Radiohalo
From Wikipedia, the free encyclopedia
- This article is about the geophysical phenomenon. For the astronomical phenomenon see radio halo.
Radiohalos are microscopic, 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 of the concentric shells are proportional to the particle's energy. They have been studied in detail by geologists since the early 1970s, but wider interest was prompted by the claims of creationist Robert V. Gentry that radiohalos in biotite are evidence for a young earth. The claims are contested by the mainstream scientific community as an example of creationist pseudoscience.
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[edit] Production
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 | 4.47e9 years | 4.196 |
| U234 | 2.455e5 years | 4.776 |
| Th230 | 75400 years | 4.6876 |
| Ra226 | 1599. years | 4.784 |
| Rn222 | 3.823 days | 5.4897 |
| Po218 | 3.04 minutes | 5.181 |
| Po214 | 163.7 microseconds | 7.686 |
| Po210 | 138.4 days | 5.304 |
| Pb206 | stable | 0 |
The final characteristics of the radiohalos occur depend upon the initial isotope. 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. A radiohalo formed from U-238 has eight concentric rings while a radiohalo formed from Po-210 only has one.
[edit] Controversy
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 have pointed out that Po-218 is a decay product of radon, which as a gas can be given off by a grain of uranium in one part of the rock and collected in another part of the rock to form a uraniumless halo. Gentry's examples rely on a radon ring that is close to the Po-210 ring and it is a bit difficult to tell them apart, and it is not certain whether the rings can be positively associated with polonium.[1]
[edit] Notes
- ^ Thomas A. Baillieul, "Polonium Haloes" Refuted 2001-2005, talk.orgins archives
[edit] References
Lide, David R. (Ed.) (2001). CRC Handbook of Chemistry and Physics, 82nd Ed.. London: CRC Press. ISBN 0-8493-0482-2.
Gentry, R.V. (October 1975). "Spectacle Haloes". Nature 258: 269–270.
S.R. Hashemi-Nezhad, J.H. Fremlin, and S.A. Durrani (October 1979). "Polonium Haloes in Mica". Nature 278: 333-335.
Gentry, R.V. (October 1973). "Radioactive Halos". Annual Review of Nuclear Science 23: 347-362.
R.V. Gentry (October 1974). "Radiohalos in a Radiochronological and Cosmological Perspective". Science 184: 62–66.
