Search for time dependence of the 137Cs decay constant

Starting from June 2011, the activity of a 137Cs source has been measured by means of a HPGe detector installed deep underground in the Gran Sasso Laboratory. Over 5100 hourly recorded energy spectra have been collected in 217 days. These data allowed the search for time variations of the decay constant with periods from a few hours to 1 year. No signal with amplitude larger than 9.6⋅10−5 at 95% C.L. has been detected. These limits are more than one order of magnitude lower than the values on the oscillation amplitude reported in literature. In particular, for 1 year period an oscillation amplitude larger than 8.5⋅10−5 has been excluded at 95% C.L., independently of the phase. The same data give a value of 29.96±0.08 years for the 137Cs half-life, in good agreement with the world mean value of 30.05±0.08 years.