Radiohalos or pleochroic halos are microscopic, spherical shells of discolouration within minerals such as biotite that occur in granite and other igneous rocks. The shells are zones of radiation damage caused by the inclusion of minute radioactive crystals within the host crystal structure. The inclusions are typically zircon, apatite, or titanite which can accommodate uranium or thorium within their crystal structures (Faure 1986). One explanation 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).
Uranium-238 follows a sequence of decay through thorium, radium, radon, polonium, and lead. These are the alpha-emitting isotopes in the sequence. (Because of their continuous energy distribution and greater range, beta particles cannot form distinct rings.)
|Isotope||Half-life||Energy in MeV|
|U-238||×109 years 4.47||4.196|
|U-234||×105 years 2.455||4.776|
The final characteristics of the radiohalo depends upon the initial isotope, and the size of each ring of a radiohalo is dependent upon the alpha decay energy. A radiohalo formed from U-238 has theoretically eight concentric rings, with five actually distinguishable under a lighted microscope, while a radiohalo formed from polonium has only one, two, or three rings depending on which isotope is the starting material (Weber 2010). In U-238 haloes, U-234, and Ra-226 rings coincide with the Th-230 to form one ring; Ra-222 and Po-210 rings also coincide to form one ring. These rings are indistinguishable from one another under a lighted microscope (Pal 2004).
Creationist Robert V. Gentry looked at halos which he said arose 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. He believes that they could not have been formed from molten rock which took many millennia to cool because polonium decays in a few minutes. As a creationist, he took this as evidence that the Earth was formed instantaneously (Gentry 1992). However, Gentry's "polonium halos" are found along microscopic cracks in rocks that also contain uranium halos (Wakefield 1988)(Collins 1997) and Po-218 is also 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 migrate to another part of the rock to form a uraniumless halo (Baillieul 2005)(Brawley 1992).
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