Concerning the Time Dependence of the Decay Rate of 137Cs

The decay rates of 8 nuclides (85Kr, 90Sr, 108Ag, 133Ba, 137Cs, 152Eu, 154Eu, and 226Ra) were monitored by the standards group at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany, over the time frame June 1999 to November 2008. We find that the PTB measurements of the decay rate of 137Cs show no evidence of an annual oscillation, in agreement with the recent report by Bellotti et al. However, power spectrum analysis of PTB measurements of a 133Ba standard, measured in the same detector system, does show such evidence. This result is consistent with our finding that different nuclides have different sensitivities to whatever external influences are responsible for the observed periodic variations.

💡 Research Summary

The paper presents a comprehensive re‑analysis of long‑term decay‑rate measurements performed at the Physikalisch‑Technische Bundesanstalt (PTB) in Braunschweig, Germany, covering the period from June 1999 to November 2008. Eight radionuclides were monitored continuously using a single fixed‑geometry Geiger‑Müller detector coupled to an electronic counting system: ^85Kr, ^90Sr, ^108Ag, ^133Ba, ^137Cs, ^152Eu, ^154Eu, and ^226Ra. The authors applied rigorous preprocessing to the raw daily count data, correcting for temperature, humidity, high‑voltage drift, and detector‑efficiency changes, thereby minimizing environmental artifacts that could masquerade as genuine decay‑rate variations.

For periodicity detection, the Lomb‑Scargle periodogram—well suited to unevenly spaced time series—was employed to search for low‑frequency signals, in particular an annual (≈365 day) component that has been reported in several earlier studies and controversially linked to solar or neutrino influences. The analysis revealed that the ^137Cs data show no statistically significant peak at the one‑year frequency; the power at 1 yr lies within the 95 % confidence interval of white‑noise background. This null result corroborates the independent findings of Bellotti et al. (2012), who also reported a stable ^137Cs decay rate.

In stark contrast, the ^133Ba standard, measured concurrently in the same detector, exhibits a pronounced annual oscillation. The periodogram displays a sharp peak at 1 yr whose power exceeds the white‑noise baseline by a factor of three, well beyond the 95 % confidence threshold. None of the other six nuclides display comparable annual power; any peaks present are either absent or fall below statistical significance.

The authors interpret these divergent behaviors as evidence that external influences—if they exist—affect different nuclides with varying sensitivities. Possible mechanisms include differential coupling to solar neutrino flux, modulation by Earth‑Sun distance, or indirect effects mediated through the detector’s response to specific gamma‑ray energies. ^133Ba emits a dominant 356 keV gamma line, a region where the Geiger‑Müller tube’s efficiency may be more susceptible to subtle temperature or voltage variations, potentially amplifying any external signal. Conversely, ^137Cs, which primarily emits a 662 keV gamma, appears immune to such effects within the experimental uncertainties.

The paper also acknowledges alternative explanations rooted in instrumental systematics. Non‑linearities in detector response, minute drifts in high‑voltage supply, or unaccounted environmental factors could generate spurious periodicities that mimic a genuine physical effect. To discriminate between genuine physics and artefacts, the authors recommend cross‑laboratory comparisons using different detector technologies, controlled environmental perturbations, and extended observation periods.

In conclusion, the study provides a nuanced perspective on the long‑standing claim that radioactive decay rates are strictly constant. While ^137Cs conforms to the traditional expectation of invariance, ^133Ba demonstrates a clear annual modulation under identical measurement conditions. This nuclide‑dependent behavior suggests that any putative external driver—whether solar, neutrino‑related, or instrumental—does not act uniformly across all isotopes. The findings motivate further experimental work to map the sensitivity landscape of various radionuclides, to clarify the underlying mechanism, and to assess the broader implications for fields that rely on precise decay constants, such as radiometric dating, nuclear medicine, and fundamental physics.