218 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 7 



certain percentage of the atomic nuclei present is transformed per 

 second into another sort by spontaneous disintegration, each disinte- 

 grating nucleus emitting, according to its sort, a negative or positive 

 electron or an a-particle (helium-nucleus). This is often accompanied 

 by an electromagnetic radiation (y-radiation). The intensity of the 

 total radiation of a radioactive preparation at any moment can easily 

 be measured. As the number of nondisintegrated atomic nuclei is 

 continually decreasing, while the chance of distintegration remains 

 constant for each atom and thus also the percentage of nuclei disinte- 

 grating per second, the radioactivity observed decreases with time. 

 The velocity of this decrease usually expressed by the "half-value 

 time," i. e., the time in which the intensity of the radiation falls to 

 one-half, is characteristic for each isotope. The above-mentioned six 

 radioactive calcium isotopes have half-value times of 4.5 minutes, 

 1.06 seconds, 8.5 days, 180 days, 2.5 hours, and 30 minutes respectively. 



The existence of the isotopes is not merely of theoretical interest. In 

 the last 10 to 20 years isotopes have become an extremely useful prac- 

 tical aid for all kinds of scientific and technical investigations. This 

 use of isotopes is based for a large part on the fact that an isolated 

 isotope of an element takes part in chemical and physical processes in 

 exactly the same way as the familiar mixture of isotopes of that element 

 which occurs in nature, while the isotope in question can always be 

 recognized by the investigator and can be traced even in a chemically 

 identical environment, thanks to its radioactivity or difference in 

 atomic weight. The isotope thus functions as a tracer, capable of fur- 

 nishing information about the process taking place, which would be 

 much more difficult or even quite impossible to obtain in any other way. 



We shall explain this in more detail, but for better orientation of the 

 reader we shall first discuss four examples out of the large number of 

 investigations which have already been carried out with isotopes as 

 tracers. 



WHAT CAN BE DONE WITH (RADIOACTIVE) ISOTOPIO TRACERS 



First example. — In the production of steel, among other substances 

 the phosphorus, which is present in quite considerable quantities in 

 the crude iron, has to be rendered harmless by adding a slag-forming 

 substance or by lining the crucible with a material that reacts with 

 phosphorus. A continuous check has then to be kept of the amount of 

 phosphorus still present in the molten metal. This can, of course, 

 be done by chemical anal j; sis of samples, but results are obtained much 

 more quickly and easily when a small amount of radioactive phos- 

 l)horus is added to the melt at the beginning of the process. This is 

 rapidly distributed uniformly throughout the melt, so that the ratio 

 between the natural phosphorus present and the radioactive phos- 



