A physicist's discovery begins an extraordinary odyssey
pride and prejudice in the scientific world.
By Dennis Crews
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Gentry's research on radiohalos had familiarized him with
a number of other unusual halo types for which no specific causative
element could be determined. Because of their rarity and unusual
sizes it was thought that they might have originated with an
unknown type of radioactivity. When the U.S. Atomic Energy commission
became aware of Gentry's research on these dwarf and giant halos,
he was invited to give seminars on his work at the Lawrence Radiation
Laboratory and at Oak Ridge National Laboratory, where scientists
were conducting an intensive search for superheavy elements.
Eventually he was invited to affiliate with Oak Ridge National
Laboratories as a guest scientist. The initial one-year invitation
ultimately stretched to thirteen years, and provided incalculable
benefits for Gentry's own research as well.
The sophisticated facilities at ORNL greatly accelerated Gentry's
study of polonium halos and before long more of his work was
in the literature. After a second article in Science,9 a report
on his investigation of lunar rock samples was published in Proceedings
of the Second Lunar Science Conference.10 Then came another significant
Advanced mass spectrometry techniques enabled Gentry to discover
that the tiny radiocenters of polonium halos were composed of
a type of lead different from any previously known. The new type
of lead, greatly enriched in certain isotopes, could not be accounted
for by uranium decay, but only by the decay of polonium. This
was another stroke of evidence against secondary origin for the
halos, and formed the basis for a third paper published in Science.11
This discovery attracted more attention than any previous report,
and soon Gentry was invited to contribute an article on radioactive
halos to the 1973 Annual Review of Nuclear Science.12
About this time a colleague of Gentry's privately suggested
that an uncommon isomeric form of radioactivity had caused the
polonium halos. Gentry investigated this hypothesis using mass
spectrometry techniques and found no experimental evidence to
support it. The results were published in Nature in August of
1973.13 By this time Gentry's credibility had been established
sufficiently enough for him to suggest there might be cosmological
implications in the existence of polonium halos. The Nature article
carefully stated: "...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 are too short to permit anything but a rapid
mineral crystallization, contrary to accepted theories of magmatic
Suggesting a rapid formation of the earth's oldest rocks,
Gentry knew, would be highly provocative to many of his colleagues.
Statements like this invited other scientists to refute his research
if it could possibly be done. It didn't take long for several
to rise to the challenge. In the June 22, 1973 issue of Science
three scientists attempted to put Gentry in his place by suggesting
that polonium halos did not really exist at all: "We cannot definitely
rule out the existence of polonium halos, but it appears that
there is no evidence requiring, or even firmly suggesting, their
existence. It was realized very early that their existence would
cause apparently insuperable geological problems since the relevant
polonium half-life is of the order of minutes. Polonium halos
would require that the polonium atoms become part of the inclusion
within minutes of the formation of the polonium and that in this
very short time the polonium must be so far removed from the
parent uranium mass that its presence or location is no longer
The issue was open now for all to see. These scientists had
identified the very root of the problem, yet in their haste to
protect popular assumptions preferred, ostrich-like, to believe
that polonium halos were not really there. Later in a review
of another of Gentry's articles Research Communications Network
noted: "To date there has been only one effort to dispute Gentry's
identification of polonium halos. As it turned out, 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
Gentry next turned his attention to polonium halos in fluorite,
a crystal that occurs in granite. Clear areas in fluorite crystals
are even more impermeable to fluid infiltration than those in
mica, which has a laminar structure. The halos in fluorite proved
virtually identical to those in mica. This discovery fueled another
article16 which was published by Science in early 1974 and provided
still more evidence against a secondary origin for the polonium
Gentry's position at Oak Ridge permitted him to use a variety
of advanced tools and techniques in his experiments, including
particle accelerators, mass spectrometers, scanning electron
microscopy and x-ray fluorescence analysis. As his research continued
to close one door after another on alternative explanations for
polonium halos, his articles in various scientific journals became
more specific in their suggestion that conventional geologic
timetables might need revision.
For several years only a few scientists ventured to challenge
the results of Gentry's research. Of those who did, most were
unfamiliar with other phases of his work which already answered
their objections. Gentry remained his own most tireless critic,
carefully exploring every step of his research for new data that
would explain polonium halos according to conventional geological
and cosmological models. With each ensuing experiment no such
data appeared, however. The only apparent explanation for the
halos remained the one Gentry had suspected from the very beginning—special
The critics' silence was not to last indefinitely. While
many of his colleagues were still trying to divine the significance
of his carefully worded conclusions, one sagely prophesied, "Gentry
can be sure that, in pressing his own decidedly radical explanations,
the sound and fury lie yet before him."17 It was only a matter
9 Gentry, 1970. "Giant Radioactive Halos: Indicators of Unknown Radioactivity?" Science, vol. 169, p. 670.
10 Gentry, 1971. "Radioactive Halos and the Lunar Environment." Proceedings of the Second Lunar Conference, vol. 1, p. 167. Cambridge: MIT Press.
11 Gentry, 1971. "Radiohalos: Some Unique Pb Isotope Ratios and Unknown Alpha Radioactivity." Science, vol. 173, p. 727.
12 Gentry, 1973. "Radioactive Halos." Annual Review of Nuclear Science, vol. 23, p. 347.
13 Gentry, 1973. "Ion Microprobe Confirmation of Pb Isotope Ratios and Search for Isomer Precursors in Polonum Radiohalos." Nature, vol. 244, no. 5414, p. 282.
14 C. Moazed, R. M. Spector, R. F. Ward, 1973. "Polonium Halos: an Alternate Interpretation." Science, vol. 180, p. 1272.
15 S. L. Talbott, 1977. "Mystery of the Radiohalos." Research Communication Network, Newsletter No. 2.
16 Gentry 1974. "Radiohalos in Radiochronological and Cosmological Perspective." Science, vol. 184, p. 62.
17 Talbott, op. cit.
For readers interested in a more comprehensive treatment of this story, Robert Gentry's book, Creation's Tiny Mystery, is available for $18 (U.S.) + S/H.
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