Chemistry Reference
In-Depth Information
of damage, known as
spontaneous fission tracks
. The tracks are extremely
small (about 0.0000003 mm wide and 0.015 mm long), too minuscule to be
visible even when viewed under an ordinary microscope. They can,
however, be seen under an electron microscope, which provides much
greater magnification: if a freshly cut and polished surface of an uranium
containing nonconducting solid is viewed under an electron microscope,
the tracks are seen where atoms of uranium-238 disintegrated near the
new surface (Fleischer et al. 1975, 1965a, 1965c).
For any uranium-containing solid, the number of uranium-238 atoms
that have disintegrated increases with age, as does the number of fission
tracks. When most solids are heated to a sufficiently high temperature, the
tracks are erased, although the extent of the erasure varies: in some solids
the tracks are erased only at very high temperatures, just below the incan-
descence temperature of the solid; in others, the tracks are erased at lower
temperatures, known as the
healing temperature of the solid
; in another
few, the temperature required for erasure is so low that most of the tracks
are gradually erased even at ambient temperature. Determining the
uranium content of a solid in which the tracks are erased only at high tem-
peratures and counting the number of tracks in the solid provide a way to
calculate its age.
Practical difficulties, related to viewing and counting the extremely
small tracks electron microscopically, however, might render this dating
method too cumbersome to be useful. Fortunately, the tracks can be
enlarged by a relatively simple chemical procedure: treating a surface of
the solid with an
etching
agent (an acid solution that preferentially dis-
solves those regions of the solid surrounding the tracks) renders the tracks
visible under an ordinary microscope. Moreover, repeated but thoroughly
controlled etching off such a surface enlarges the tracks enough to be
easily countable under a regular optical microscope, as shown in Figure 25
(a). This procedure makes the fission track method a valuable method for
dating archaeological materials and tools (Westgate et al. 1997; Wagner
and van den Haute 1992).
Having enlarged and counted the tracks, all that is required to deter-
mine the age of a suitable solid by the
fission track method
is the deter-
mination of two values:
1.
The track density, that is, the number of tracks per unit surface area
2.
The concentration of uranium in the solid
The track density can be easily determined on a newly cut, polished, and
etched surface by counting, under an optical microscope, the number of
etched tracks in a measured area of the solid. The uranium concentration
can be determined by a number of analytical techniques. Following these
