Radiohalos in a Radiochronological and Cosmological Perspective

Science, vol. 184, pp. 62-66, April 5, 1974.

Abstract. New photographic evidence, data on halo ring sizes, and x-ray fluorescence analyses provide unambiguous evidence that polonium halos exist as a separate and distinct class apart from uranium halos. Because of the short half-lives of the polonium isotopes involved, it is not clear how polonium halos may be explained on the basis of currently accepted cosmological models of Earth formation.

I have examined some 10⁵ or more radiohalos, mainly from Precambrian granites and pegmatites located in several continents. In addition to U and Th halos, originally studied (1, 2) for information on the constancy of the α-decay energy E_α and the decay constant λ, I have discussed X halos (2, 3), dwarf halos (3), and giant halos (4), and explained how these remain prime candidates for identifying unknown α-radioactivity and, not impossibly, unknown elements as well.

I have also reported (5) on a class of halos which had been tentatively attributed (6, 7) to the α-decay of ²¹⁰Po, ²¹⁴Po, and ²¹⁸Po. Earlier investigators (2, 7-10), possessing only a sparse collection of Po halos, at times confused them with U halos or invented spurious types such as "emanation" halos (2) or "actinium" halos (8) to account for them. (Figure 1, a to d, is a schematic comparison of U and Po halo types with ring radii drawn proportional to the respective ranges of α-particles in air.) To explain Po halos, Henderson (7) postulated a slow accumulation of Po isotopes (or their respective β-decay precursors) from U daughter product activity. I demonstrated that this secondary accumulation hypothesis was untenable and showed, using the ion microprobe (3), that Po halo radiocenters (or inclusions) exhibit anomalously high ²⁰⁶Pb/²⁰⁷Pb isotope ratios which are a necessary consequence of Po α-decay to ²⁰⁶Pb.

Recently, these ion microprobe results have been questioned, Henderson's results misinterpreted, Po halos considered [p. 240] to be only U halos, and allusions made to the geological difficulties that Po halos would present if they were real (11) [see (12) for comments].

Admittedly, compared to ordinary Pb types, the Pb isotope ratios of Po halos are unusual, but new ion microprobe analyses have confirmed (13) my earlier results (3). It is also apparent that Po halos do pose contradictions to currently held views of Earth history.

For example, there is first the problem of how isotopic separation of several Po isotopes [or their β-decay precursors (13)] could have occurred naturally. Second, a straightforward explanation of ²¹⁸Po halos implies that the 1-μm radiocenters of very dark halos of this type initially contained as many as 5 × l0⁹ atoms (a concentration of more than 50 percent) of the isotope ²¹⁸Po (half-life, 3 minutes), a problem that almost defies reason. A further necessary consequence, that such Po halos could have formed only if the host rocks underwent a rapid crystallization, renders exceedingly difficult, in my estimation, the prospect of explaining these halos by physical laws as presently understood. In brief, Po halos are an enigma, and their ring structure as well as other distinguishing characteristics need to be made abundantly clear.

In order to ascertain the E_α corresponding to a specific halo radius, I have produced a new series of standard sizes against which halo radii may be compared without relying on estimates derived from ranges of α-particles in air. Standard sizes may be prepared by irradiation of halo-bearing mineral samples with ⁴He ions (4); the coloration bands thus produced show varying sizes (as measured from edge to coloration extinction) which are dependent on energy, total dose, and dose rate, the latter two factors not being accounted for in other comparative methods.

I made more than 350 irradiations 1 to 10⁴ seconds in duration using ⁴He ions with energies ranging from 1 to 15 Mev, on over 40 samples of biotite, fluorite, and cordierite (14). Selecting the band sizes which correspond to the energies of the ²³⁸U α-emitters (see Table 1) permits a direct comparison with new as well as previous (1, 9, 10, 15) U halo measurements in biotite, fluorite, and cordierite. Figure 1e shows a coloration band in biotite produced by 7.7-Mev ⁴He ions, and Fig. 2a shows a densitometer profile of Fig. 1e.

[Larger picture] Fig. 1. The scale for all photomicrographs is 1 cm ≃ 25.0 μm, except for (h') and (r'), which are enlargements of (h) and (r). (a) Schematic drawing of 238U halo with radii proportional to ranges of α-particles in air. (b) Schematic of 210Po halo. (c) Schematic of 214Po halo. (d) Schematic of 218Po halo. (e) Coloration band formed in mica by 7.7-Mev 4He ions. Arrow shows direction of beam penetration. (f) A 238U halo in biotite formed by sequential α-decay of the 238U decay series. (g) Embryonic 238U halo in fluorite with only two rings developed. (h) Normally developed 238U halo in fluorite with nearly all rings visible. (h') Same halo as in (h) but at higher magnification. (i) Well-developed 238U halo in fluorite with slightly blurred rings. (j) Overexposed 238U halo in fluorite, showing inner ring diminution. (k) Two overexposed 238U halos in fluorite showing inner ring diminution in one halo and obliteration of inner rings in the other. (l) More overexposed 238U halo in fluorite, showing outer ring reversal effects. (m) Second-stage reversal in a 238U halo in fluorite. The ring sizes are unrelated to 238U α-particle ranges. (n) Three 210Po halos of light, medium, and very dark coloration in biotite. Note the differences in radius. (o) Three 210Po halos of varying degrees of coloration in fluorite. (p) A 214Po halo in biotite. (q) Two 218Po halos in biotite. (r) Two 218Po halos in fluorite. (r') Same halo as in (r) but at higher magnification.

[p. 241]