Radiohalo

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This article is about the geophysical phenomenon. For the astronomical phenomenon see radio halo.

Radiohalos are microscopic, spherical shells of discolouration 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 discolouration 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. See:http://www.talkorigins.org/faqs/po-halos/gentry.html .

Contents

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 discolour 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 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.

Christian Schnier has suggested that anomalous radiohalos result from the decay chain of unidentified Superheavy elements in the earth's crust. He postulates the presence of two long-lived superheavy elements, one with an atomic mass of between 280 and 300, and another with an atomic mass greater than 300.

Controversy

See also: Creation geophysics

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.[1]

Gentry's work has been continued and expanded by the creationist Radioactivity and the Age of the Earth (R.A.T.E.) project that was operating between 1997 and 2005. Radiohalos were studied as part of the R.A.T.E. project by creationists such as Andrew Snelling of Answers in Genesis, Russell Humphreys, John Baumgardner and Steven A. Austin at the Institute of Creation Research as well as others at the Creation Research Society. However, Lorence G. Collins, J. Richard Wakefield and others have repeatedly and soundly rebutted the radiohalo evidence for a young earth in peer-reviewed publications.

References

  1. ^ Thomas A. Baillieul, "Polonium Haloes" Refuted 2001-2005, talk.origins archives


Citations

Favoring a young earth interpretation

  • Gentry, R.V. (October 1975). "Spectacle Haloes". Nature 258: 269–270. 
  • Gentry, R.V. (October 1973). "Radioactive Halos". Annual Review of Nuclear Science 23: 347-362. 
  • Gentry, R.V. (October 1974). "Radiohalos in a Radiochronological and Cosmological Perspective". Science 184: 62–66. 

Disputing a young earth interpretation

  • S.R. Hashemi-Nezhad, J.H. Fremlin, and S.A. Durrani (October 1979). "Polonium Haloes in Mica". Nature 278: 333-335. 
  • Schnier, C (August 2002). "Indications for the existence of superheavy elements in radioactive halos". Journal of Radioanalytical and Nuclear Chemistry, 253: 209-216. 
  • Ellenberger, C.L., with reply by Gentry, R.V. 1984. "Polonium Halos Redux," Physics Today. December 1984. pp. 91-92
  • Ellenberger, C.L. 1986. "Absolute Dating," unanswered surrebuttal to Gentry, Physics Today. March 1986. pp. 152, 156
  • Osmon, P., 1986, "Gentry’s pleochroic halos: Creation/Evolution," Newsletter, Feser, Karl D., Editor, v. 6, no. 1, Concord College, Athens, West Virginia
  • Schadewald, R., 1987. "Gentry’s tiny mystery, Creation/Evolution" Newsletter, Fezer, Karl D, Editor, v. 4, no. 2 & 3. Concord College. Athens. West Virginia, p 20.
  • Wakefield, J. R., 1987-88, "Gentry’s Tiny Mystery - unsupported by geology," Creation/Evolution, v. 22, p. 13-33.
  • Wakefield, J. R., 1988, "The geology of 'Gentry’s Tiny Mystery,'" Journal of Geological Education, v. 36, p. 161-175.
  • Moazed, Cyrus; Richard M. Spector; Richard F. Ward, 1973, Polonium Radiohalos: An Alternate Interpretation, Science, Vol. 180, pp. 1272-1274.
  • Odom, L.A., and Rink, W.J., 1989, "Giant Radiation-Induced Color Halos in Quartz: Solution to a Riddle," Science, v. 246, pp. 107-109.
  • York, D., 1979, Pleochroic Halos and Geochronology, EOS, v. 60, no. 33, pp. 617-618, Aug. 14, 1979 (publication of the American Geophysical Union).
  • Henderson, G. H., A quantitative study of pleochroic halos, V, The genesis of halos, Proc. Roy. Soc. , A, 173, 250-264, 1939.
  • Henderson, G. H., and F. W. Sparks, A quantitative study of pleochroic halos, IV, New types of halos, Proc. Roy. Soc., A, 173, 238-249, 1939.

Other

  • Lide, David R. (Ed.) (2001). CRC Handbook of Chemistry and Physics, 82nd Ed.. London: CRC Press. ISBN 0-8493-0482-2. 

External links

Favoring a young earth interpretation

Disputing a young earth interpretation

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