When a neutron strikes the nucleus of uranium-235 (²³⁵U) or
plutonium-239 (²³⁹Pu), it may cause the nucleus to split into two
roughly equal fragments, releasing neutrons and energy. This is the
well-known process of neutron-induced fission, the method in which
nuclear energy is produced in both reactors and bombs.[14] The most
common uranium isotope, ²³⁸U, also breaks up by fission, but does so all
by itself, without the need for any external neutrons. That process is
_spontaneous fission_ and it goes on at random, very much like
radioactive decay. It is a relatively rare process and the fission
half-life is long—about 10 million aeons (10¹⁶ years). That means that
only about one spontaneous fission occurs in uranium-238 for every 2
million alpha decays. That is enough to make a useful clock, however,
because ²³⁸U is present almost everywhere. (See Table III on page 19.)
Imagine an atom of ²³⁸U in some mineral. When the atom suddenly
fissions, it breaks in two with considerable energy, and the two fission
fragments rip like cannon balls through the surrounding crystalline
structure in opposite directions, creating havoc along the way. They
travel a distance something like 10 microns (4 millionths of an inch)
before they are finally slowed down and stopped by all their collisions
with other atoms. Each fragment’s path remains behind as an intensely
damaged tube through the crystal.
The process was known for a long time before anyone was able to find
these fission tracks (the damaged tubes) in the crystals. Finally, about
1960, three young physicists, R. L. Fleischer, P. B. Price, and R. M.
Walker, working at the General Electric Research Laboratory, fell upon
the idea of etching freshly broken surfaces of crystals with acid. They
reasoned that a region so intensely disturbed by the passage of a
fission fragment should be etched more easily and deeply than the
undisturbed surrounding crystal. That idea turned out to be correct, and
fission tracks have now been found in almost every common mineral (since
almost all minerals contain small amounts of uranium).
[Illustration: _Tracks of uranium fission from a fossil antelope bone
fragment from Hopefield, Cape Province, South Africa._]
The fission clock method works this way: A cleavage face or a polished
surface of a crystal or glass fragment is etched with a suitable
solvent. Different acids work best for different materials, and a
suitable procedure must be developed especially for each substance. The
etching brings out the fission tracks so they can be seen (usually as
little conical pits) and counted under a microscope.
After this, the sample is exposed to a known amount of slow neutrons[15]
in a nuclear reactor. New fissions are produced, but this time only in
²³⁵U (which is present in all natural uranium in the proportion of 1
atom of ²³⁵U to 137.7 atoms of ²³⁸U), because slow neutrons do not
produce fissions in ²³⁸U. After the neutron irradiation, the same
surface is etched again, and the new tracks counted. The old tracks,
having been etched twice, now appear larger and thus can be
distinguished from the new ones that were caused by ²³⁵U fission.
The rate at which ²³⁸U decays by fission, λ_{f}, is known, as are the
rate it decays by alpha decay, λ_{α}, and the total number of slow
neutrons, _n_, to which the sample was exposed in the reactor. The age
of the crystal or glass can then be calculated:
t = (1)/(λ_{α}) ln (1 + (_n_N_{s})/(N_{i}) × constant)
where
ln = the natural logarithm (log to the base _e_),
N_{s} = the number of atoms in the sample.
N_{i} = the number of atoms of ²³⁵U in the irradiated sample,
and the constant has the value:
½ (λ_{α} × 582 × 10⁻²⁴)/(λ_{f} × 137.7) = 4.25 × 10⁻¹⁸
Fission-track dating is a brand new technique, still only partly
developed. It has enormous range and is applicable to numerous minerals;
these advantages imply that it is likely to become very useful.
[Illustration: _An atomic absorption spectrophotometer is used to
measure the amount of potassium in samples of mica dissolved in acid._]