Radon-220, decay

Radon (Rn) and Radon Decay Products Radon is a radioactive gas formed in the decay of uranium. The radon decay products (also called radon daughters or progeny) can be breathed into the lung where they continue to release radiation as they further decay. 

FIGURE 31.1 Radon decay showing half-lives of products. (Adapted from U.S. EPA, Radon-Resistant Construction Techniques for New Residential Construction—Technical Guidance, EPA/625/2-91/032, U.S. Environmental Protection Agency, Washington, DC, February 1991.)... 

Radon and its decay products were a major focus of meetings held in Capri, Italy, in 1983 and in Maastricht, the Netherlands, in 1985. These meetings reported on ongoing national surveys in European countries, on laboratory and field studies on the properties of radon decay products, and on the models for relating the airborne radioactivity concentrations to the human lung. 

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. 

McLaughlin, J.P., Population Doses from Radon Decay Products in Ireland, this volume (1987). 

Establishment of data quality programs which will result in consistent and reliable radon and radon decay products measurements for both individuals and governmental agencies. 

Numerous radon and radon decay measurements in houses are now being made by a large number of private and governmental organizations. In order to assure valid and consistent measurements, it is important that proven methods be used following standardized procedures. To address this need, EPA issued "Interim Indoor Radon and Radon Decay Product Measurement Protocols" and established a Radon/Radon Progeny Measurement Proficiency program. 

In February 1986, EPA issued a document (Ronca-Battista et al., 1986) titled "Interim Radon and Radon Decay Product Measurement Protocols," describing seven methods for measuring radon and its decay products in houses. The methods addressed are those that have been evaluated by EPA and found to be satisfactory other methods may be added as they are reviewed by EPA. In addition, portions of the document may be revised as new information and data becomes available. 

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. 

An equilibrium factor of 0.35, derived from measurements made during the local surveys, has been assumed to typify conditions in UK dwellings. This value has been used to convert the average radon concentrations measured in the national survey to potential alpha-energy concentration of radon decay-products. On average, persons in the UK spend 75% of their time in their homes and 15% of their time elsewhere indoors (Brown, 1983). The occupancy factor of 0.75, together with an equilibrium factor of 0.35, results in an annual exposure of 1.3 10"5 J h m"3 (0.0037 Working Level Months,... 

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. 

In order to compare exposures to radon decay-products with those to other forms of ionising radiation, it is useful to assess the effective dose equivalent expressed in sieverts (Sv). A conversion coefficient of 15 Sv per J h m"3, equivalent to 5.5 mSv per WLM, has been recommended (UNSCEAR, 1982). With this conversion factor, the... 

Table II. Annual average exposure to radon decay-products in UK dwellings from the national survey...

The average concentration of radon in outdoor air in the UK is 2.6 Bq m"3. Comprehensive data on the equilibrium factor in outdoor air in the UK is not available. Assuming equilibrium, the average exposure to radon decay products received by a member of the UK population during the 10% of time spent in the open is 0.0036 WLM, an annual effective dose equivalent of 0.02 mSv. 

O Riordan, M.C., A.C. James, S. Rae and A.D. Wrixon, Human Exposure to Radon Decay Products Inside Dwellings in the United Kingdom, NRPB-R152, HMSO, London (1983). 

Commission of the European Communities., Results of the Second CEC Intercomparison of Active and Passive Dosemeters for the Measurement of Radon and Radon Decay Products, EUR Report 10403 EN (1986). ... 

Hofmann, W., Cellular Lung Dosimetry for Inhaled Radon Decay Products as a Base for Radiation - Induced Lung Cancer Risk Assessment. Radiat. Environ. Biophys. 20 95-112 (1982). 

Figure 5. The areal exhalation rate from the porous sample in Figure 2, enclosed in three different exhalation cans. Two of them ( a1 and 0 ) are completely radon-tight and the third Cb1) has a radon leak rate constant v, numerically equal to the radon decay rate constant (v=A= 2.1 10" s" ). The cans are closed at time zero. The radon exhalation evolution as a function of time is discussed in the text (theory).

Porstendorfer, J. and T.T. Mercer, Influence of Electric Charge and Humidity upon the Diffusion Coefficient of Radon Decay Products, Health Phvs. 37 191-199 (1979). 

Raabe, O.G., Concerning the Interactions that Occur Between Radon Decay Products and Aerosols, Health Phvs. 17 177 (1969). 

Rudnick, S.N., W.C. Hinds, E.F. Maher, and M.W. First, Effect of Plateout, Air Motion and Dust Removal on Radon Decay Product Concentration in a Simulated Residence, Health Phvs. 45 463-470... 

Card, J.W, and K. Bell, Radon Decay Products and their Application to Uranium Exploration, CIM Bull., 72 81-87 (1979). 

The concentration of radon decay products and therefore the factors F and fp are influenced by the basic processes of the attachment, recoil and deposition (plateout) and by room specific parameters of radon emanation and ventilation (Fig. 1). ... 

Surface deposition is the most important parameter in reduction of the free and aerosol attached radon decay products in room air. If V is the volume of a room and S is the surface area available for deposition (walls, furniture etc), the rate of removal (plateout rate) q is vg S/V, always assuming well mixed room air. vg is the deposition velocity. 

Taking into account the results of wind tunnel experiments the average deposition velocities for the free (vq = 2 m h 1) and the attached (v = 0.02 m h"1) radon decay products can be derived... 

The plateout rates of the free radon decay products on room surfaces are about 200 times higher than the values of the aerosol radon daughters and have a great influence on the radon daughter activity concentrations indoors. 

Bruno, R.C., Verifying a model of radon decay product behaviour indoors, Health Phvs. 45 471-480(1983)... 

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