Evidence against solar influence on nuclear decay constants

Abstract

The hypothesis that proximity to the Sun causes variation of decay constants at permille level has been tested and disproved. Repeated activity measurements of mono-radionuclide sources were performed over periods from 200 days up to four decades at 14 laboratories across the globe. Residuals from the exponential nuclear decay curves were inspected for annual oscillations. 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.

Keywords: Decay constant; Half-life; Neutrino; Radioactivity; Sun; Uncertainty.

Fig. 1

A. Annual average residuals from exponential decay for ²²⁶Ra activity measurements with an IC at PTB from 1983 to 1998. The line represents relative changes in the inverse square 1/R² of the Earth–Sun distance, normalised to an amplitude of 0.15%. B. Same for ²²⁶Ra activity measurements with the Vinten IC of NPL from 1993 to 2016, after renormalisation per calendar year.

Fig. 2

A. Annual average residuals from exponential decay for ¹³⁴Cs activity measurements with the IG12 IC at the JRC from 2010 to 2016. B. Same for ²²Na.

Fig. 3

Amplitude of average annual oscillations in the decay rates of ²⁴¹Am and ¹⁵²Eu measured by γ-ray spectrometry with 8 HPGe detectors at SCK between 2008 and 2016. The index refers to the detector number. A mixed ²⁴¹Am– ¹⁵²Eu point source was measured 166–466 times in a fixed geometry at about 11 cm from the endcap using the 59 keV line of ²⁴¹Am and the 122 keV, 779 keV and 1408 keV lines of ¹⁵²Eu.