Evidence against solar influence on nuclear decay constants
Denis E. Bergeron, Ryan P. Fitzgerald, Leticia S. Pibida, Stefaan Pomme
The exponential decay of radionuclides as a function of time is a cornerstone of nuclear physics and radionuclide metrology. Decay constants for spontaneous radioactive decay are considered invariable in time and space. This convenient trait allows projecting activity values to a point of time in the past or the future. In the past decade, evidence has been published of annual modulations in measured radioactive decay rates and new physics was postulated claiming that decay constants are susceptible to solar influence. Here we disprove the hypothesis that the seasonal variations in Earth-Sun orbital distance affects decay constants at the permille level. We collected a unique set of repeated decay rate measurements of mono-radionuclide sources performed over periods from 200 days up to four decades at 13 laboratories across the globe. Systematic deviations from a purely exponential decay curve differ from one data set to another and are attributable to instabilities in the instrumentation and measurement conditions. The most stable activity measurements of alpha, beta-minus, electron capture, and beta-plus decaying sources set an upper limit of 0.0006 % to 0.008 % to the amplitude of annual oscillations in the decay rate. Oscillations in phase with Earth's orbital distance to the Sun could not be observed within a 10-6 to 10-5 range of precision. There are also no apparent modulations over periods of weeks or months. Consequently, there is no indication of a natural impediment against sub-permille accuracy in half-life determinations, renormalisation of activity to a distant reference date, application of nuclear dating for archaeology, geo- and cosmochronology, nor in establishing the SI unit becquerel and seeking international equivalence of activity standards.