A study of radioactive and nonradioactive aerosol behaviour

1 1 1 1 2 2P. Otahal , J. Vosahlik , I. Burian , L. Nemecek , V. Zdimal , J. Ondracek

1The National Institute of Nuclear, Chemical and Biological Protection,v.v.i., Kamenna 71, 26231, Milin, Czech Republic2Institute of Chemical Process Fundamentals of the ASCR, v.v.i., Rozvojova 135, 16502 Prague 6, Czech Republic

otahal@sujchbo.cz

INTRODUCTIONAerosols belong to the most dangerous forms of radioactive and nuclear substances, due to their ability to spread. Radioactive aerosols are liquid or solid particles suspended in air with diameters ranging from 0.001 to 100 microns; these particles are either formed by a radioactive material or the radioactive material is attached to the surface of a non-radioactive particle.

MATERIAL AND METHODS

Fig. 1: Experimental set-up during injection of aerosol particles to the radon-aerosol chamber

The experiments were carried out in a radon-aerosol chamber at SÚJCHBO (see figure 1). This chamber is a device for studying physical behaviour of radioactive and non-radioactive aerosols. The inner atmosphere of the radon-aerosol chamber is separated from the ambient atmosphere by a sophisticated air-exchange system which enables to perform the experiments at pre-defined climatic and ventilation conditions [1].Different aerosol atmospheres were produced by three aerosol generators. Figure 2 demonstrates a common aerosol spectrum generated by a CW (Carnauba Wax) generator. The summary of main parameters of the generated particles is presented at the table 1.

Tab. 1: Summary information about generating aerosol particles

[1] Burian I.,Otahal P., Vosahlik J., Pilecka E. (2011): Czech primary radon measurement equipment, Radiation Protection Dosimetry, Vol. 145, No. 2-3, pp. 333-336

[2] Baron A. P., Willeke K. (2001): Electrical Techniques. In Aerosol Measurement: Principles, Techniques, and Applications,Wiley, New York, pp. 540-542

ACKNOWLEDGEMENT: A pilot study of behaviour of radioactive and nonradioactive aerosol particles was supported by the project of the Czech Ministry of the Interior No. Vf20112015013 - Research of modern methods of detection and identification of hazardous CBRN substances and

materials, methods of hazard reduction and decontamination; research of modern method for personal protection and elements of critical infrastructure. The authors would like to thank Prof. R. Holub for his valuable help and knowledge support in the theory of radioactive aerosol.

Particle number size distribution in the size range 10 nm - 750 nm were measured by a Scanning Mobility Particle Sizer (SMPS 3934). It consists of two parts: an Electrostatic Classifier (EC - 3071) and by a Condensation Particle Counter (CPC 3025). An Aerodynamic Particle Sizer Spectrometer (APS - 3321) determined the size distribution over the complementary size range of 500 nm - 20 µm. Airborne radioactivity after sampling was measured by a Canberra multichannel alpha-spectrometer (Model 7401 VR). Photon dose estimation was determined by a portable multichannel gama-spectrometer Falcon (Model 5000).Radon and its short lived decay products were used as an inner

85radioactive source and a cartridge with a Kr was used as a point source of ionization radiation.

THEORETICAL BASISA number of processes contribute to the charging of aerosol particles: static electrification, photoemission, thermionic emission, charging by small gas ions, and self-charging of radioactive aerosols [2]. There are two possibilities: in one case the radioactive material is embedded into an aerosol particle. In the second case the radioactivity is on the surphace of nonactive particle. There is another possibility to redistribute the charge: using a flux of photons (from „outside“ of the particle). Processes such as coagulation and/or deposition may be significantly affected by redistribution of electrostatic charge of aerosol particles.

Fig. 2: Demonstration of reproducibility of aerosol production by CW generator during preliminary tests

INTRODUCTION RESULTSThe main objective of the experiments was to compare the behaviour of

85aerosol in a field of ionizing radiation generated by Kr and the behaviour of aerosol particles in an environment of extreme concentrations of radon. The results of these experiments were compared with the behaviour of non-radioactive aerosols. Aerosol particles were generated in three different size classes characterized by the size mode (50 nm, 180 nm and 2 µm). Experimental results are summarized in the following figures (see figure 3, 4).

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Fig. 3: The time course of the total concentration of aerosol particles in the chamber for different types of aerosol particles

In this work, we studied physical behaviour of radioactive and non-radioactive aerosols. The experiments were carried out in a radon-aerosol chamber at SUJCHBO. Three types of aerosol were generated

– CW, AS and DEHS. The aerosol was filled into the chamber either as it 85was generated, or after charge neutralization by Kr. In part of the

experiments, the aerosol in the chamber was mixed with Rn and its short lived decay products. In other experiments it was exposed to an elevated photon stream.It has been found that: irradiations were performed by high concentration of radon (and its decay products). In this case the non-radioactive aerosols was of course covered by radioactivity.In the second case an elevated photon stream was used. One can predict a difference in the of behaviour of aerosols when the aerosol was charged by irraditation „from outside“. Hoewer, we did not observed any mode change or an increasing plate-out.

T yp e C arnauba w ax A m m onium

su lfa te

D E H S

on N aC l

In d ica tion C W A S D E H S

T yp e o f gen erator C arnauba w ax A G K 2000 M A G 3000

M od e o f s ize

d istr ib u tion

[ n m ]

180 50 2000

G S D 1 ,6 1 ,2 1 ,6

Fig. 4: The time course of size mode distributions of aerosol particles in the chamber for different types of aerosol particles

Plate-out rtype of aerosol particles. In some cases particles were influenced by

-3radon (concentration about 1.5 MBq.m ). The dose rate coresponding to this concentration is approximately 3 µGy/s. Most of the energy the alpha particle is absorbed in the mass of air. Taking in to account number

-3 concentrations of about 5000 particles per cm , the ratio between the -6mass of aerosol particles to the air mass is around 3.3E . Flowig through

85Kr neutralizer, the aerosol stream was irradiated by 15 µSv. When we compared the behaviour of aerosol particles and different exposer (Figure 4) almost no difference has been found. The same conclusion holds for the decrease of the total concentration (see Fig. 5).

ate of total concentration of aerosol particles depends on the

Fig. 5: The time course of the total concentration of aerosol particles in the chamber for ammonium sulphate with / without Kr85 neutralizer and / without the presence of radon

CONCLUSION

REFERENCES

0,00E+00

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dN

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m3]

Diameter [nm]

CW5Total Conc. [#.cm-3]: 9.01E+4Mode [nm]: 126.3GSD: 1.60

CW6Total Conc. [#.cm-3]: 9.44E+4Mode [nm]: 131.0GSD: 1.62

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CW with Rn CW with Kr85

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