• Sow, Mamadou
• Gensdarmes, Francois
• Dougniaux, Gregoire

Abstract

Nuclear accidents, such as Chernobyl or Fukushima, led to release of radioactive aerosols into the environment, which are measured all over the world. During this long-range transport, aerosols undergo an electrical self-charging due the radioactivity they carry. The specific self-charging rate for particles containing β emitter radionuclides is always considered equal to the specific activity of the particles (Bq/particle) (Clement & Harrison, 1992; Gensdarmes et al., 2001). This assumption is supported by the fact that radionuclides usually considered (Kim et al., 2017), such as 137Cs, 132Te or 131I, have a high mean energy β decay which leads to an electron path length in matter larger than the particle diameter. This study aims to quantify the influence of this assumption by performing calculations of the specific self-charging rate of particles considering the full energy spectrum of β emitter radionuclides. The specific self-charging is treated as the electron escape probability from the particle. Calculations of electron transport in particle matter are realized with Geant4 toolkit (Agostinelli et al., 2003). The particles studied are single spheres with diameters ranging from 20 nm to 200 µm. The simulations are realized for pure iron particles (density 7.87 g/cm3) in vacuum environment (i.e. no interaction of electrons with matter outside the particle). The β emitter radionuclide considered for the calculation, 132Te, is selected from the case studied by Kim et al. (2017). We assume that the radionuclide is homogeneously distributed in the whole particle and random electron emission occurs. The maximum energy of β decay of 132Te is 240.1 keV. The mean energy of β decay is roughly equal to one third of the maximum energy. To calculate characteristic path length of electron in matter, usually a lower value, equal to a quarter of the maximum energy, is considered in order to take account of the higher coefficients of linear energy transfer for low energy electrons of the spectrum (Gensdarmes et al., 2001). The escape probability of electrons is defined for each particle diameter by the ratio of the electrons that exit the particle by the total number of generated electrons. 105 electrons are generated for each diameter and energy considered. An energy size bin of 1 keV is considered for calculation with the full spectrum. The assumption of escape probability equal to 1 for β emitter radioactive aerosol studied here leads to an overestimation of specific particle self-charging rate for diameters below 20 µm. The calculations based only on the mean energy could not give accurate results; the discrepancy lies between 10 % and 25 % in comparison to that obtained with the full spectrum for diameters between 1 µm and 10 µm. Specific attention has to be paid for other radionuclides with low energy spectrum like tritium.

Topics

• density
• impedance spectroscopy
• simulation
• iron
• random