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 biotite caused by the inclusion of radioactive grains in the rock or by deposition of radioactive material in them. One widely accepted theory is that the discolouration is caused by alpha particles emitted by the nuclei; the radius of the concentric shells are proportional to the particle's energy (Henderson & Bateson 1934). The phenomenon of radiohalos has been known to geologists since the early part of the 20th century, but wider interest was prompted by the claims of creationist Robert V. Gentry that radiohalos in biotite are evidence for a young earth (Gentry 1992). The claims are contested by the mainstream scientific community as an example of creationist pseudoscience (Wakefield 1988).

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[edit] Production

Uranium-238 follows a sequence of decay through thorium, radium, radon, polonium, and lead. These are the alpha-emitting isotopes in the sequence. (According to the standard theory, beta particles do not discolour the rock, although Baillieul (2005) suggests the need to reexamine the possible role of beta emission.)

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,[citation needed] 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 theoretically eight concentric rings, with five actually distinguishable, while a radiohalo formed from polonium can have only one, two, or three rings depending on which isotope is the starting material.[citation needed] However, a halo formed from Ra-222 is not distinguishable from one formed from Po-218.[citation needed]

Christian Schnier (Schnier 2002) 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.

[edit] Controversy

See also: Creation geophysics

Robert V. Gentry studied halos which appeared to have arisen from Po-218 rather than U-238 and concluded that solid rock must have been created with these polonium inclusions, which decayed with a half-life of 3 minutes.[citation needed] They could not have been formed from molten rock which took many millennia to cool (the standard theory) because polonium decays in a few minutes. This is taken by creationists as evidence that the Earth was formed instantaneously (Gentry 1992).

Critics of Gentry, including Thomas A. Baillieul (Baillieul 2005) and John Brawley (Brawley 1992), 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. Apparently a large number of radon atoms are caught or adsorbed at a particular point. This has not been proved experimentally, but is supported by the fact that Gentry's "polonium halos" are found along microscopic cracks in rocks that also contain uranium halos (Wakefield 1988).

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 (Wieland 2003). 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, Collins (1997), Wakefield (1988) and others have repeatedly and soundly rebutted the radiohalo evidence for a young Earth in peer-reviewed publications.

[edit] Citations

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

[edit] Disputing a young earth interpretation

  • Durrani, S.A. & Fremlin, J.H. (1979), “Polonium Haloes in Mica”, Nature 278: 333-335, October 1979 .
  • C, Schnier (2002), “Indications for the existence of superheavy elements in radioactive halos”, Journal of Radioanalytical and Nuclear Chemistry, 253: 209-216, August 2002 .
  • 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.
  • 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.

[edit] Other

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

[edit] External links

[edit] Favoring a young earth interpretation

[edit] Disputing a young earth interpretation