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 E_α 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)].

Fig. 3. Scanning electron microscope-x-ray fluorescence spectra of (a) the fluorite (CaF2) matrix, (b) a U halo radiocenter in fluorite characteristic of Fig. 1, l and m, and (c) a 218Po halo radiocenter in fluorite characteristic of Fig. 1r.

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.