Fine structure of the absorbed dose rate monitored in Zagreb, Croatia, in the period 1985–2011

We report on the fine structure of the absorbed dose rate D which was measured and recorded on a daily basis at the Institute for Medical Research and Occupational Health in Zagreb, Croatia, throughout the period 1985–2011. After the Chernobyl accident, D increased steeply by a factor of 3.5, but this is the only prominent feature in the D versus time (t) curve. In the absence of accidental conditions, the D(t) is flat and amounts to 30–35 pGy/s. Despite the apparent plainness of D(t), its Fourier transform reveals several periodic modulations hidden in the noise. Some of the corresponding periods (6 and 12 months) can be related to seasonal atmospheric changes but this is not the case with the other periods identified (9.3, 13.7, 15.7, 20, 31, and 39 months). These are found to agree well with literature data on periodicities in solar activity, which implies that they are most probably linked to variations in the atmospheric production of 7Be by cosmic rays.

► Absorbed dose rate D was monitored on a daily basis over twenty-six years.
► D was constant (30–35 pGy/s) always except after the Chernobyl accident.
► D as a function of time comprises well defined periodicities.
► Non-seasonal periodicities in D agree with those in solar activity.
► Non-seasonal modulations of D are linked to production of 7Be by cosmic rays.