Page 281 - Radiochemistry and nuclear chemistry

Uses of Radioactive Tracers 265

shown that a very rapid exchange takes place between atoms on a metal surface and the
metal ions in solution. While the exchange is a function of the nature of the surface, within
minutes it may involve atoms several hundred layers deep. The depth of penetration of
sorbed radioactive isotopes can be obtained from a measurement of the absorption of the
radiation or by measuring the radioactivity removed by cutting, or grinding, away thin
layers. With the same technique the diffusion of atoms in their own solid matrix can be
studied. For example, using single crystals of silver suspended in a solution containing
silver nitrate labeled with l l0mAg (/T~, tt/2 249.76 d) it has been possible to demonstrate
different rates of diffusion into different faces of the crystal. The surface area of solids can
also be determined by measurement of the sorption of radiotracers which do not penetrate
into the specimen.
If a radioactive gas is incorporated in a crystalline compound the amount of gas released
(the emanation ability) can be measured as a function of the temperature. It is found that
the emanation increases considerably at certain temperatures, indicating structural changes
in the solid at those temperatures. Studies of diffusion and emanation play a valuable role
in understanding the mechanism of sintering and in the formation of new solid compounds.
This has been of practical importance in the cement and glass industries, in the production
of semiconductors, in the paint industry, etc. Studies of surface reactions are of practical
importance for flotation, corrosion, metal plating and finishing, and detergent action to
name only a few applications.
The distribution of a radioactive dement or compound in a composite matrix can made
visible either to the naked eye or under a microscope by means of autoradiography. The
technique is based on the blackening of photographic films when exposed to nuclear
radiation (of. w167 and 7.10). The technique is best illustrated by an example.
Lead is an unwanted impurity in stainless steel even in very small amounts. In order to
investigate the mechanism of its incorporation, 212pb was added to a steel melt. After
cooling, the ingot was cut by a saw and the fiat surface machine-polished and etched in an
electrolytic bath (electro-polishing). This provided a very flat and "virgin" surface. A
photographic film was placed firmly with even pressure against the metal surface, and the
film was exposed in darkness in a cool room for about a week. After development of the
film, darkened spots caused by the radiation from 212pb showed where on the metal surface
lead was present. By taking the results into account in the production process, the negative
effect of lead in the raw material could be reduced.
In another technique, a polished surface of the specimen (metal, mineral, etc.) was dipped
into a solution containing a radioactive reagent, which selectively reacts with one of the
constituents of the surface. A mineral was dipped into potassium ethyl xanthate labeled with
35S (/T 87.5 d); the xanthate reacted selectively with sphalerite (zinc blende), ZnS, in the
sample. The distribution of the xanthate, as shown by the autoradiograph, indicated the ZnS
distribution in the mineral. The low/~-energy of 35S, Ema x 0.2 MeV, was an advantage to
the technique because the resolution of the autoradiograph increases with decreasing particle
range.