past, see this fine article in the New Scientist’s site. The most recent material on the issue, however, appears to be Nucl-Ex/0701019 , where Steve Lamoreaux and collaborators revise their own previous estimates, and now put a more stringent (although admittedly model-dependent) bound on the variation of alpha,
-0.11 <delta(alpha)/alpha < 0.24
in 10^-7 units, at 2-sigma level. So the latest data is consistent with the fine structure constant being, in fact, a constant, but small changes are not ruled out yet. Indeed, the matter is complex, and the final word might not have been said on the Oklo reactors yet…
While we wait for it, why not considering that a sizable number of respected, mainstream physicists have been devolving years of research time in the attempt at determining a variation of alpha, which is basically at the same level of heresy as what Louise Riofrio has been proposing for a while ?
What I mean to say the lesson is: let the data speak one way or another rather than preemptively throw the first stone! No true scientist (bureaucrats and lackeys do not belong to the category of course) can ever be completely crackpot-free. We all, in fact, share an attraction to the bold, revolutionary idea. Be it a varying speed of light or a tiny but nonzero amount of energy filling empty space, it still fascinates us, until it is proven false.