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).
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, λf, is known, as are the rate it decays by alpha decay, λα, 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 |
| λα |