An Invitation to Join a National Laboratory

In addition to my research on polonium halos, I had continued to study some unusual halo types known as the dwarf and giant halos. Their rarity and uncommon sizes suggested they might have originated with an unknown type of radioactivity. In late 1968 the U.S. Atomic Energy Commission (AEC) first became aware of my research on dwarf and giant halos through a contact I initiated with the scientist who was then Chairman of the AEC. Subsequently, arrangements were made for me to give a seminar on my research at the Lawrence Radiation Laboratory (now the Lawrence Berkeley Laboratory) and the Oak Ridge National Laboratory (ORNL). Both laboratories were among several around the world which were then initiating a search for superheavy elements—chemical elements with atomic weights heavier than any previously discovered in nature. Because the dwarf and giant halos seemed to be evidence of unknown radioactivity, I was invited to affiliate with ORNL as a guest scientist and join them in their search for superheavy elements. This one-year opportunity, which stretched to thirteen years, greatly accelerated my research.

Before joining the Oak Ridge National Laboratory the AEC wrote letters of introduction enabling me to visit two well-known Soviet scientists who were involved in the search for superheavy elements. My trip to the Soviet Union in the spring of 1969 included stops in Moscow and Dubna, where the Soviet nuclear laboratory equivalent of ORNL is located.

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My move to ORNL occurred in July 1969. By 1970 I had completed a series of new experiments on giant halos using the advanced scientific instrumentation available there. A manuscript detailing those results was prepared for publication. After it passed the standard internal review process at ORNL, it was submitted to Science. With minor revisions this report was published in August 1970 as "Giant Radioactive Halos: Indicators of Unknown Alpha-Radioactivity?" (Gentry 1970; Appendix). Eight possible explanations for the origin of the giant halos were examined, but at that time none, including superheavy elements, could be identified as the final solution. The origin of the giant halos remained an enigma, and this attracted attention to my research.

Search for Halos in Lunar Rocks

Soon after joining ORNL as a guest scientist, I submitted a proposal to the National Aeronautics and Space Administration (NASA) to search for halos in rocks returned from the Apollo 11 mission to the moon. This proposal was accepted by NASA, and a search was made of the thin sections of the lunar rocks then available. No halos were found. This is not surprising when one considers that the minerals which most often contain halos (in earth rocks) are generally absent from the lunar rocks returned on the Apollo missions. In addition, most of those lunar rocks had recrystallized from molten material produced by meteorite impact. Any halos, if they had existed, would have been destroyed in this process. My summary report on these investigations was published in the Proceedings of the Second Lunar Science Conference (Gentry 1971a).

Polonium Halo Analysis

The same advanced analytical techniques employed to study the giant halos were also adaptable to the study of polonium halos. Most of my earlier research on polonium halos had involved the optical microscope, in combination with chemical etching and neutron irradiation techniques. These procedures were quite useful in showing that uranium was generally absent around the polonium halos, but they could not reveal the composition of the halo centers. With the equipment available at ORNL, I analyzed the centers of the halos, the tiny specks where the radioactive atoms themselves were originally encased. Using advanced mass spectrometry techniques I discovered that polonium halo radiocenters contained a composition of the [p. 45] chemical element lead which was different from any previously known. This new type of lead, greatly enriched in the isotope ²⁰⁶Pb, could not be accounted for by uranium decay; yet it was exactly that expected on the basis of the decay of polonium in the halo center. These experimental results, along with others obtained on the puzzling dwarf halos, formed the basis of another report published in Science in 1971 (Gentry 1971b).

I expected the discovery of this new type of lead in polonium halo radiocenters to attract more attention to my work on polonium halos than my previous reports. Evidence that this had happened came in 1972 when I received an invitation to contribute a review article on radioactive halos for the Annual Review of Nuclear Science (ARNS). My review article was published in the 1973 edition (Gentry 1973). My ARNS article briefly discussed (1) limitations in the original arguments used to establish a uniform radioactive decay rate over geological time, (2) characteristics of a number of unusual types of radioactive halos (dwarf and giant halos) whose origin was still under investigation, and (3) evidence for the existence of primordial polonium halos featuring the results of my most recent experiments at Oak Ridge. In that article I again drew attention to the implications associated with their existence:

Now the reason for the various attempts to account for Po halos by some sort of secondary process is quite simple; the half-lives of the respective Po isotopes are far too short to be reconciled with slow magmatic cooling rates for Po-bearing rocks such as granites (t½ = 3 min for ²¹⁸Po). (Gentry 1973, 356).

A Novel Theory of Polonium Halo Origin

About the time that I was preparing the ARNS review article, a colleague who had become interested in my work privately suggested an alternative explanation of polonium halos. He speculated that an uncommon (isomeric) form of radioactivity might have been the source of the polonium. Some colleagues and I used mass spectrometry techniques to investigate this possibility but found no experimental evidence to support it. (Chapter 5 cites the results of a renowned nuclear physicist who later excluded the isomer hypothesis on the basis of his theoretical studies.) Our results were published in Nature in August of 1973 (Gentry et al. 1973; Appendix). The following quote from that report shows how attention was again focused on the implications of the polonium halos in Precambrian granites:

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. . . A straightforward attempt to account for the origin of these Po haloes by assuming that Po was incorporated into the halo inclusion at the time of host mineral crystallization meets with severe geological problems: the half-lives of the polonium isotopes (t½ = 3 min for ²¹⁸Po) are too short to permit anything but a rapid mineral crystallization, contrary to accepted theories of magmatic cooling rates. (Gentry et al. 1973, 282—italics mine)

Suggesting a rapid synthesis of the earth's basement rocks was like raising a red flag before some of my colleagues. Such statements invited scientists to refute my results if it could be done.