Posted by jlwile on August 20, 2012
Almost three years ago, I wrote about how I had changed my mind on radioactive half-lives. Throughout my scientific education (from high school through graduate school), I had it pounded in my head that radioactive half-lives are constant. There is so much energy involved in radioactive decay that there is just no way to change the fundamental rate at which a given radioactive isotope decays without taking extreme measures that don’t generally occur in nature. This was considered a scientific fact, and to question it was just not reasonable.
Over the years, however, more and more evidence has been piling up indicating that this scientific “fact” is simply not true. Some of the most surprising evidence has come from Brookhaven National Laboratory (BNL) and a German lab known as the Physikalisch-Technische Bundesanstalt (PTB). The group at BNL had been studying the radioactive decay of silicon-32, and they noticed that the half-life of the decay periodically increased and decreased based on the time of year. The half-life was shortest in the winter and longest in the summer. The variations were very small, but they were measurable. The PTB group was studying the decay of radium-226, and they noticed the exact same behavior. In the end, both groups concluded that the half-lives of these two isotopes were changing slightly in direct correlation with the minor variation in the distance between the earth and the sun. Thus, they concluded that the sun was affecting the rate of decay in those two isotopes.1
This conclusion was bolstered by a fortunate coincidence in which the BNL group was measuring the radioactive decay of manganese-54 before, during, and after the solar flare that occurred on December 13, 2006. They noticed that the half-life of that isotope’s radioactive decay increased more than a day before the solar flare occurred. In addition, the behavior repeated itself on December 17, when another solar flare occurred.2 Based on these two papers, it seemed obvious that the sun was exerting some influence over the half-lives of at least some radioactive isotopes.
Obviously, of course, others tried to replicate these results, and they weren’t always successful. A group at the University of California Berkeley analyzed their data for several different radioactive isotopes but saw no correlation between their half-lives and the seasons.3 However, a reanalysis of the same data seemed to show some variation correlated with the distance between the earth and the sun, although it was much weaker than what was seen by BNL and PTB. The authors of the reanalysis suggested that perhaps the influence of the sun was different for different isotopes. Since different isotopes have different half-lives, it makes sense that they would respond differently to an outside influence such as the sun.4
Well, some new data have come to light, and as far as I can tell, they confirm that at least for some radioactive isotopes, the sun is affecting the value of their half-lives.
These data come from a third lab, The Ohio State University Research Reactor. This lab wasn’t doing experiments to determine whether or not radioactive half-lives are affected by the sun. Instead, as a part of their standard routine, they use a detector to measure the background radiation in the lab. On a weekly basis, however, they calibrate that detector by using it to measure the amount of radiation coming from a standard chlorine-36 isotope. So here we have the same detector being used to measure background radiation as well as the activity of a chlorine-36 isotope. The paper shows that once again, the chlorine-36 isotope’s half-life varies annually, and once again, it is shortest in the winter and longest in the summer.5
Not only is this a different lab presenting the same results with yet another isotope, there is an extra piece of data in this study that makes the case rather ironclad. One of the obvious objections to all observations of annual changes in half-life is that these changes also correlate to the seasons. Since the seasons bring changing weather (which results in changes in the operations of a lab’s environmental controls), it is possible that all these observations are the result of changes in the detectors’ surroundings. If the surroundings change, the detectors’ responses might change, and that could account for what is being observed.
In this case, however, the detector was being used to detect both the background radiation and the activity of the chlorine-36 isotope. In Figure 4 of the paper, the authors show that the variation exists when the detector is measuring the chlorine-36 source, but it does not exist when the detector is measuring the background radiation. If the observed variation in half-life were the result of changes in the detector’s response, you would expect to see it in the background measurements as well. Since you don’t, it seems very clear that the changes observed in the half-life of the chlorine-36 isotope are real.
