Search for time modulations in the decay rate of 40K and 232Th

TL;DR

This study measured the decay rates of 40K and 232Th over 500 days to investigate claims of annual or monthly modulations, finding no evidence of such time-dependent variations in decay constants.

Contribution

It provides high-sensitivity measurements of 40K and 232Th decay rates over extended periods, testing previous claims of decay rate modulations with no observed effects.

Findings

No evidence of decay rate modulations at the level of a few parts in 10^5.

Decay rates remained constant during solar flares in May 2013.

Results challenge previous reports of periodic decay rate variations.

Abstract

Time modulations at per mil level have been reported to take place in the decay constant of about 15 nuclei with period of one year (most cases) but also of about one month or one day. In this paper we give the results of the activity measurement of a 40K source and a 232Th one. The two experiments have been done at the Gran Sasso Laboratory during a period of about 500 days, above ground (40K) and underground (232Th) with a target sensitivity of a few parts over 10^5. We also give the results of the activity measurement at the time of the X-class solar flares which took place in May 2013. Briefly, our measurements do not show any evidence of unexpected time dependence in the decay rate of 40K and 232Th.