Thoron decay products

Menon, V.B., P. Kotrappa and D.P. Bhanti, A Study of the Attachment of Thoron Decay Products to Aerosols using an Aerosol Centrifuge,... 

Bigu, J., On the Effect of a Negative Ion-Generator and a Mixing Fan on the Attachment of Thoron-Decay Products in a Thoron Box, Health Phys., 46, no.4, 933-939, (1984). 

This paper deals with the plate-out characteristics of a variety of materials such as metals, plastics, fabrics and powders to the decay products of radon and thoron under laboratory-controlled conditions. In a previous paper, the author reported on measurements on the attachment rate and deposition velocity of radon and thoron decay products (Bigu, 1985). In these experiments, stainless steel discs and filter paper were used. At the time, the assumption was made that the surface a-activity measured was independent of the chemical and physical nature, and conditions, of the surface on which the products were deposited. The present work was partly aimed at verifying this assumption. 

In the environment, thorium and its compounds do not degrade or mineralize like many organic compounds, but instead speciate into different chemical compounds and form radioactive decay products. Analytical methods for the quantification of radioactive decay products, such as radium, radon, polonium and lead are available. However, the decay products of thorium are rarely analyzed in environmental samples. Since radon-220 (thoron, a decay product of thorium-232) is a gas, determination of thoron decay products in some environmental samples may be simpler, and their concentrations may be used as an indirect measure of the parent compound in the environment if a secular equilibrium is reached between thorium-232 and all its decay products. There are few analytical methods that will allow quantification of the speciation products formed as a result of environmental interactions of thorium (e.g., formation of complex). A knowledge of the environmental transformation processes of thorium and the compounds formed as a result is important in the understanding of their transport in environmental media. For example, in aquatic media, formation of soluble complexes will increase thorium mobility, whereas formation of insoluble species will enhance its incorporation into the sediment and limit its mobility. 

Radon and thoron decay products as small ions... 

The subject of this particular volume relates to aerosol particle physics including aerosol characterisation, the formation mechanism, the aerodynamic size distribution of the activity and aerosol residence time, instrumentation techniques, aerosol collection and sampling, various kinds of environmental (atmospheric aerosols), particularly radioactive aerosols and the special case of radon decay product aerosols (indoors and outdoors) and the unattached fl ac-tion, thoron decay product aerosols, the deposition patterns of aerosol particles in the lung and the subsequent uptake into human subjects and risk assessment. 

Radioactive aerosols can be classified in the following categories (a) Radioactive aerosols associated with radioactive nuclides of cosmogenic origin, such as Be, Na, and (b) radon and thoron decay product aerosols associated with Po Pb, Pb, Pb, and °Po, (c) fission product radionuclide aerosols associated with Sr, Sr, Cs, Ru, isif 132-pg i40g (-(j) radioactive aerosols associated with the operation of high-energy ac-... 

Gmndel and Porstendorfer (2004) showed that the long-lived radon decay products Pb and Po are almost all (93-96%) adsorbed on aerosol particles in the accumulation size range and only 4-7% of their activities are attached on nuclei with diameters smaller than 60 nm. AMAD-values of 558 nm for Pb and 545 nm for Po were measured, i.e. significantly larger values than those of the short-lived radon and thoron decay products. 

Kotrappa, P, Bhantin, D.P., Dhandayutham, R. (1975). Diffusion sampler useful for measuring diffusion coefficients and unattached fraction of radon and thoron decay products. Health Phys. 29, 155—162. 

Gaggeler, H.W., lost, D.T., Baltensperger, U., Schwikowski, M. (1995). Radon and thoron decay product and Pb measurements at Jungfraujoch, Switzerland. Atmos. Environ. 29 (5), 607-616. 

Figures 5.2 and 5.3 show the effective dose per unit exposure of the tracheo-bronchial (bronchial and bronchiolar) and pulmonary or alveolar region as a function of aerosol particle diameter calculated for the radon decay products Po, " Pb, and " Po and the thoron decay products Pb, Bi and Po. A tissue weighting factor of the lung and the radiation weighting factor of 0.12 and 20, respectively, are taken into account. The effective dose from a radioactive mixture can be deduced by adding the effective doses of each decay product.

Figure 5.6 shows the unattached fraction of radon (/ ,Rn) and thoron (/p,Tn) decay products as a function of the particle concentration of atmospheric aerosols. The fp values as a function of the particle concentration, Z, are measured by means of a condensation nuclei counter (CNC). Many working places have aerosol sources due to human activities and combustion and technical processes with a high particle concentration, Z > 4 X 10 particles cm , and therefore fp values below 0.01. The fp values are higher than 0.1 for places with particle concentrations <4 x 10 particles cm . This is the case in poorly ventilated rooms (ventilation rate <0.5 h ) without additional aerosol sources, rooms with an operating air cleaner and poorly ventilated underground caves. For the unattached thoron decay products in indoor air, the unattached fraction is estimated by the equation... 

In Figure 5.18, a spectrum for a radon gas source is shown. Fifty channels are available for the spectrum. The maximum of the Po peak is situated in channel 24 and the maximum of the " Po peak is situated in channel 40. The energy difference between the peaks is 1.685 MeV, giving a resolution of 105 keV per channel. The energy cut-off at channel 0 of the spectmm amounts to 3.58 MeV. The spectrum is divided into five ROIs, which are stored and available for analysis. These five ROIs allow a separation of the radon decay products Po, " Po and the thoron decay products. In Figure 5.18, an estimated peak tailing for the two radon decay product peaks Po and " Po has been drawn. It is seen that the main peak intensity is situated well inside the chosen ROI. Two modes are available for the calculation of the activity concentration of the radon gas. The fast mode uses the peak areas of Rn and Po and the slow mode uses in addition the " Po peak. 

Porstendorfer, J. (2001). Physical parameters and dose factors of the radon and thoron decay products. Radiat. Prot. 

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