Characterization of the “bGeigie Nano” instrument used in Citizen Science dose-rate monitoring

Safecast, a Citizen Science project devoted to monitoring ambient dose rate, initiated in Japan 2011 after the Fukushima NPP accident and soon spread worldwide. Its standard instrument is bGeigie Nano, featuring a pancake-type thin-window GM-sensor coupled to a GPS receiver and data-storage, in a sturdy plastic case allowing field use. Recorded ADR tracks are shown on a publicly accessible map on Safecast.org, containing almost 200 M records by 2022. Interpretability of results depends on quality assurance of the measurement process, covering both metrological characterization of the instrument and its practical use—relevant because users are citizens in general not familiar with metrological procedures, measurement statistics, the concept of representativeness, etc. Here we focus on the former aspect. In field use, the source of GM response is internal background (BG), ambient gamma-rays and secondary cosmic radiation (SCR). Through dedicated experiments, mainly performed on lakes (where the terrestrial gamma component is largely absent), we quantified the BG and SCR response and investigated the variance of response between instruments. We investigated conformity to count-rates Poisson statistics and the occurrence of spurious extreme signals, which can lead to artefacts in ADR maps. Impact of experimental results on practice and uncertainties are discussed.