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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
|ROBERT V. GENTRY|
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,
- 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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