2055 words - 8 pages

Radon: Our Major Source of Radiation DoseIntroductionRadon-222 is a natural, radioactive isotope of element number 86, thatoccurs in the uranium-238 decay chain (see table 1). Its immediate parent isradium-226, and radon-222 itself decays by alpha particle emission through aseries of short -lived decay products (mainly isotopes of polonium, lead andbismuth) to lead-210 and on eventually to stable lead-206.Element Radiation Half-Life

Uranium

238 alpha 4,460,000,000 years

Thorium

234 beta 24.1 days

Protactinium

234 beta 1.17 minutes

Uranium

234 alpha 247,000 years

Thorium

230 alpha 80,000 years

Radium

226 alpha 1,602 years

Radon

222 alpha 3.82 days

Polonium

218 alpha 3.05 minutes

Lead

214 beta 27 minutes

Bismuth

214 beta 19.7 minutes

Polonium

214 alpha 1 microsecond

Lead

210 beta 22.3 years

Bismuth

210 beta 5.01 days

Polonium

210 alpha 138.4 days

Lead

206 none stable

Table 1. Uranium

238 Decay Chain

Two other isotopes of radon occur in nature; radon-220, which occurs in thethorium-232 decay chain (table 2) and radon-219 in the uranium-235 chain(table 3). Both these isotopes have half-lives of under a minute and are lessimportant than radon-222 which is the subject of the rest of this essay.Element Radiation Half-Life

Th

232 alpha 14,000,000,000 years

Ra

228 beta 5.76 years

Ac

228 beta 6.13 hours

Th

228 alpha 1.9 years

Ra

224 alpha 3.66 days

Rn

220 alpha 55.6 seconds

Po

216 alpha 0.145 seconds

Pb

212 beta 10.6 hours

Bi

212 beta 61 minutes

Po

212 alpha 0.3 microseconds

Tl

208 beta 3 minutes

Pb

208 stable 22.3 years

Table 2. Thorium-232 Decay Chain

U

235 Alpha 704,000,000 years

Th

231 Beta 1 day

Pa

231 Alpha 32,500 years

Ac

227 Alpha 21.8 years

Fr

223 Beta 21.8 minutes

Ra

223 Alpha 11.4 days

Rn

219 Alpha 4 seconds

Po

215 Alpha 1.78 milliseconds

Pb

211 Beta 36 minutes

Bi

211 Alpha 2.15 minutes

Tl

207 Beta 4.77 minutes

Pb

207 Stable

Table 3. Uranium-235 Decay ChainRadon is one of the group 0 elements, the noble gases, and is, therefore,chemically virtually inert. However, it has been reported that fluorinereacts with radon, forming a fluoride.On average, one part of radon is present in 1 x 1021 parts of air. At roomtemperature radon is a colourless gas; when cooled below its freezingpoint (-71 C), radon exhibits a brilliant phosphorescence which becomesyellow as the temperature is lowered and orange-red at the temperatureof liquid air. Liquid radon boils at -61.7 C.It is formed in the environment by the decay of the trace amounts ofnaturally occurring radium present in all rocks and soils. With a half-life of3.8 days, it can migrate considerable distances through the ground, andescape into the air. Outside, radon quickly disperses and levels are low -about 4 Bq m-3, but indoors levels are higher - about 20 Bq m-3 onThe radioactivity of radon is measured in Becquerels per cubic metre of air(Bq m-3)....

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