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 Eα 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 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
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 × 109 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 Eα 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,
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.
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.
238U halo in fluorite with only two rings
(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
(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
(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.
The coloration extinction boundary is
poorly defined near threshold coloration;
only a few very light bands in biotite could
be reliably measured. Reproducible
measurements were obtained in the plateau
region (14), where variations in band size
are minimal. Darker halos in biotite
generally have slightly larger radii than
lighter halos (3, 4). Also, reversal effects in
some biotites immediately exterior to the
terminus of a halo ring cause apparent
diminution of the radius. Therefore, while
there are differences between the sizes of
medium coloration hands (Table 1, column
2) and the radii of U halos in biotite (Table 1,
columns 8, 9, and 10) that could be
interpreted in terms of an actual change in
Eα and λ (16), such differences more likely
arise from a combination of dose and
reversal effects (15, 17), producing slightly
diminished radii. Diminution of U halo
radii may also result from attenuation of α-particles within the small but relatively
dense zircon radiocenters. Even though
slight differences between band sizes and
U halo radii do exist in biotite, the
idealized U halo ring structure (Fig. 1a)
compares very well with an actual U halo
in biotite (Fig. 1f).
Biotite and fluorite are good halo
detectors, but fluorite is superior because
the halo rings exhibit more detail, often
have smaller radiocenter diameters (< 1
μm), and have almost negligible size
variations due to dose effects in the
embryonic to normal stages of
development. Figure 1g shows an
embryonic U halo in fluorite with only the
first two rings fully developed; the other
rings are barely visible because, due to the
inverse square effect, threshold coloration
has not been reached. Figure 1h shows a U
halo in fluorite in the normal stage of
development, when nearly all the rings
are visible. This halo closely approximates
the idealized U halo in Fig. 1a. Under high
magnification even separation of the 210Po
and 222Rn rings may be seen. Figure 1i
shows another U halo in fluorite, with a
ring structure that is clearly visible but not
adequate for accurate radius
In Table 1, columns 4, 11, and 12, the
fluorite band sizes agree very well with the
U halo radii measured in this mineral by
myself and Schilling (9). This suggests that
the differences between U halo radii and
band sizes in biotite are not due to a
change in Eα However, experimental
uncertainties in measuring U halo radii
preclude establishing the constancy of λ to within 35
percent, and under certain assumptions U
halos provide no information at all in this
While halos with point-like nuclei which
show well-defined, normally developed
rings (as in Fig. 1h) can be used to
determine the Eα's of the radionuclides in
the inclusion, there are pitfalls in
ascertaining what constitutes a normally
developed ring. In contrast to the easily
recognizable U halos in fluorite in Fig. 1, g
to i, the overexposed fluorite U halo in
Fig. 1j shows a diminutive ghost inner ring,
which could be mistaken for an actual 238U
ring. Figure 1k shows two other partially
reversed U halos, one of which shows the
diminutive inner ring, while in the other all
the inner rings are obliterated. The U halo
in Fig. 1l is even more overexposed, and
encroaching reversal effects have given
to another ghost ring just inside the
periphery. Figure 1m shows a still more
overexposed U halo; in which second-stage reversal effects have produced
spurious ghost rings that are unrelated to
the terminal α-particle ranges.
Since this association of the halos in Fig.
1, l and m, with U α-decay cannot be easily
proved by ring structure analysis alone, I
have utilized electron-induced x-ray
fluorescence to confirm this identification.
Figure 3a shows the prominent Ca x-ray
lines of the fluorite matrix (the F lines are
below detection threshold) along with
some background Ag and Rh lines which
are not from the sample, but are produced
when back-scattered electrons strike a Ag-Rh alloy pole piece in the sample chamber.
Figure 3b, the x-ray spectrum of a halo
radiocenter typical of the halos in Fig. 1, l
and m, clearly shows the x-ray lines due to
U (as well as a small amount of Si) in
addition to the matrix and background
peaks. A more detailed analysis (18)
reveals that the Uζ line masks a small
amount of Pb probably generated by in situ
The variety of U halos shown in Fig. 1, g
to m, establishes two points: (i) only a thorough search will reveal the
numerous variations in appearance of U
halos, and (ii) unless such a search is
made, the existence of halos originating
with α-emitters other than 238U or 232Th
could easily be overlooked.
So far, three criteria have been used to
establish the identity of U halos: (i) close resemblance of actual halos in
biotile (Fig. 1f) and fluorite (Fig. 1h) to the
idealized ring structure
(Fig. 1a), (ii) identification
of lines in x-ray fluorescence spectra,
and (iii) agreement between U halo radii
and equivalent band sizes (very good in
fluorite and fair in biotite and cordierite).
Using the third criterion (either band sizes
or U halo radii) I can determine Eαfor a
normally developed fluorite halo ring to
within ± 0.1 Mev. For biotite halos, U halo
radii may form a suitable standard for
determining Eα for rings that show reversal
or other effects characteristic of U halos in
the same sample. If good U halos are not
available, and if the halos with variant sizes
show well-developed rings without reversal
effects, then the band sizes form a suitable
standard for Eα determination when
coloration intensities of variant halos and
band sizes are matched.
Fig. 2. Densitometer profiles of the photographic negatives of (a) Fig. 1e, (b)
Fig. 1f, (c) the light 210Po halo in Fig. 1n, (d) the medinm 210Po halo in Fig.
1n, (e) the dark 210Po halo in Fig. 1n,
and (f) Fig. 1p.
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
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).
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
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
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