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Appendix: "Perspective"
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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?
ROBERT V. GENTRY |
Chemistry Division,
Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37830 |
References and Notes
- 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.
- 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.
- R. V. Gentry. Science 173, 727 (1971).
- ——, ibid. 169, 670 (1970).
- ——, ibid. 160, 1228 (1968).
- G. H. Henderson and F. W. Sparks, Proc. R. Soc. Lond. Ser. A Math. Phys. Sci. 173, 238 (1939).
- G. H. Henderson, ibid., p. 250. A fourth type attributed to 226Ra
α-decay is in error.
- S. Iimori and J. Yoshimura, Sci. Pap. Inst. Phys. Chem. Res. Tokyo 5, 11 (1926).
- 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.
- C. Mahadevan, Indian J. Phys. 1, 445 (1927).
- C. Moazed, R. M. Spector, R. F. Ward, Science 180, 1272 (1973).
- 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, λα/λf;
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).
- R. V. Gentry, S. S. Cristy, J. F. McLaughlin, J. A. McHugh, Nature (Lond.) 244, 282 (1973).
- 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 × l013 4He 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.
- D. E. Kerr-Lawson, Univ. Toronto Stud. Geol. Ser. No. 27 (1928), p. 15.
- From α-decay theory, dλ/λ ≃
(3/2)(ZR)½ (dR/R) +
(2Z/E½) (dE/E), where Z is
the atomic number, R is the nuclear radius in 10−15 m,
and E (= Eα) 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.
- 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.
- R. V. Gentry, in preparation.
- ——, in Proceedings of the Second Lunar Science Conference (MIT Press,
Cambridge, 1971), vol. 1, pp. 167-168.
- ——, Annu. Rev. Nucl. Sci. 23, 347 (1973).
- 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
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