222 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



ordinary hydrogen, which has a weight of approximately one unit 

 of atomic mass, called hydrogen 1. The other is approximately twice 

 as heavy and is called heavy hydrogen, or hydrogen 2. We can also 

 make a still heavier hydrogen 3. 



Both hydrogen 1 and hydrogen 2 are stable; that is, they do not 

 change with time, or disintegrate, or give off radiation. Hydrogen 3, 

 on the other hand, is radioactive and disintegrates or decays to a 

 stable isotope of helium. In disintegrating, hydrogen 3 gives off 

 radiation. 



Five isotopes are known for the element carbon, only two of which 

 are stable and naturally occurring. The other three are radioactive 

 and have to be made. Generally speaking, most naturally occurring 

 isotopes are stable, whereas most radioactive isotopes have to be made. 

 There are, however, exceptions particularly in the case of the heavy 

 elements. 



NATURALLY OCCURRING RADIOISOTOPES 



The historical sequence of events leading to today's widespread 

 availability of radioisotopes is unique. It was the naturallj'' occurring 

 radioelement uranium which even before the turn of the century led 

 to the discovery of radioactivity. This subsequently led to the dis- 

 covery of some 45 other naturally occurring radioisotopes, including 

 such important isotopes as radium and radon, whose uses are familiar. 

 Approximately 50 years later the same radioelement, uranium, led to 

 the design and operation of the nuclear reactor, today's mass producer 

 of man-made radioisotopes. Just as radioactivity proved the key to 

 the development of nuclear science, uranium proved the key to the 

 availability of radiomaterials. But we are getting ahead of our story. 



In 1913 Hevesy and Paneth conducted the first tracer experiment 

 when they used minute amounts of naturally occurring radioactive 

 lead to study the solubilities of sparingly soluble lead salts. Later 

 these investigators used the same naturally occurring radioactive lead 

 to study the absorption and translocation of that element in plants. 

 This was in 1923. Other studies of a similar nature were conducted 

 in the years that followed, but none of them were very broad in 

 scope. The reason was simple. There just were not any radioactive 

 counterparts for most of the elements usually found in plant and 

 animal systems. No naturally occurring radioisotopes for those ele- 

 ments existed, and no one knew how to make them. Here then was 

 a technique that admittedly had unlimited possibilities but that 

 could not be used because the materials to do the job were not available. 



MAN-MADE RADIOISOTOPES 



Then came the key to a whole new era for radioactivity. In 1934 

 1. Joliot-Curie and her husband, F. Joliot, while bombarding light 



