The x-ray data in Fig. 3c are unambiguous and should remove any doubt that previously reported ²⁰⁶Pb/²⁰⁷Pb mass ratios (3, 13) actually are Pb isotope ratios, and in fact represent a new type of Pb derived specifically from Po α-decay. In summary, the combined results of ring structure studies, mass spectrometric analyses, and electron induced x-ray fluorescence present a compelling case for the independent existence of Po halos. The question is, can they be explained by presently accepted cosmological and geological concepts relating to the origin and development of Earth?

References and Notes

1. G. H. Henderson, C. M. Mushkat, D. P. Crawford, Proc. R. Soc. Lond. Ser. A Math. Phys. Sci. 158, 199 (1934); G. H. Henderson and L. G. Turnbull, ibid. 145, 582 (1934); G. H. Henderson and S. Bateson, ibid., p. 573.
2. J. Joly, ibid. 217, 51 (1917); Nature (Lond.) 109, 480 (1920). I have examined Joly's collection and found that he associated certain Po halos with U halos and incorrectly associated the ²¹⁰Po halo as originating with Rn α-decay.
3. R. V. Gentry. Science 173, 727 (1971).
4. ——, ibid. 169, 670 (1970).
5. ——, ibid. 160, 1228 (1968).
6. G. H. Henderson and F. W. Sparks, Proc. R. Soc. Lond. Ser. A Math. Phys. Sci. 173, 238 (1939).
7. G. H. Henderson, ibid., p. 250. A fourth type attributed to ²²⁶Ra α-decay is in error.
8. S. Iimori and J. Yoshimura, Sci. Pap. Inst. Phys. Chem. Res. Tokyo 5, 11 (1926).
9. A. Schilling. Neues Jahrb. Mineral. Abh. 53A, 241 (1926). See translation, Oak Ridge Natl. Lab. Rep. ORNL-tr-697. Schilling, as did Joly, erroneously designated ²¹⁰Po halos as emanation halos. As for explanation of the 14.0-μm, 14.4-μm, and 15.8-μm rings which Schilling attributed to UI, UII, and Io, I can state that one of the rings at 14.0 μm and 14.4 μm is a ghost ring. I also rarely observe a light at about 16 μm, but do not presently associate this ring with ²³⁰Th (Io) α-decay.
10. C. Mahadevan, Indian J. Phys. 1, 445 (1927).
11. C. Moazed, R. M. Spector, R. F. Ward, Science 180, 1272 (1973).
12. Moazed et al. (11) stated that because they could not find halos with dimensions matching those of Henderson's type B halo (the ²¹⁴Po halo in my terminology) such halos do not exist; however, Henderson gave both measurements and photographic evidence (6, figure 4, facing p. 242). They then inferred that a different halo (a U halo) must be the equivalent of the type B halo, although the radii of the inner ring of Henderson's type B halo and the outer second ring of their halo were significantly different (20 compared to 22.3 μm). They concluded that all Po halos are only U halos, ] without having U halos with normal ring structure available for comparison. I showed (5) that Po halos and U halos are distinguished by the number of fossil fission tracks after etching; that is, few, if any, compared to a cluster of 20 to 100 tracks. I also showed that the threshold coloration dose is directly obtainable by converting a U halo fossil fission-track count (20 to 100) to the number of emitted α-particles by using the ²³⁸U branching ratio, λ_α/λ_f; this contradicts the supposition that such data are unknown to two orders of magnitude. Ion probe analyses of U halos show that a high U isotopic ratio can not be responsible for a small induced fission-track count. Furthermore, contrary to a statement by Moazed et al., Henderson was able to distinguish reliably between his type B and type C halos (6, pp. 246-248).
13. R. V. Gentry, S. S. Cristy, J. F. McLaughlin, J. A. McHugh, Nature (Lond.) 244, 282 (1973).
14. The irradiated biotite samples were cleaved in about 5-μm sections for microscopic examination. The coloration threshold (CT) for 30-μm biotite sections varied from 3 × 10¹³ to 6 × l0^{13 4}He ions per square centimeter. Band sizes monotonically increased with dose to about 100 CT but were reproducible in a plateau region around 10 to 20 CT. Because band sizes were unpredictable at high beam intensities it was necessary to use beams of only about 10 na/mm².
15. D. E. Kerr-Lawson, Univ. Toronto Stud. Geol. Ser. No. 27 (1928), p. 15.
16. From α-decay theory, dλ/λ (3/2)(ZR)^½ (dR/R) + (2Z/E^½) (dE/E), where Z is the atomic number, R is the nuclear radius in 10⁻¹⁵ m, and E (= E_α) is the α-decay energy in million electron volts. A particle of mass m and charge z has a range r (halo radius), given by the espression r = constant × E²/mz². Then dλ/λ 43(dR/R) + 46(dr/r). If the difference between the halo radius and the coloration band size at 4.2 Mev is real, then Δr = −0.4 μm and dλ/λ 46(−0.4/13) = −1.4. Since the minimum uncertainty in making comparative range measurements is Δr = 0.1 μm, it is actually impossible to establish the constancy of λ (for ²³⁸U) from radiohalo data any better than dλ/λ 46(0.1/13) = 0.35. Also, if dE/E = 0 while dR/R 0, then dλ/λ 0. In such a case, halos furnish no proof that λ is constant.
17. Some inner ring coloration in Fig. 1f results from other α-emitters in the U decay chain. Fission track analysis shows that the dose of α-particles from ²³⁸U is only about 10¹³ per square centimeter, about ten times less than the ⁴He ion dose for medium coloration.
18. R. V. Gentry, in preparation.
19. ——, in Proceedings of the Second Lunar Science Conference (MIT Press, Cambridge, 1971), vol. 1, pp. 167-168.
20. ——, Annu. Rev. Nucl. Sci. 23, 347 (1973).
21. This work was sponsored by the Atomic Energy Commission under contract with Union Carbide Corporation, and by NSF grant GP-29510 to Columbia Union College, Takoma Park, Maryland.

2 July 1973; revised 26 December 1973