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Comments by Leading Scientists Before the demise of the journal, Pensée, the editor—in preparation for a planned article on Gentry's work—approached a number of leading scientists for their assessment of polonium halos. The following responses were received during the first month or so of 1975. PROFESSOR TRUMAN P. KOHMAN, Department of Chemistry, Carnegie-Mellon University, Pittsburgh. "I do not believe that 'Gentry's contentions' can be regarded as of 'rather startling nature.' However, some of his experimental findings (like those of his predecessors) are quite difficult to understand, and the ultimate explanations could be interesting and even surprising. Many persons probably do not take them seriously, believing either that there is something wrong with the reported findings or that the explanations are to be found in simple phenomena which have been overlooked or discarded. . . . I believe it can be said that Gentry is honest and sincere, and that his scientific work is good and correctly reported. It would be very hard to believe that all, or any, of it could have been fabricated PROFESSOR EDWARD ANDERS, Enrico Fermi Institute, University of Chicago. "His [Gentry's] conclusions are startling and shake the very foundations of radiochemistry and geochemistry. Yet he has been so meticulous in his experimental work, and so restrained in his interpretations, that most people take his work seriously. . . . I think most people believe, as I do, that some unspectacular explanation will eventually be found for the anomalous halos and that orthodoxy will turn out to be right after all. Meanwhile, Gentry should be encouraged to keep rattling this skeleton in our closet for all it is worth." DR. EMILIO SEGRE, Istituto Di Fisica "Guglielmo Marconi," Università Degli Studi, Rome. "The photos [of radiohalos] are remarkable, but their interpretation is still uncertain." PROFESSOR FREEMAN DYSON, Institute for Advanced Study, Princeton. "Supposing that the results of Gentry are confirmed, what will it mean for theory? I do not think it will mean any radical changes in geology or cosmology. It is much more likely that the explanation will be some tricky point in nuclear physics or nuclear chemistry that the experts have overlooked. That is of course only my personal opinion and I am accustomed to being proved wrong by events. (I just lost a $10 bet that Nixon would be in office till the end of 1974. I will be glad to lose this one too.)" ACADEMICIAN G. N. FLEROV, Joint Institute for Nuclear Research, Moscow. "We made sure that [Gentry] carried out his investigations very thoroughly. . . . Therefore his data deserve serious attention. . . . It is not excluded that [polonium halos] have been formed as a result of the extremely rare combination of geochemical, geological and other conditions, and their existence does not contradict the logically grounded system of concepts involved in the history of Earth formation." DR. PAUL RAMDOHR, Emeritus Professor of Mineralogy, Heidelberg University, Heidelberg. "The very careful and timetaking examinations of Dr. Gentry are indeed very interesting and extremely difficult to explain. But I think there is no need to doubt 'currently accepted cosmological models of Earth formation'. . . . Anyhow, there is a very interesting and essential question and you could discuss it, perhaps with cautious restrictions against so weighty statements like the one above in quotes. It would be interesting and good if more scientists would have more knowledge of the problems." PROFESSOR EUGENE P. WIGNER, Department of Physics, Rockefeller University, New York. "Even though I know Dr. Gentry personally, I am not sufficiently familiar with his scientific results to be able to judge them. Personally, however, I have a very high regard for him." DR. E. H. TAYLOR, Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee. "I can attest to the thoroughness, care and effort which Gentry puts into his work. . . . In a general way these puzzling pieces of information might result from unsuspected species or phenomena in nuclear physics, from unusual geological or geochemical processes, or even from cosmological phenomena. Or they (or one of them) might arise from some unsuspected, trivial and uninteresting cause. All that one can say is that they do present a puzzle (or several puzzles) and that there is some reasonable probability that the answer will be scientifically interesting."

POLONIUM HALOS

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).

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