Wouldn’t you think that the decay rates of isotopes found on Earth would remain fairly constant under controlled conditions? Statistically-speaking one would be able to make a pretty good prediction about a radioactive element’s decay rate at any point in the future, regardless of external influences. However, a group of researchers have found the radioisotope decay rates of radium (226Ra) and silicon (32Si) varies periodically. This may not seem strange at first, but when measured, this fluctuation in decay rate has a period of approximately a year. Does this relate to the Earth’s position in its orbit? Does this mean radioactive decay rates are influenced depending on how far the element is from the Sun? Perhaps decay rates are not as predictable as we think…
Generally speaking, the decay rates of radioisotopes should remain pretty constant regardless of external forces or drivers. However, in the 1980’s, scientists in Brookhaven National Labs in the US and at the Physikalisch-Technische Bundesandstalt in Germany found some strange and unexpected variations in the decay rates of silicon-32 and radium-226. No cause was found and a pattern didn’t appear to exist. That was until Jere Jenkins and colleagues from Purdue University, Indiana, made a stunning discovery.
Before we go into what they have uncovered, we’ll briefly discuss radioactive decay:
Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide [...] This is a random process on the atomic level, in that it is impossible to predict when a given atom will decay, but given a large number of similar atoms, the decay rate, on average, is predictable. The SI unit of radioactive decay (the phenomenon of natural and artificial radioactivity) is the Becquerel (Bq). – Wikipedia: radioactive decay
In the case of 32Si, this isotope decays via beta-emission (i.e. high energy electrons or positrons are generated) and the 226Ra isotope decays via alpha-emission (i.e. high-energy helium nuclei containing two protons and two neutrons). Regardless of the type of emission, the radioactive decay rate should be predictable and certainly should not be influenced by an external driver, as the 2008 publication explains:
For 32Si and 226Ra, which decay by beta- and alpha-emission, respectively, fluctuations in the counting rates (in the absence of strong external electromagnetic fields) should thus be uncorrelated with any external time-dependent signal, as well as with each other. – Jenkins et al. 2008
With this in mind, the paper’s findings may seem pretty strange. Not only is there a periodic variation in the decay rates of the 1980’s 32Si and 226Ra independent samples, their periods appear to be correlated. But the best bit is yet to come; guess how long the observed period is? One year.
As the Earth orbits the Sun, its orbital radius varies slightly. At closest approach (perihelion), the Earth is approximately 147,000,000 km (0.98 AU) from the Sun and at aphelion the Earth is about 152,000,000 km (1.02 AU) from the Sun. This means that the Sun-Earth distance varies by approximately 5 million kilometres (0.04 AU); could this be the reason behind the annual modulation in decay rates? It would certainly explain the annual periodicity and it would also explain why all the independent samples are correlated. So what mechanism would be sensitive to this small variation in distance?
The Perdue group have a couple of ideas. Firstly, they refer to work recently done by John Barrow and Douglas Shaw which takes the standpoint that the fundamental constants of nature may not be fundamental nor constant. In this vane, they indicate that that the fine structure of the fabric of space-time may alter with distance from the Sun, thereby varying the “fundamental constants” slightly. Perhaps the decay rates of radioactive isotopes are influenced by this variation too.
Another idea is that decaying particles may be affected by neutrino flux. As you move further away from the Sun, the neutrino flux will decrease (following a 1/r2 decay), perhaps the annual modulation in neutrino flux is to blame. There appears to be some observational evidence for the neutrino explanation too. During the December 13th 2006 solar flare, the Perdue team measured a variation in decay rate. Some research suggests neutrino emission is altered by flare activity, so it would be interesting to see whether there is a flare-neutrino-decay rate link.
So the moral of this story is? The predictable nature of radioactive decay has just become a little more unpredictable…