Technique for estimating the charge number of individual radioactive particles using Kelvin probe force microscopy

The Fukushima Daiichi Nuclear Power Plant accident in Japan resulted in the emission of many radioactive cesium (Cs)-containing particles that have charges on the surface due to self-charging. Charged aerosol particles are efficiently deposited inside human airways, leading to adverse health effects. To evaluate these effects, we developed a technique for estimating the charge number (n(p)) of radioactive particles by measuring the surface potentials (V-p) of individual radioactive particles using Kelvin probe force microscopy. The V-p values of the individual CsCl particles were highly correlated with the surface n(p), indicating that V-p is a measure of the charged aerosol state. To further examine the V-p-n(p) relationship, a simplified capacitance model was developed to estimate the ratio of V-p to n(p) per unit area of particles. Although the calculated V-p was proportional to the n(p), consistent with our experiment, the calculated ratio was higher than those determined experimentally. The magnitude of this ratio may depend on the conductivity, microphysical properties and chemical composition of the particles. Despite these uncertainties, the experimentally determined V-p-n(p) relationship of the CsCl particles was used to estimate the n(p) of the radioactive and non-radioactive particles from the measurement of the V-p of these particles. It was demonstrated that the n(p) of the radioactive particles was much higher than that of the non-radioactive particles, suggesting that radioactive particles are efficiently charged by self-charging. These charged radioactive particles may strongly cause adverse human health effects owing to their efficient deposition in human airways.