Influence of the beta energy decay spectrum and particle size on the aerosol specific self-charging rate of radioactive aerosol

International audienceNuclear 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

Impact of the coarse indoor non-radioactive aerosols on the background radon progenies' compensation of a continuous air monitor

International audienceThis paper addresses the problem of false positive alarm when using a continuous air monitor (CAM) in decommissioning sites of nuclear facilities. CAMs are used to measure airborne activity and play an important role in the radiation protection of workers likely to be exposed to radioactive aerosols. Monitors usually sample aerosols on a membrane filter. Radioactive particles sampled are detected through the alpha and beta decays that they emit. These latter ionizing particles are measured online by spectrometry thanks to a Passivated Implanted Planar Silicon detector (PIPS). Alpha and beta decays, in this context, come mainly from the natural radon progeny (218Po, 214Pb, and so on) and, in the case of radioactive contamination, also from artificial radionuclides such as 239Pu or 137Cs. The aim of the CAM is to alert the workers when the artificial airborne activity occurs, always considering the presence of a variable background due to the natural particulate airborne activity. The CAM-specific algorithm considers this background dynamically and continuously, often by using a constant parameter. However, non-radioactive aerosols are also sampled on the membrane filter. These latter make the discrimination more difficult as they lead to the deterioration of the alpha-energy spectrum. In this paper, the effect of coarse non-radioactive aerosols on the CAM response is highlighted with four aerosol size-distributions. The evolution of the background is characterized as a function of the aerosol mass sampled, with the example of a simple algorithm. Thus, in this paper, results show a positive correlation of the background with the aerosol mass sampled by the CAM. In addition, results highlight at least two different evolutionary trends according to the aerosol size distribution. An explanation of these evolutions is given by considering the penetration profile of the natural radioactive aerosols in the granular deposit above the CAM filter. The main consequence of these results is that the background could not be considered as proportional to radon progeny as it is currently used

Impact de la masse de particules sur le comportement d'un moniteur de mesure de la contamination atmosphérique (cam)

International audienceCet article présente la caractérisation du comportement d'un moniteur de mesure de la contamination particulaire radioactive atmosphérique en condition de chantier de démantèlement simulée en laboratoire. Les premiers résultats présentés dans ce papier mettent en avant une mauvaise adaptation de la compensation dynamique du bruit de fond dans des conditions de fonctionnement non prises en compte dans les référentiels IEC