Indeed, some have decided that the case is so strong that they have proposed using this effect as an advanced warning system for solar flares. Solar flares can be very harmful to satellites, electronic systems, and even the power grid. An advanced warning system for solar flares would allow those in charge of such systems to take steps that might mitigate the damage.
Now while I do agree that the data strongly indicate the sun exerts an effect on the half-lives of some isotopes, there are still a lot of unanswered questions. For example, based on what has been observed so far, the solar effect exists only for one class of radioactive isotopes: those that emit beta particles.* Studies have shown no effect for isotopes that emit alpha particles. Why? In addition, it is clear that the variation in half-life is dependent on the isotope – some isotopes’ half-lives vary more than others. While it makes sense that this should be the case, we still don’t know why.
Of course, that brings me to the biggest question of all: “How does the sun exert an effect on some radioactive half-lives?” The answer right now is that we simply don’t know. Some have suggested that the neutrinos coming from the sun might be affecting the isotopes in some way. However, neutrinos are notoriously hard to detect specifically because they don’t interact with matter very well. Thus, it is hard to understand what they could be doing to the isotopes. Also, there is at least one experiment that casts some doubt on that explanation.
While the answers to these questions remain open, I think we can safely conclude one thing: we obviously don’t know as much about radioactive decay as we thought we did! After all, based on what we know about radioactive decay, most scientists concluded that radioactive half-lives are always constant. We now know that this is simply not true. As a result, we should be very skeptical of any conclusion that requires the assumption of constant radioactive half-lives!
1. JH Jenkins, E Fischbach, JB Buncher, JT Gruenwald, DE Krause, and JJ Mattes, “Evidence of correlations between nuclear decay rates and Earth-Sun distance,” Astroparticle Physics 32(1):42-46, 2009.
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2. JH Jenkins, and E Fischbach, “Perturbation of nuclear decay rates during the solar flare of 2006 December 13,” Astroparticle Physics 31(6):407-411, 2009.
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3. E. B. Norman, E. Browne, H. A. Shugart, T. H. Joshi, and R. B. Firestone, “Evidence against correlations between nuclear decay rates and Earth-Sun distance,” Astroparticle Physics 31(2):135-137, 2009.
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4. D. Javorsek, P. A. Sturrock, R. N. Lasenby, A. N. Lasenby, J. B. Buncher, E. Fischbach, J. T. Gruenwald, A. W. Hoft, T. J. Horan, J. H. Jenkins, J. L. Kerford, R. H. Lee, A. Longman, J. J. Mattes, B. L. Morreale, D. B. Morris, R. N. Mudry, J. R. Newport, D. O’Keefe, M. A. Petrelli, M. A. Silver, C. A. Stewart, and B. Terry, “Power spectrum analyses of nuclear decay rates,” Astroparticle Physics 34(1):173–178, 2010.
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5. Jere H. Jenkins, Kevin R. Herminghuysen, Thomas E. Blue, Ephraim Fischbach, Daniel Javorsek II, Andrew C. Kauffman, Daniel W. Mundye, Peter A. Sturrock, Joseph W. Talnagi, “Additional experimental evidence for a solar influence on nuclear decay rates,” Astroparticle Physics http://dx.doi.org/10.1016/j.astropartphys.2012.07.008 2012.
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* A reader asked for clarification on this point, since radium-226 emits alpha particles, not beta particles. While radium-226 does have a rare double-beta decay option, the real issue is that when radium-226 emits an alpha particle, the result (radon-222) is not stable and also decays. In fact, a whole chain of decays ensue, and some of those decays are beta decays. The practical result, then, is that a radium-226 source contains lots of different isotopes, all of which are decaying according to their own energetics. When the varying half-life effect was first seen, it was thought that the effect was caused by the alpha decay of radium-226. However, other isotopes that decay by alpha emission show no such effect. As a result, the authors have shown that the effect seen with the radium-226 source is actually a result of the beta decay of the isotopes produced in the decay chain.
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