NOTE: Long after this article was published, new experimental data was published indicating that the effect is not real.
Over the course of my scientific career, I have been drug, kicking and screaming all the way, to the conclusion that radioactive half-lives have probably not been constant over the course of earth’s history. Because of this, I have written about observations that indicate the half-lives of certain isotopes seem to depend on the distance between the earth and the sun. The essence of the story is that investigators have been measuring the activity of certain isotopes over several years, and there seems to be a periodic variation in their half-lives. The half-lives increase and decreased based on the season. In addition, when a solar flare was observed, a marked decrease in the half-life of one isotope was observed. As I stated in my previous post on this subject, I think the researchers have done a good job eliminating the possibility that the observed variations are due to some artifact of the experimental procedure.
So if the observed variations in half-lives are real, what is causing them? Well, the sun emits tiny particles called neutrinos as a result of the nuclear fusion that is powering it. The distance between the earth and sun would affect how many of those neutrinos hit the earth. The closer the earth is to the sun, the more neutrinos would hit the earth. In addition, the number of neutrinos hitting the earth increases during a solar flare. The observations indicate that in both cases (during solar flares and when the earth is closest to the sun), radioactive half-lives
increase decrease. In other words, radioactive decay slows down speeds up when the sun is hitting the earth with more neutrinos. Based on this reasoning, some nuclear scientists have proposed that neutrinos coming from the sun are somehow inhibiting accelerating radioactive decay. [ADDITION (5/10/17): A colleague informed me that I had the proposed neutrino effect backwards, so I corrected the wording, as indicated by the deletion marks and boldfaced type.]
The viability of that explanation was recently tested by a rather clever experiment, and the results of the test indicate that neutrinos are probably not responsible for the observed variation in half-lives.
R.M. Lindstrom and colleagues analyzed the decay of Au-198, a radioactive isotope of gold. They took samples of the isotope and fashioned them into different shapes: spheres and flat foils. Since Au-198 actually emits a neutrino when it decays, the gold isotopes in the sphere are exposed to neutrinos emitted by other gold isotopes in the sphere. This isn’t the case for the isotopes in the flat foil, so in the end, the gold isotopes in the spheres are exposed to significantly more neutrinos than the gold isotopes in the flat foils. If neutrinos inhibited radioactive decay in some way, the gold isotopes in the spheres should have a higher half-life than the gold isotopes in the flat foils. When the measured half-lives were compared, however, they were found to be identical within experimental error.1
So if the observation that radioactive half-lives depend on the distance between the earth and the sun is correct, the reason is probably not related to solar neutrinos. Interestingly enough, in their desire to protect radioactive dating methods, some outlets are reading far too much into the results of this study. For example, physorg.com reports on this study with the following headline:
Research shows radiometric dating still reliable (again)
Actually, the study doesn’t address the reliability of radioactive dating at all. Neither does it address whether or not the observed variations in radioactive half-lives are real. It merely says that if the half-lives of certain isotopes do depend on the distance between the earth and the sun, it is probably not because of solar neutrinos.
In the end, then, here’s where we are on this issue: The authors who have observed variations in radioactive half-lives based on the distance between the earth and the sun as well as the sun’s activity have produced strong evidence that what they have observed is a real effect. However, the most popular proposed explanation for the effect is probably not correct. Thus, if the observed variation in radioactive half-lives is real, it is more of a mystery than ever.
1. R.M. Lindstrom, et al., “Study of the dependence of 198Au half-life on source geometry,” Nuclear Instruments and Methods in Physics Research A622:93-96, 2010
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