Fig. 2. Fossil α-recoil centers in the vicinity of a 210Po halo (phase contrast).

The mean fossil α-recoil densities were 12.7 × 10⁶ and 11.6 × 10⁶ α / cm² for the Canadian and Irish micas, respectively, regardless of where the α-recoil count was taken. For a given etch period these results are reproducible within ± 10 percent. The fission-track density exhibited a random distribution in each piece of mica except (as expected) near the ²³⁸U halos. The α-recoil:fission-track ratios were about 2.5 × 10³ and 3.0 × 10³, respectively, for the Canadian and Irish micas. Huang and Walker (16) have shown that the background α-recoil density in micas is due to both uranium and thorium α-decay; by using 100 Å and 10 μ for the alpha-recoil and fission-track ranges, respectively, one can determine that uranium alone contributes an α-recoil:fission-track ratio of about 2.2 × l0³, any excess being due to thorium. Figure 2 portrays a ²¹⁰Po halo (Irish mica) showing the distribution of α-radioactivity (fossil α-recoil centers) in the vicinity.

As far as the experimental analysis is concerned, there is no detectable difference in the microscopic distribution of α-radioactivity (with respect to background density) near either the uranium or the polonium halos. [I note that thin clefts, which usually result near the edges of the mica from weathering (but not within the bulk of the mica), are easily detected by an acid etch since α-recoil tracks appear throughout the extent of the cleft area.] This finding seems to imply that there was no gross transport of α-radioactivity to the polonium-halo inclusions (i) by way of laminar flow of solutions (through thin clefts) disequilibrated as to uranium daughter-product activity, or (ii) by diffusion of radon, since an increased α-recoil density, higher than background by several orders of magnitude, should be evident within a l0-μ radius of the halo inclusions in either case. This last value is a conservative estimate, for I have considered only the decay of ²¹⁸Po atoms en route to an inclusion. Furthermore, autoradiographic experiments on the samples of Canadian mica containing ²³⁸U,²³²Th, and polonium halos showed only the normal background distribution of α-tracks, indicating that if excess activity now exists it is below the detection level of the method.

Thus, as far as the experimental analysis is concerned, I cannot confirm Henderson's model for the secondary origin of the polonium halos. To the question of what mode of origin is consistent with the relatively short half-lives of the polonium isotopes (or their β-decaying precursors), I can say only that other mechanisms are under study.

Whatever hypothesis is invoked, to explain the origin of the polonium halos, must also explain both the one found by Henderson (19) [due to a combination of isotopes from both the thorium series (²¹²Po and ²¹²Bi) and the uranium series (²¹⁰Po)] and a halo presumably due to ²¹¹Bi (12) from the ²³⁵U series. Perhaps most interesting of all is the occurrence of 20,000 to 30,000 ²¹⁸Po and ²¹⁰Po halos per cubic centimeter in a Norwegian mica—without the ²¹⁴Po halos.

References and Notes

1. J. Joly, Phil. Mag. 13, 381 (1907).
2. O. Mugge, Zentr. Mineral. 1907, 397 (1907) (see Oak Ridge National Laboratory ORNL-tr-757).
3. J. Joly, Phil. Trans. Roy. Soc. London Ser. A 217, 51 (1917); P. Ramdohr, Geol. Rundschau 49, 253 (1960) (see ORNL-tr-758).
4. C. O. Hutton, Amer. J. Sci. 245, 154 (1947).
5. J. Joly, Nature 109, 480 (1922); F. Lotze, ibid. 121, 90 (1928).
6. G. Gamow. Phys. Rev. Letters 19, 759 (1967).
7. J. Joly, Proc. Roy. Soc. London Ser. A 102, 682 (1923).
8. S. Iimori and J. Yoshimura, Sci. Papers Inst. Phys. Chem. Res. 5, 11 (1926); A. Schilling, Neues Jahrb. Mineral. Abhandl. 53A, 241 (1926) (see ORNL-tr-697).
9. J. S. van der Lingen, Zentr. Mineral. Abt. A 1926, 177 (1926) (see ORNL-tr-699); C. Mahadevan, Indian J. Phys. 1, 445 (1927); H. Hirschi, Vierteljahresschr. Naturforsch. Ges. Zuerich 65, 209 (1920) (see ORNL-tr-702); E. Wiman, Bull. Geol. Inst. Univ. Uppsala 23, 1 (1930); G. H. Henderson, Proc. Roy. Soc. London Ser. A 173, 238 (1939).
10. G. H. Henderson, Proc. Roy. Soc. London Ser. A 173, 250 (1939).
11. R. V. Gentry, Appl. Phys. Letters 8, 65 (1966); Earth Planetary Sci. Letters 1, 453 (1966).
12. ——, Nature 213, 487 (1967).
13. Observations on this and other class-II halos will be reported.
14. I thank Larry Kobren, Goddard Space Flight Center, for the electron-microprobe analysis. Also I thank Truman Kohman, Carnegie-Mellon University, for suggesting the micro-probe experiments and for valuable discussions concerning the origin of the halos.
15. R. L. Fleischer, P. B. Price, R. M. Walker, Science 149, 383 (1965).
16. W. H. Huang and R. M. Walker, ibid. 155, 1103 (1967).
17. J. Boyle and R. V. Gentry, in preparation.
18. G. H. Henderson, Proc. Roy. Soc. London Ser. A 145, 591 (1934).
19. I thank G. C. Milligan and other members of the geology and physics departments of Dalhousie University, Halifax, for the loan of Henderson's halos and microphotographs. The halo referred to is in this collection.
20. I thank Paul Ramdohr, University of Heidelberg, for this particular specimen. Also I thank R. R. Gorbatschev (Uppsala), B. Loberg (Stockholm), D. E. Kerr-Lawson (Swastika, Ontario), J. H. J. Poole (Trinity College), and J. A. Mandarino (Royal Ontario Museum) for other mica specimens containing halos. I also thank H. L. Price for assisting in the α-recoil analysis and John Boyle. Oak Ridge National Laboratory, for the α-recoil experiments. For more extensive investigation I would appreciate contributions of samples of biotite from as many Precambrian localities as possible.

26 April 1968