The last three alpha decay steps in the uranium-238 decay series (see glossary above) involve the successive decay of polonium-218 [p. 236] (218Po), polonium-214 (214Po), and polonium-210 (210Po). In contrast to the decay of the parent uranium, these steps occur very quickly; the half-lives of the three forms of polonium are 3.05 minutes, 164 microseconds, and 140 days, respectively. Polonium, therefore, is not thought to be observed in nature except as a daughter product of uranium and thorium decay.
That is where the enigma begins. For Gentry has analyzed numerous polonium halos possessing, in some cases, the rings for all three polonium isotopes; in other cases the rings for 214Po and 210Po; and in other cases, the ring for 210 alone—but none of these halos exhibits rings for the earlier uranium-238 daughters. These halos are evidence for parentless polonium, not derived from uranium.*
[* Gentry has also found halos with rings from polonium-218, -214, or -210, combined with a ring from polonium-212 which is in the thorium decay series. This last form of polonium is also parentless— that is, there are no halo rings for thorium itself or its other daughters.]
But the question then arises, How did the polonium inclusions ever become embedded in the host rocks (more specifically, in Earth's oldest—Precambrian—rocks)? On the conventional view, these rocks slowly cooled and crystallized out of the primordial magma (molten rock) over millions of years. Under such circumstances, any polonium (with its extremely short half life) that was incorporated into the solidifying rocks would have completely decayed long before the crystalline rock structure was established. No halos could have formed, for they consist precisely of radiation damage to this crystalline structure. Polonium rings should exist only in conjunction with the other uranium series rings. But since the actual halos were caused by parentless polonium, they require nearly instantaneous crystallization of the rocks, simultaneously with the synthesis or creation of the polonium atoms.
Gentry, well aware that this conclusion is unthinkable to most, has buttressed it with impressive experimentation: fission track and neutron flux techniques (3) reveal no uranium in the inclusions that could have given rise to the polonium—a conclusion more recently confirmed by electron microscope x-ray fluorescence spectra (4); fossil alpha recoil [p. 237] analysis (3) demonstrates that neither polonium nor other daughter products migrated from neighboring uranium sources in the rock, which agrees with calculations based on diffusion rates (5); ion microprobe mass spectrometry yields extraordinarily high 206Pb/207Pb isotope ratios that are wholly inconsistent with normal decay modes (6), but which are exactly what one would expect as a result of polonium decay in the absence of uranium.
To date there has been only one effort (7) to dispute Gentry's identification of polonium halos. As it turned out (4), that effort might better never have been written, the authors having been impelled more by the worry that polonium halos "would cause apparently insuperable geological problems," than by a thorough grasp of the evidences. Challenges to Gentry's interpretation of the polonium halos have been more noteworthy. English physicist J. H. Fremlin wrote in Nature (November 20, 1975) that "The nuclear geophysical enigma of the 210Po halos is quite fascinating, but the explanation put forward is not easy either to understand or to believe." Fremlin proposed two possible explanations:
Geologic transfer. If there are uranium inclusions reasonably close to polonium halos, then it is possible that one or more of the uranium daughter products migrated from the uranium site to a new location, where subsequent decay gave rise to the polonium halo. Since the daughter products have much shorter half-lives than uranium, we would not expect to find any quantity of them remaining at the site of the halo. The polonium would therefore appear to be "parentless." The difficulty with this view is that transfer of uranium daughters in minerals occurs so slowly that the daughters would decay long before they could migrate any significant distance (3, 5).
If the sophisticated experimentation cited above proved telling against the transfer hypothesis, Gentry and several co-workers delivered a yet more conclusive blow in a very recent paper: polonium halos derived by geologic transfer from uranium sources have now actually been found in coalified wood deposits (8). Their presence here was to be expected: prior to coalification the wood was in a gel-like condition permeated by a uranium-bearing solution. Such a material "would exhibit a much higher transport rate as well as unusual geochemical conditions which might favor the accumulation of 210Po"—quite different from the situation in mineral rocks. Further, of these uranium-derived polonium halos, none were found due to 218Po, and only three could conceivably (but doubtfully) be attributed to 214Po, in contrast to numerous 210Po halos. The half-life of 210Po we will recall, is 140 days, whereas the half-life of those forms of polonium which failed to generate halos in the coalified wood is a few minutes or less. So even under the ideal conditions in this wood, the short-half-lived 218Po and 214Po were not able to migrate rapidly enough from the parent uranium to form "parentless" halos. Clearly, then, such migration could not account for the 218Po and 214Po halos Gentry has found in Precambrian minerals, where the diffusion rate is very much lower even than in wood (5).
Isomer precursors. Two atoms with identical nuclear composition but different radioactive behavior are termed "isomers." For example, 212Po (in the thorium decay series) decays to 208Pb by emission of an alpha particle with an energy of 8.78 MeV. However, about one out of every 5500 212Po atoms emits an alpha particle with a much higher energy of 10.55 MeV. These rarely occurring, higher-energy 212Po atoms are isomers, and they are apparently explained by some variation in nuclear structure. The suggestion has been made, therefore, that polonium halos may result from the presence of heretofore unknown isomers which are long-lived and which decay* into polonium. These isomers ("precursors" of polonium) would circumvent the cosmological problem caused by the short-half-life polonium.
[* by beta-emission]
However, not only are such isomers unknown, but a careful search has revealed the presence of no elements which might qualify as the required isomers (4, 5). "Experimental results have ruled out the isomer hypothesis" (5).
Earth Science Associates