ISOTOPIC TRACERS 163 



The development of tracer experimentation 



Tracer experiments were used for the first time by Hevesy in 1923. 

 He used several naturally occurring radioactive isotopes of lead to trace 

 the path by which materials moved from one place to another within 

 plants. In 1934 the famous Curies demonstrated the possibility of pro- 

 ducing artificial radioisotopes (a contraction of "radioactive isotopes"), 

 and even before the beginning of World War II several isotopes had been 

 produced artificially. Most of the potential tracers were not yet available 

 in adequate quantities, however, so only a few isotopic tracer experi- 

 ments were performed. After World War II great quantities of many 

 different isotopes became quite readily available, and laboratories all 

 over the world adopted this new tool. Some difficulties were encountered, 

 and, in fact, some problems not particularly amenable to solution by the 

 use of tracers were investigated. By now the "fad" has passed, and we 

 have settled down to a judicious use of this most valuable technique. 

 Tracer materials are now available in almost any conceivable form, and 

 the instruments used for detection have reached a high state of develop- 

 ment. 



Selection of tracer isotopes 



So many isotopes have been produced artificially and are potentially 

 available that the investigator is faced with a choice of isotopes. For 

 example, there are six isotopes of carbon, some stable, some radioactive. 

 If a convenient radioactive isotope is available it usually is chosen as 

 a tracer because the detection of radioactivity is easier than the detection 

 of stable isotopes. Two principles are considered in selecting from among 

 several possible radioisotopes: the rate of disintegration and the type of 

 disintegration and radioactive emission. 



Any radioactive isotope disintegrates at a characteristic rate. Any 

 single atom has a certain probability of decomposing, regardless of how 

 many similar atoms are in the vicinity. This probability means that in a 

 unit of time a certain constant fraction of the total will disappear. Thus, 

 dN/dt = —KN, where N is the number of atoms, t is time, and K is a 

 constant representing the fraction of the total number of atoms disin- 



