16.3 Counting
techniques
Close examination must now be given to the
assumptions involved in fission track dating. The first of these, noted above,
is that the induced track count is performed on identical material to the
spontaneous track count. Several different experimental methods are available
which attempt to reach this ideal. Different approaches may be best for
different types of sample material.
16.3.1 Population method
This expression was coined by Naeser (1979a), but was effectively the method adopted by
the earliest workers (e.g. Price and Walker, 1963). The term refers to the fact
that spontaneous and induced tracks are counted in different splits or
sub-populations of material, which are nevertheless assumed to sample the same
population. This depends on the material having a homogeneous distribution of
uranium between the two splits. The method has proved particularly successful
for dating glass and apatite, but unsuccessful for sphene
and zircon, where uranium distribution is very variable both within and between
grains.
To
apply the population method, the sample is separated into two splits (Fig.
16.6). One is irradiated with thermal neutrons along with the standard (flux-monitor).
Both spontaneous and induced tracks are to be registered under spherical (4B) spatial geometry. Therefore, after
irradiation of the induced-track split, both splits are mounted in epoxy,
ground, polished and etched under identical conditions. This reveals an
internal surface of the material and also removes any extraneous superficial
tracks generated by uranium-bearing dust particles. Track densities are counted
in both splits. The induced-track density is calculated by subtracting the spontaneous-track
density (un-irradiated sample) from the total track density (irradiated
sample).
Fig. 16.6. Schematic
illustration of the population method of fission track analysis. After Naeser and Naeser
(1984).
The
population method should be statistically tested by counting track densities in
numerous grains or glass shards in each split. Alternatively, if a large piece
of glass or mineral is available it can be cut or cleaved so that the two faces
to be counted are nearly identical sections through the sample. The latter
method was adopted by Price and Walker (1963) in their analyses of muscovite.
Price and Walker took the extra precaution of irradiating the split for
spontaneous-track counting in a cadmium box (which screens out thermal
neutrons) so that both splits should be treated as nearly identically as
possible prior to etching. However, this precaution has now been dispensed
with.
In
the analysis of apatite, pre-irradiation heating of the induced-track split has
been found advantageous to erase all spontaneous tracks by thermal annealing (see below). This allows the induced track density to be
determined directly in the irradiated split. However, this procedure may be
problematical in dating glass because it may affect the etching properties of
the irradiated split, leading to systematic track counting errors.
16.3.2 External detector method
In this technique (Fleischer et al., 1965a) the uranium content of
the material to be dated is determined by inducing counts in an external
detector rather than in the sample material itself. The sample is ground,
polished, etched and counted, after which a sheet of detector material is
placed in intimate contact with the etched surface. This must be done with
absolute cleanliness to exclude uranium-bearing dust grains (see above). The
external detector is commonly a low-uranium mica or a
plastic such as lexan. After irradiation, the
external detector is removed from the sample, etched and counted (Fig. 16.7).
Fig. 16.7. Schematic
illustration of the external detector method of fission track analysis, as
described by Naeser (1979a). In this version
the counting of spontaneous tracks is performed after irradiation, unlike the
sequence described in the text. After Naeser
and Naeser (1984).
The
advantage of the external detector method is that both spontaneous and induced
tracks are generated by the same sample material. Hence, it is suited to the
analysis of material with a very heterogeneous distribution of uranium. The
main disadvantage of the method is that the spontaneous and induced tracks are
recorded under different spatial geometry conditions (Fig. 16.8). Spontaneous
tracks are generated in the interior of the rock, and can therefore be formed
by uranium atoms both above and below the etched plane (spherical or 4B geometry). In contrast, tracks
induced in the external detector come out from the surface of the analysed
material and are therefore generated with approximately one-half the frequency
(hemi-spherical or 2B geometry). Reimer et al.
(1970) questioned whether the efficiency of induced-track formation is exactly
50%, or whether small biasses are
introduced. However, subsequent experiments (discussed by Hurford
and Green, 1982) showed that in most cases the ideal efficiency of 50% is
achieved.
Fig. 16.8. Schematic
illustration of the difference between 4B
and 2B geometry in track
formation.
16.3.3 Re-etching and Re-polishing
The re-etching technique, described by Price
and Walker (1963), is similar to the external detector method in that a sample
is irradiated after polishing,
etching and counting of spontaneous tracks. However, the sample itself is now
re-etched and re-counted to determine the induced-track density by subtraction.
As for the external method, induced tracks are formed with only 50% efficiency
(2B geometry). The
disadvantage of this method is that spontaneous-track pits will be unduly
enlarged after the second etch, and may obscure some induced tracks. It is
consequently less popular than the external method.
The
re-polishing technique (Naeser et al., 1989) is an improvement on the re-etching method, and
yields results similar to the ‘mirror image’ population method (Price and
Walker, 1963). The sample is polished, etched and counted for spontaneous-track
density. After irradiation it is re-polished to a depth of at least 20 :m to reveal a new internal face with
4B track
geometry. This is then etched and counted to determine the induced-track
density by subtraction. The method has the advantage that both spontaneous and
induced tracks are recorded under identical geometry, and spontaneous tracks
are not over-enlarged by double etching. Also, surface contamination during
irradiation is not a problem. The spontaneous and induced tracks are not
generated by exactly the same sample material, but the two etched surfaces are
so close together that uranium inhomogeneity in the
grain as a whole is unlikely to significantly bias the data. A disadvantage
compared with the normal population method is that the two etching steps are
performed separately, and may therefore vary slightly in efficiency.
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