Sub-daily periodic radon signals in a confined radon system

• Radon within a confined experimental volume is monitored from several directions.
• Short-term signals (2–3 h periodicity) are superimposed on diurnal signals.
• Short-term and Diurnal signals exhibit directionality in their variation pattern.
• Short-term signal driver is earth-bound; diurnal signal is related to solar irradiance.
• Short-term signals appear ∼10 h before Tohoku earthquake and continue for 2 days.

Signals from radon in air enclosed in a tight canister are recorded by five gamma detectors located around the horizontal plane and along the vertical axis. At steady state conditions (diffusion input = radon decay) the primary variation is of daily radon (DR) signals with amplitudes of around 20–25%. The DR signal, with a rounded form, is characterized by periodicities of 24-, 12- and 8-h (i.e. 1, 2 & 3 CPD). Similar DR variation patterns occur in the east and west sensors whereas inverse DR patterns are recorded by the north and south sensors. Short term (ST) signals, having saw tooth form and periods of 2–3 h (frequencies in the range of 9–12 CPD) are observed at all five sensors and are superimposed on the DR signals with relative amplitudes of around 20%. They exhibit differing forms and phase at the different sensors, located at different directions around the canister. The latter is similar to the spatial manifestation of form and phase of the DR signal in such experiments, indicating a communality of the driving mechanism. At this stage a geophysical explanation cannot be presented for the ST signals. In this respect a peculiar observation is that their extraordinary occurrence coincides in time with the Tohoku Earthquake (Mw = 9.0; 11 March 2011).