Redirected from Radiohalos
Uranium follows a sequence of decay through thorium, radium, radon, polonium, and lead. These are the alpha-emitting isotopes in the sequence. (Beta particles do not discolor the rock.)
Isotope | Half-life | Energy in MeV |
U238 | 4.46e9 years | 4.19 |
U234 | 2.45e5 years | 4.77 |
Th230 | 75400 years | 4.68 |
Ra226 | 1599 years | 4.78 |
Rn222 | 3.82 days | 5.49 |
Po218 | 3.1 minutes | 6.00 |
Po214 | 164 microseconds | 7.69 |
Po210 | 138 days | 5.30 |
Pb206 | infinite | 0 |
Radiohalos occur with the uranium rings and without them. The U-234 and Ra-226 rings coincide, with the Th-230 ring merely thickening it, so it is hard to tell which one of those isotopes started the halo, but it is easy to tell a polonium halo from a uranium halo.
Robert V. Gentry[?] studied these halos and concluded that the rock must have formed within three minutes if the halo was formed by Po-218. This is taken by creationists as evidence that the earth was formed instantaneously.
Critics of Gentry's theory claim that radon, given off by a grain of uranium elsewhere in the rock, collected at a point in the rock and formed a uraniumless halo. The radon ring is close to the Po-210 ring and it is a bit difficult to tell them apart.
External links:
wikipedia.org dumped 2003-03-17 with terodump