Radiohalos are spherical shells of discoloration in rocks, such as granite, or wood caused by the inclusion of radioactive grains in the rock or by deposition of radioactive material in them. The discoloration is caused by alpha particles emitted by the nuclei; the radius depends on the particle's energy.
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.
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