Appendix: "Giant Radioactive Halos"
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Very recent mass spectrometric studies in which the Ion Microprobe Mass Analyzer (IMMA) (Applied Research Laboratories) was used revealed an isotope
ratio for Pb207 to Pb206 of about 0.16 for the halo inclusions as contrasted with a value of about 0.35 for the bulk monazite crystals
(13),which occur adjacent to
the mica (both values were uncorrected for common Pb). If subsequent work shows that this difference cannot be attributed to common Pb, this result might
suggest that a closer examination be made of possible high-energy isomers, namely, an isomer in a chain decaying to Pb.
The possibility that the giant halos originate with a postulated superheavy element
(14) in the region from atomic numbers 110 to 114 seems remote, since these
elements (i) would not be expected to occur in monazites and (ii) would be expected to exhibit spontaneous fission activity either directly or indirectly (that is,
to decay by way of alpha emission to the known spontaneous fission region below atomic number Z = 105)
(15). As noted earlier, some giant halo inclusions do
not exhibit background fission tracks. However, of special interest in this context are very recent theoretical calculations by Bassichis and
Kerman (16), which
indicate an island of superheavy element stability at somewhat higher Z (around 120). If such an element exists, it might be expected to occur in a pegmatitic
|ROBERT V. GENTRY|
Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37830
References and Notes
- G. H. Henderson and S. Bateson, Proc. Roy. Soc. London Ser. A Math. Phys. Sci.145, 563 (1934).
- E. Wiman, Bull. Geol. Inst. Univ. Uppsala 23, 1 (1930); H. Hirschi, Vierteljahresschr. Naturforsch. Ges. Zuerich 65, 209 (1920) (see Oak Ridge Nat. Lab. Rep.
ORNL-tr-702); J. Joly, Proc. Roy. Soc. London Ser. A Math. Phys. Sci. 102, 682 (1923); R. V. Gentry, Earth Planet. Sci. Lett. 1, 453 (1966); J. S. van der
Lingen, Zentralbl. Mineral. Geol. Palaeontol. Abt. A 1926, 177 (1926) (see Oak Ridge Nat. Lab. Rep. ORNL-tr-699).
- R. V. Gentry, Appl. Phys. Lett. 8, 65 (1966).
- G. Hoppe, Geol. Foren. Stockholm Forhandl. 81, 485 (1959) (see Oak Ridge Nat. Lab. Rep. ORNL-tr-756).
- I thank Dr. F. Young, Physics Department, University of Maryland, for cooperation in the Van de Graaff experiments.
- L. Kobren, Goddard Space Flight Center, National Aeronautics and Space Administration, and C. Feldman, Oak Ridge National Laboratory, performed the
electron micro-probe analyses, which revealed typical monazite constituents.
- G. G. Laemmlein, Nature 155, 724 (1945).
- I assumed that (i) (α,p) reactions in the inclusion occur mainly with phosphorus and have a cross section of 0.1 barn, (ii) weight fractions are 0.25, 0.05,
and 0.2 for Th, U, and P, respectively, and (iii) 0.2 is the fraction of U atoms decayed (≈ 0.1 from fission-track analysis on the mica); calculations then
show that the integrated proton flux from (α,p) reactions in an inclusion 25 μm in diameter is at least a factor of 102 below that required to produce
threshold coloration (≈ 1013 alpha particles per square centimeter from Van de Graaff irradiation) in a giant halo of radius 75 μm. Similar
considerations hold for (α,p) reactions on nuclides of low Z in the surrounding matrix.
- Variations of 1 μm in halo radii are noted even with point-like inclusions, possibly resulting from maximum ionization (coloration) occurring at slightly
less than end-point range. In larger inclusions the halo radius appears to increase several micrometers as the halo develops because a greater fraction of the
alpha particles are being emitted within the outermost micrometer of the inclusion. Nonuniform halo boundaries also occur and may result from a nonuniform
distribution of radioactivity in the inclusion or may be related to one of the unusual development modes previously described herein.
- I thank G. N. Flerov, Director, Laboratory for Nuclear Reactions, Joint Institute for [p. 228] Nuclear Research, Dubna, U.S.S.R., for this suggestion.
- R. V. Gentry, Science 160, 1228 (1968).
- T. P. Kohman, personal communication; U.S. At. Energy Comm. Rep. No. NYO-844-76 (1969), p. 74.
- This work was performed by C. Andersen, Applied Research Laboratories, Goleta, Calif. In the IMMA a finely focused (10 μm) O2-ion beam is used to
sputter material directly into a mass spectrometer.
- G. T. Seaborg, Annu. Rec. Nucl. Sci. 18, 53 (1968).
- Mass spectrometric analyses of mica containing the giant halo inclusions, performed at Ledoux & Company, Teaneck, N.J., and the GCA Corporation,
Bedford, Mass., indicated that, if superheavy elements are present, their abundance must be less than 200 parts per million. The IMMA analysis of the monazite
inclusions revealed the presence of what are almost certainly molecular ions with a mass of 303 (possibly CaThP), 310 (possibly La2O2) and somewhat higher
- W. H. Bassichis and A. K. Kerman, Phys. Rev., in press.
- I thank P. Ramdohr, University of Heidelberg, and H. de la Roche, National Center for Scientific Research, Nancy, France, for the Madagascar mica samples; I
thank J. Boyle, Oak Ridge National Laboratory, for valuable assistance with the experiments. The research performed at Oak Ridge National Laboratory was
sponsored by the U.S. Atomic Energy Commission under contract with Union Carbide Corporation. Part of the research was performed at Columbia Union
College, Takoma Park, Maryland, from which I am presently on leave of absence.
12 February 1970; revised 9 June 1970
Reprinted from SCIENCE
14 August 1970, Volume 169, pp. 670-673
|Copyright© 1970 by the|
American Association for the Advancement of Science
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