Size distribution radon decay products

Raes, F. A. Jannsens, and H. Vanmarcke, Modeling Size Distributions of Radon Decay Products in Realistic Environments, this volume (1987). 

A Model for Size Distributions of Radon Decay Products in Realistic Environments... 

The environmental conditions for each of the cases considered below are summarized in Table III all these parameters are constant in time. The build up of the nucleation mode of the stable particles and the build up of both the nucleation and accumulation modes of the radon decay products is calculated, and the results are given after a process time of one hour. Figures 1 to 5 show the size distributions of stable and radioactive particles, and Table IV gives the disequilibrium, the equilibrium factor F, the "unattached fraction" f and the plate-out rates for the different daughters. 

The calculated size distribution of newly attached decay products is shown as curve C in Fig. 1.9. The activity median diameter is 0.16 /zm. With passage of time, the distribution would be shifted to larger particle sizes, as coagulation proceeds. George (1972) used diffusion batteries to measure the size distribution of nuclei carrying radon decay products and found activity median diameters (AMD) averaging 0.18,0.11, and 0.30 /um in a city basement, fifth floor room, and rural outside air, respectively. 

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. 

A plot of the attachment coefficient, p, of radon decay product ions is shown in Figure 2.3 (Chamberlain, 1991). The line is Equation (2.2) with the diffusion coefficient value, D = 7 X 10 m s Vm = 44 ms and a = 1. In the natural aerosol size distribution, typical of well-populated country districts, Junge s (1963) natural aerosol size distribution includes particles such as sea salt and resuspended dust which extend the distribution at the large-diameter end, the rate constant for attachment = 2.1 x 10 s , and since A = 1.7 x 10 m for the Junge s aerosol, the corresponding value of the attachment coefficient is P = 1.2x10 m s . Measured values for the attachment coefficient p for outdoor aerosols... 

In all dosimetric models, the dominant parameter related to dose is the activity size distribution of the radon decay products in air as the original deposition destination and amount of... 

Average values of the relative size distribution of the unattached radon decay products in terms of potential alj a energy concentration (PAEC). The measured values were approximated by a sum of i log-normal distributions, char-actmsed by the activity median aerodynamic diameter, AMAD, noted as AMDui, geometric standard deviation, Ogui. and activity fraction, fpvd- n = numba- of measurements Z = aerosol parhcle concentrations Cq = radon concentration RH = relative humidity... 

Fig. 5.7. Relative size distribution in terms of potential alpha energy concentration, PAEC, of the unattached radon decay product clusters measured in indoor air.

Besides cluster formation, the radon decay products attach to the existing aerosol particles within 1-100 s, forming the radioactive aerosols of the radon decay products. Results of the activity size distribution measurements carried out at different places in outdoor air, dwellings and workplaces are presented in Table 5.2. In general, the activity size distribution of the radon... 

Parameters of the activity size distribution of the aerosol-attached short-Uved radon decay products in air at different locations. Activity median aerodynamic diameter, AMAD, noted as AMD geometric standard deviation, Og fraction of the mode, fpi. The indices i = n, a and c represent the nucleation (Aitken nuclei), accumulation and coarse particles modes, Z = aerosol particle concentrations... 

Fig. 5.9. Activity size distribution of the radon decay products in outdoor air.

Fig. 5.10. Typical relative activity size distribution of the potential alpha energy concentration, PAEC, of the radon decay product aerosols in room air. Ventilation <0.5 h , without aerosol sources. AMAD noted as AMDn = 40 nm, Sg = 1.7, fpn = 0.2 AMDa = 200 nm, , Sga = 2.2, /pa = 0.8.

Fig. 5.11. Relative activity size distribution of the radon decay product aerosols in air containing a high particle concentration of combustion aerosol from diesel engines and cigarette smoke. —, mine air (working + diesel engine) AMDa = 201 nm. - - room air (+ cigarette smoke) AMDa = 270 nm.

The dose conversion factor, DCF, in effective dose per exposure unit is calculated by taking into account the dose function of the particle diameter (Figure 5.4) and the radon decay product characteristics. The dose conversion factors for living and work places with typical activity size distributions, as a function of the unattached fraction, fp, are illustrated in Figures 5.13 and 5.14, respectively. The value of the dose conversion factor, DCFae, for fp = 0 represents the dose contribution of the radon product aerosols. The values of dose conversion factor in Figure 5.14 are based on aerosol conditions which are typical for many workplaces. 

The four-stage low-pressure cascade impactor incorporates both the impactor and wire screen methods (Tokonami et al., 1997). This system can measure the activity size distribution of radon decay products in a low level environment within 90 min. Figure 6.9 shows a block diagram of the activity-weighted size distribution instrument. In the first air inlet, unattached radon decay products are collected on a metal wire screen (300 mesh openings 118.2 cm wire diameter 3.75 x 10 cm). A silicon semiconductor detector, SSD, is set opposite the metal wire screens where both collection and detection are concurrent. Output signals from the silicon semiconductor detector are sent through a preamplifier, PA, and the internal amplifier of a multichannel analyser, MCA, and then to the multichannel analyser. 

As an example, the activity size distributions of the aerosol-attached fraction of all shortlived radon decay products are shown in Figure 6.13. The errors (dashed lines) included the statistical errors of the counting rates and the uncertainties in the efficiencies of the detector of the on-line a-cascade impactor in the several stages. The fitted distribution and its parameters are also shown in Figure 6.13. The AMAD of Po is about 10% smaller and Og is about... 

Fig. 6.13. Measured and fitted activity size distributions of the short-lived radon decay products obtained by an online a-iintactor.

A model has been developed to calculate the size distributions of the short lived decay products of radon in the indoor environment. In addition to the classical processes like attachment, plate out and ventilation, clustering of condensable species around the radioactive ions, and the neutralization of these ions by recombination and charge transfer are also taken into account. Some examples are presented showing that the latter processes may affect considerably the appearance and amount of the so called unattached fraction, as well as the equilibrium factor. 

The amount of particles determine the quantity of decay products that stay in the air (equilibrium fraction, F) and the fraction of activity associated with the "unattached or ultrafine mode of the size distribution (fDot) These decay products are certainly harmful at high concentrations but we cannot yet detect the effects at normal levels because the vast majority of lung cancer death are due to smoking. Models predict that potentially 9000 lung cancer deaths per year in the United States are due to indoor radon. Methods are currently available and new methods are being developed and tested for lowering the levels of radon in indoor air. 

Activity size distributions of the radon product decay aerosols 3.4.1. Lead-214 and aerosol size distributions... 

Summary of mean monthly activity median aerodynamic diameters (AMADs) and geometric standard deviations (ffg) of radon ( Rn) and thoron ( °Rn) decay products size distributions in ambient aerosols... 

The size distribution of Rn decay product aerosols in ambient air exerts a marked effect on the calculated dose to lung tissue following inhalation of radon-contaminated air because of the alpha radiation from its decay products ( Po, " Po). 

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