Radioactive isotopes archaeological dating
By comparison with the tree rings in the extremely old bristle-cone pines, however, a corrected carbon date can be found for objects over about 1500 years old.
The trees are themselves dated by the carbon-14 method using dead parts in the bark.
This method of dating can be used with success to determine not only the ages of animal remains but also those of wood, paper, cloth and other organic material.
One difficulty with this method is that it has to be assumed that the cosmic ray intensity has remained constant, and in fact this has been found not to be the case.
These materials have a variety of uses and a selection of these are listed below.(a) dating geological specimens, using uranium, rubidium or bismuth; (b) dating archaeological specimens, using carbon 14(c) paper or plastic thickness measurement using beta radiation(d) treatment of tumours;(e) sterilisation of foodstuffs;(f) nuclear pacemakers for the heart;(g) liquid flow measurement;(h) tracing sewage or silt in the sea or rivers;(i) checking blood circulation and blood volume;(j) atomic lights using krypton 85;(k) checking the silver content of coins;(I) radiographs of castings and teeth;(m) testing for leaks in pipes;(n) tracing phosphate fertilisers using phosphorus 32(o) smoke alarms (p) sterilisation of insects for pest control.
(a) Iodine-131 with a half-life of 8.0 days and activity of 8 C may be taken as liquid or in a capsule.(b)Technetium-99 with a half-life of 6 hours gives gamma-rays of 140 ke V energy.(c) Iodine-123 is suitable for medical studies since it gives no beta- radiation.(d) Cobalt-60 sources of up to 10 000 curies have been used; such a source gives 200 R per minute at 1 m. The very long half-lives of these isotopes make them particularly suitable for finding the age of rocks.
Generally, however, they are useful either because we can detect their radioactivity or we can use the energy they release.
We know these steps because researchers followed the progress of the radioactive carbon-14 throughout the process.(Recall that tritium, H, is a radioactive isotope of hydrogen.) Tracers can also be used to follow the steps of a complex chemical reaction.