Here's an interesting conundrum involving nuclear decay rates.
We think that the decay rates of elements are constant regardless of the ambient conditions (except in a few special cases where beta decay can be influenced by powerful electric fields).
So that makes it hard to explain the curious periodic variations in the decay rates of silicon-32 and radium-226 observed by groups at the Brookhaven National Labs in the US and at the Physikalisch-Technische Bundesandstalt in Germany in the 1980s.
Today, the story gets even more puzzling. Jere Jenkins and pals at Purdue University in Indiana have re-analyzed the raw data from these experiments and say that the modulations are synchronized with each other and with Earth's distance from the sun. (Both groups, in acts of selfless dedication, measured the decay rates of silicon-32 and radium-226 over a period of many years.)
In other words, there appears to be an annual variation in the decay rates of these elements.
Jenkins and co put forward two theories to explain why this might be happening.
First, they say a theory developed by John Barrow at the University of Cambridge in the UK and Douglas Shaw at the University of London, suggests that the sun produces a field that changes the value of the fine structure constant on Earth as its distance from the sun varies during each orbit. Such an effect would certainly cause the kind of an annual variation in decay rates that Jenkins and co highlight.
Another idea is that the effect is caused by some kind of interaction with the neutrino flux from the sun's interior, which could be tested by carrying out the measurements close to a nuclear reactor (which would generate its own powerful neutrino flux).
It turns out, that the notion of that nuclear decay rates are constant has been under attack for some time. In 2006, Jenkins says the decay rate of manganese-54 in their lab decreased dramtically during a solar flare on 13 December.
And numerous groups disagree over the decay rate for elements such as titanium-44, silicon-32 and cesium-137. Perhaps they took their data at different times of the year.
Keep em peeled because we could hear more about this. Interesting stuff.
Ref: arxiv.org/abs/0808.3283: Evidence for Correlations Between Nuclear Decay Rates and Earth-Sun Distance