ISOTOPIC TRACERS 165 



emitters, and in this case the /3-emitter is likely to be chosen because the 

 less-penetrating and less-energetic ^ particles are less dangerous to per- 

 sonnel. 



As an example, let us examine the six isotopes of carbon: C^^, C^\ 

 Ci2^ (-13^ Qi, ^^^ C15 (-12 i3 ji^g abundant or normal isotope; C^^ is 



stable and occurs naturally (about 1 per cent); C^^ is radioactive, a jS- 

 emitter, and occurs naturally as traces. The radioisotopes C^**, C^\ C^^, 

 and C^^ have all been produced artificially and have the characteristics 

 listed in Table 12-1. Carbon, of course, is the biological element, if any 

 element can so qualify. C^^ was the obvious choice as a tracer because of 

 its long half-life and because of its relatively safe production of ^~ parti- 

 cles. The first radiocarbon tracer experiments were performed with C^^ 

 because it was produced earlier, but as soon as C^'* became abundant it 

 was immediately adopted. 



Table 12-1. Radioactive Isotopes of Carbon 



Isotope Half-life Type of Emanation 

 CIO 19 sec |3+,Y 



Cii 20 min P+ 



C14 5700 years p- 



C^^ 2.4 sec (3, Y 



Commonly used tracers 



Biologists usually deal with relatively few of the elements, chiefly 

 those in the lower one-third to one-half of the periodic table. Most of 

 these elements have at least one convenient radioisotope. The important 

 exceptions are nitrogen and oxygen, both of which are extremely impor- 

 tant in biology. Table 12-2 lists the isotopes most commonly used as 

 tracers in biology. 



In addition to these commonly used tracers, several other radioisotopes 

 are important in biology. Co^ is used frequently as a y source for experi- 

 ments on the effects of ionizing radiation. Masses of Co^ up to several 

 hundred curies can be used in properly shielded pieces of apparatus 

 into which biological or chemical materials can be introduced for varying 

 lengths of time. A cesium isotope (Cs^^") is used for similar purposes, 

 offering easier handling but somewhat less energetic y radiation. Radio- 

 active rubidium (Rb*^^) is sometimes used as a tracer, not for Rb^'' but for 

 potassium, which it resembles chemically. Some cells accumulate Rb 



