Increased radioactivity during precipitation measured by the HiSPARC experiment

HiSPARC is an extensive network of cosmic ray detection stations mainly across The Netherlands and United Kingdom. Its primary goal is to analyse cosmic ray air showers by sampling their footprints using multiple detectors. Also the detection rates per single detector are recorded. These 'singles rates' fluctuate in time. Large fluctuations are caused by precipitation. During rain an excess of ²²²Rn daughter nuclei, scavenged by raindrops, falls to the ground. This results in an increase in the number of gamma rays from the decay of ²¹⁴Bi. The excess of events can be described by a simple decay model that takes fluctuations of the rain intensity into account. (paper)