The Use of Radioactive Elements as Tracers

A valuable technique for research is the use of both radioactive and non-radioactive isotopes as tracers. By the use of these isotopes an element can be observed in the presence of large quantities of the same element. 

For example, one of the earliest uses of tracers was the experimental determination of the rate at which lead atoms move around through a crystalline sample of the metal lead. This phenomenon is called self-diffusion. If some radioactive lead is placed as a surface layer on a sheet of lead, and the sample is allowed to stand for a while, it can then be cut up into thin sections parallel to the original surface layer, and the radioactivity present in each section can be measured. The presence of radioactivity in layers other than the original surface layer shows that lead atoms from the surface layer have diffused through the metal. 

The process can be carried out by running a solution of ammonium nitrate into the nuclear reactor, where it is exposed to neutrons. The carbon that is made in this way is in the form of the hydrogen carbonate ion, HCO.3 , and it can be precipitated as barium carbonate by adding barium hydroxide solution. The samples of radioactive carbon are very strongly radioactive, containing as much as 5% of the radioactive isotope. 

It has been found convenient to introduce a special unit in which to measure amounts of radioactive material. The unit of radioactivity is called the curie. One curie of any radioactive substance is an amount of the substance such that 3.70 x 10 atoms of the substance undergo radioactive disintegration per second. 

The curie is a rather large unit. One curie of radium is approximately one gram of the element. (The curie was orginally defined in such a way as to make a curie of radium equal to one gram, but because of improvement in technique it has been found convenient to define it instead in the way given above.) 

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