Radon, Rn-222 (half-life = 3.82 days), is a daughter product of radium, Ra-226, which in turn is derived from the longer-lived antecedent, U-238. Thoron, Rn-220 (half-life = 56 seconds) is a daughter of thorium as shown in Figure 2.2, Th-232, which is present in larger amount in the earth’s crust than radon.
Because of thoron’s short half-life, it is essentially all gone before it leaves the ground, and is of no significant radiobiologic consequence. These radionuclide series are present in slowly decreasing amounts in the environment (geologic time scale), due to radioactive decay of their parents, which has been known and understood since the end of the last century.
Figure 2.2 Principal decay scheme of thorium-232 – (Source NRPB, 1994)
Figure 2.3 Principal decay scheme of uranium-238 – (Source NRPB, 1994)
The half-life of 3.82 days for Radon-222 is long compared with its first four decay products, as shown in Figure 2.3, and these decay products in air are unlikely to be very far removed from equilibrium with the parent gas. Under these circumstances it can be demonstrated that nearly all the dose to lung tissue arises from the inhaled decay products. In the case of radon-220, with a half-life of 54.5 s, this will be close to equilibrium with its decay products, polonium-216 of half-life 0.16 s. However, the next member of this decay scheme is lead-212 with a half-life of 10.6 h and, under all practical ventilation rates, lead-212 and bitmuth-212 will be far from equilibrium with the parent radon-220. This indicates that is some situations the dose to lung from airborne radon-220 may be significant.
In the general environment it is the inhalation of the decay products of radon-220 (lead-212 and bitmuth-212) which pose a potential hazard, here the second difference between the decay products of radon-222 and radon-220 manifests itself. The longest half-life of the short-lived decay products of radon-222 is 26.8 minutes (lead-214) and hence the decay products of radon-222 deposited in the bronchial tree will largely decay in lung before biological removal mechanisms are effective.
|Main radiation energies and intensities|
Figure 2.4 Principle decay properties of radon-222 and short-lived decay products – (Source NRPB_1993)
As shown in Figure 2.4 the beta and gamma radiation’s may be ignored, since their contribution to both lung dose equivalent and effective dose equivalent are small compared to those from alpha radiation (ICRP,1987).