While coloration surrounding minute veins in the mica is an indication of the flow of radioactive solutions (very weak solutions may show no staining whatsoever), it does not follow that halos that formed around small nuclei in the conduits were necessarily derived from radioactivity in solution. For example, polonium, uranium, and thorium halos also form around very small inclusions, with no visible conduit or crack in the mica connecting the halo nuclei, and it is certainly not clear that these halos are of hydrothermal origin.
An attempt to determine whether the halo nuclei were capable of acting as selective fixation sites for certain radionuclides, by electron-microprobe analysis of the halo inclusions, failed because of the small size involved. However, refinement of techniques may lead [p. 224] to clarification of the nature of the inclusions (14). Thus a more sensitive technique is required for testing of the hypothesis regarding genesis of the polonium halos from a uranium-bearing solution.
Fission-track techniques (15) may serve this purpose. Uranium-238 fissions spontaneously, and the damaged regions in the host mineral, produced by the fission fragments, can be enlarged sufficiently by acid etching for visibility under an optical microscope. Immersion of biotite samples, containing the polonium and uranium halos in hydrofluoric acid for a few seconds and subsequent observation of the areas in the vicinity of the inclusions reveal a striking difference: the polonium halos are characterized by complete absence of fission tracks, whereas the uranium halos always show clusters of fission tracks.
To eliminate the possibility that fission tracks may have been annealed out of the sample, I have irradiated mica specimens containing the uranium and polonium halos with a neutron flux of 5 × 1017 neutrons per square centimeter and again etched the mica. The uranium halos show, as expected, marked increase in the number of fission tracks emanating from the central inclusion, due to neutron-induced 235U fission, whereas the polonium halos are again completely devoid of tracks (12).
If a uranium solution had been in a conduit feeding the central inclusions of the polonium halos with daughter-product activity, about 70 fission tracks per centimeter of conduit would be expected by use of Henderson's model (10). This result depends on such parameters as the uranium concentration in the solution, the rate of flow (conservatively I have assumed that the solution ceased to flow when the polonium halos formed), and the total number of polonium atoms (5 × 108) necessary to form a well-developed 218Po halo. This last value I determined by observing the degree of coloration in uranium halos as a function of the number of fission tracks emanating from the halo nucleus, the total number of α-particles required for production of a halo being computed as eight times the number of fission tracks times the ratio of the half-lives for spontaneous fission and alpha decay for 238U. While fission tracks are observed along stained conduits, in general I cannot correlate the distribution of fission tracks along clear conduits with the presence of polonium halos.
Polonium halos are also found randomly distributed throughout the interior of large mica crystals far removed from any conduit. (A limited survey may indicate halos occurring within certain cleavage planes, but more extensive search shows this is not the case.) The question now arises of whether the source of the short-half-life radioactivity, characteristic of such polonium halos, was due to (i) the laminar flow of a non-uranium-bearing solution, containing disequilibrium amounts of daughter-product α-activity, through a thin cleft parallel to the cleavage plane, or (ii) the diffusion of gaseous radon through the mica. The latter case has been considered (8), but only recently has the discovery of α-recoil tracks in micas (16) enabled quantitative checking of either of these mechanisms. This technique is based on the fact that an atom recoiling from α-emission impinges on the host mineral and forms a damaged region large enough to produce a pit which is visible in phase contrast when etched with hydrofluoric acid.
The original experiment (16) determined that a series of multiple recoils, such as is expected in the sequential α-decay of 238U and 232Th, yields α-recoil tracks. Two additional points necessary for a complete α-recoil analysis—(i) whether a single α-recoil produces a track, and (ii) whether α-recoil pits form in a sample placed in contact with an α-emitter—have now been resolved.
Several samples of mica were annealed for removal of background α-recoil pits; three different concentrations of dilute solutions of americium (5 percent 241Am and 95 percent 243Am) were evaporated on separate samples, and an α-count was taken. The daughter products of the americium isotopes have very long half-lives, so that any α-recoil pits occurring reflect only single α-decay. The higher α-count samples yielded correspondingly higherα-recoil densities within the area of deposition, accompanied by almost complete absence of tracks outside the radioactive zone. Thus was established the existence of one α-track from a single α-recoil (17).
Corresponding α-recoil densities were also noted in annealed mica samples placed in contact with the americium-coated samples. It follows that any excessα-radioactivity in micas may be effectively determined by analysis of the samples by the α-recoil technique.
The procedure for ascertaining the extent of increased α-activity consists in measuring background fossil α-recoil track densities in areas far removed from the halos themselves, and in comparing these values with the densities near the halos for determination of the degree of excess α-activity. Samples of Precambrian mica from Canada and Ireland (18), containing uranium and polonium halos, were investigated by etching in 48 percent hydrofluoric acid for about 15 to 50 seconds. As in earlier experiments, 238U halos revealed the presence of fission tracks emanating from the central inclusions, whereas no fission tracks were noted from the central inclusions of the polonium halos.
The experimental procedure was to photograph in phase contrast a given etched area, enlarge, and count anywhere from several hundred to 1000 α-recoil centers for each density measurement. The enlargement factor was determined by photographing the rulings of a stage micrometer, using each objective. Replicate measurements were made on several areas with different [p. 225] halo types. The background fossil α-recoil density was measured before a count was made in the mica cleavage plane about 5 to 10 μ directly above the halo nucleus. The mica was then cleaved until the central inclusion appeared on the surface; the mica was etched again and another count was made to enable a density comparison of three separate regions.
Earth Science Associates