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

Abstract

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.

Editor:

• Jingkun Jiang

Acknowledgments

We thank Associate Professor T. Shimizu of the Department of Physics and Information Engineering at Keio University for her advice regarding the theoretical model of Kelvin probe force microscopy (KPFM). The surface potential of the aerosol particles was measured using a scanning probe microscope (SPM-9600) at the central laboratory of Keio University. We also thank M. Matsuda, M. Azami, and K. Kitamura of Shimadzu Corporation for their assistance with KPFM measurements.

Data availability statement

The data that support the findings of this study are openly available at https://keio.box.com/s/9cdkot0j99ryhpy5fuv8hcpl3u5w5pka.

Additional information

Funding

This work was supported by the Japan Society for the Promotion of Science KAKENHI Grants (JP26340010, JP17H01864, JP18K19856, JP20K19959, and JP20H00636), Ministry of Education, Culture, Sports, Science and Technology, Japan Nuclear Safety Institute, Project for Promotion of Nuclear Science and Technology and Human Resource Development with the Combined Wisdom, International Cooperation Type Decommissioning Research Program (Japan-UK Joint Research; R02I030-1).