Therefore, if halos result from the α-decay of 210Po to 206Pb, their appearance should resemble the idealized schematic (Fig. 1b), and the light and dark halos of this type in biotite should exhibit radius variations consistent with the differences between lower and higher coloration band sizes (Table 1, columns 2, 3, 6, 14, and 15). Further, such halos, whether very light or very dark, should appear without any outer ring structure, as illustrated in Fig. 1n. Compare also the densitometer profiles of the halo negatives of Fig. 1f (the U halo) and Fig. 1n shown in Fig. 2b and Fig. 2, c to e, respectively. Fig. 1o shows three similar halos in fluorite; here, irrespective of coloration differences, the halo radii are the same and correspond to the Eα of 210Po (Table 1, columns 4, 6, and 20). Accordingly, the halos in Fig. 1, n and o, are designated 210Po halos. (Actually I should emphasize that since not all biotites exhibit the same coloration responses, the radius measurements in Table 1 are strictly valid only for the particular micas I used. I did try to illustrate a range of responses by utilizing four different biotites for the U halo and the three Po halo types.)
By analogy, the moderately developed biotite halo in Fig. 1p shows a marked resemblance to the idealized halo that would form from the sequential α-decay of 214Po and 210Po (see Fig. 1c). Table 1, columns 2, 3, 6, 7, 16, and 17, shows the correspondence of the radii with band sizes. The prominent unmistakable feature of the 214Po halo is the broad annulus separating the inner and outer rings [see the densitometer profile of Fig. 1p shown in Fig. 2f and figures 7 to 9 in (6)]. With respect to comments in (11) it should be noted that the 214Po halo can easily be distinguished from a U halo.
The last correspondence to be established is the resemblance of the two three-ring halos in biotite (Fig. 1q) and two similar halos in fluorite (Fig. 1r) to the idealized 218Po halo (Fig. 1d) showing the ring structure from the sequential α-decay of 218Po 214Po, and 210Po. In biotite such halos may appear very light to very dark with radii correspondingly slightly lower and higher (excluding reversal effects) than those measured for medium coloration bands (compare Table 1, columns 2, 3, 18, and 19). Cursory examination of inferior specimens of this halo type could lead to confusion with the U halo, especially in biotite, where ring sizes vary slightly because of dose and other effects. However, good specimens of this type are easily distinguished from U halos, even in biotite. In fluorite, where the ring detail is better, a most important difference between 238U and 218Po halos is delineated, that is, the presence of the 222Rn ring in the U halo (Fig. 1a) in contrast to its absence in the 218Po halo (Fig. 1d). For example, note the slightly wider annulus (3.9 μm) between the 210Po and 218Po rings of the 218Po halo compared to the equivalent annulus (3.0 μm) in the 238U halo (Fig. 1, a, d, h, h', r, and r'). This is evidence that the 218Po halo indeed initiated with 218Po rather than with 222Rn or any other α-decay precursor in the U chain. As further proof, Table 1 (columns 4, 11, 12, and 21 shows that the 218Po halo radii agree very well with equivalent band sizes and U halo radii in this mineral. Additional Po halo types also exist (3) but are quite rare. [As yet I have found no halos at all in meteorites or lunar rocks (19)].
The preceding discussion has shown [p. 243] that Po halos can be positively identified by ring structure studies alone. That x-ray fluorescence analyses also provide quite convincing evidence is seen in Fig. 3c, where I show for the first time the x-ray spectra of a Po halo radiocenter (specifically, a 218Po halo). Comparison of Fig. 3, b and c, reveals that the Pb in the Po halo radiocenter in fluorite did not arise from in situ decay of U. [Longer runs have shown small amounts as Se as well as U in some Po halo radiocenters (18).] On the other hand, the presence of Pb is to be expected in a 218Po halo radiocenter because the decay product is 206Pb. That the parent nuclide was 218Po and not a β-decaying isomer precursor (13, 20) follows from half-life considerations of the U halo U/Pb ratio (> 10); the proposed isomer, if formed at nucleosynthesis, should now be detectable in Po halo radiocenters. No trace of this isomer has yet been found, and I thus view the isomer hypothesis as untenable.
Earth Science Associates