Cosmic muon flux is recorded by two plastic scintillator detectors: one located at ground level and the other located in the Belgrade underground laboratory (25 m w.e.). Power spectral analysis of the raw detector data for the period from January 1st 2002 to December 31st 2002 is performed. Similar periodicities are found in both data sets (ground and underground), but with different amplitudes. Main periodicity in the underground detector data has a 26.5 days period, while the highest amplitude signal in the ground detector data has a 243 ± 45 days period. Other signals present in the time series correspond to higher harmonics of the solar rotation period. There is also a wave with a 34.4 days period, whose significance is confirmed by different methods of spectral analysis. The same periodicity is recently found in some processes on the Sun.
Observation of the shadowing of cosmic rays by the Moon using a deep underground detector
