Around 1986, a 4-year long experiment to measure the radio-active decay rate of Silicon 32, revealed an unexpected variation of that rate with the time of the year.
This is anathemic to the conventional science of immutable radioactive decay rates – indeed some of the most accurate clocks have been based on decay rates.
Thus this was one of my favourite unexplained physics results, requiring some new physics – maybe it was caused by solar neutrino flux, the sun being a little closer during certain seasons?
Later studies even suggested some decay rate reverse correlation with pre-emergence of sun-spots.
However, recent experimental measurements have been specifically designed to rule out any seasonal variation in the performance of radiation detectors, because studies have shown that the decay variations of different elements are highly correlated, but detector-specific. Disappointingly, if such a word is appropriate in science, the definitive studies with the most advanced equipment now rule out any variation above the 0.01% level.
But there is a way to alter a radioactive decay rate – see my blog on the Quantum Zeno effect.