Potential alpha energy of unattached decay products

Ventilation in centrally heated dwellings, though much less than in those with open fires, usually gives 0.1 to 0.3 air changes per hour, so ventilation and deposition on surfaces are about equally effective in limiting the concentration of attached decay products in dwellings. 

Unattached decay products plate-out much more rapidly. From Fig. 1.10, vu can be expected to be of the order 1 mm s-1 in rooms with normal air movement. Bigu (1985) measured plate-out of unattached decay products of both 222Rn and 220Rn in a chamber of volume 26 m3. He found vu varying from 0.9 to 2.4 mm s-1 (222Rn products) and from 0.6 to 5.3 mm s-1 (220Rn products) depending on whether or not a fan was operated in the chamber. 

Vanmarcke et al. (1987) found that a value vu = 1.9 mm s-1 gave the best fit in model calculations applied to their measurements in dwellings. If vu = 2 mm s—1 is taken as a reference value, with a surface/ volume ratio of 3 m-1, then XDu = 6 x 10-3 s-1 = 20h 1. 

14 Potential alpha energy of unattached decay products 

218Po than 214Pb atoms. Hence most of the unattached potential alpha energy, Epu, is associated with 218Po. 

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