A new study analyzing data from 1991-1996 beta decay measurements detects seasonal variations in beta decay rates of the same amplitude and phase over the course of the year across many different radioactive isotypes (the statistical significance of the result is eleven sigma).
A number of prior studies have seen hints of seasonal variation in beta decay rates, but none have been so definitive.
The researchers suggest an impact of solar neutrinos, whose flux through Earth varies seasonally, as a likely cause of the effect. They aso consider why the methodology used in the experimental results they study was more sensitive to these effects than other experiments which have not disclosed such a clear trend.
Footnote: The result also suggests an engineering application - either bombarding atoms with neutrinos, or shielding them from neutrinos, to tweak their beta decay rates, either making stable isotypes radioactive, or making unstable isotypes temporarily stable while in those conditions.