Radon decay products distributions

There are a series of papers that focus on the behavior of the radon decay products and their interactions with the indoor atmosphere. Previous studies (Goldstein and Hopke, 1983) have elucidated the mechanisms of neutralization of the Po-218 ionic species in air. Wilkening (1987) reviews the physics of small ions in the air. It now appears that the initially formed polonium ion is rapidly neutralized, but can become associated with other ions present. Reports by Jonassen (1984) and Jonassen and McLaughlin (1985) suggest that only 5 to 10% of the decay products are associated with highly mobile ions and that much of the activity is on large particles that have a bipolar charge distribution. 

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

The selection of the 1980-82 measurements (Swedjemark and MjOnes, 1984) was made on dwellings built before 1976 and with the aim of determining dose distributions and the collective dose to the Swedish population from the exposure of the short-lived radon decay products. This was done by using the statistical selection made by the National Institute for Building Research intended for an energy study of the Swedish stock of houses. From a selection of 3 100 houses in 103 municipalities, 2 900 were inspected. The data was found to be in substantial conformity with data from the land register and the population census of 1975. For the study of the radon concentration 752 dwellings were selected at random. 

The parameters of the frequency distributions of radon decay-product exposure are given in Table 3. This table combines the UK data from the national survey with those from local surveys. For the local surveys, the number of dwellings shown is fewer than the number surveyed actively, because only those that completed the follow-up passive survey have been included. 

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... 

Fig. 5.4. Effective dose by the inhalation of radon decay products as a function of the particle diameter at different relative cancer sensitivity distributions between the bronchial, wbb, bronchiolar, Wbb, and alveolar, wai, region of the thoracic lung (wr = 20, U) = 0.12). v = 0.75 (basal cells + secretory cells).

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... 

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