416 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



2. The activation cross section a should be high. This is characteristic 

 of the element and is a main determinant as to which elements can or can- 

 not be adequately analyzed by the method. 



3. The half-life, as expressed by the decay constant, should fall between 

 a few minutes and several days. It can be shown that an irradiation 

 period equal to several half-lives is required to give the maximum activity 

 available and that an irradiation period equal to one half-life gives about 

 one-half the maximum value. This means that prohibitively long irra- 

 diation periods are required for radioisotopes of long half-lives. On the 

 other hand, the radioisotope starts to suffer a net loss of activity after 

 removal from the pile, and if the half-life is too short, it may be impossible 

 to get the measurements made before the desired activity has been 

 reduced below detectable amounts. This decay factor has not been 

 included in Eq. (10-1). 



4. The lower the atomic weight M, the greater will be the sensitivity. 



5. The higher the abundance of the target element G, the greater will 

 be the sensitivity. 



6. There is an important factor that is not contained in Eq. (10-1), 

 namely, the efficiency with which the induced radioactivity can be meas- 

 ured. An estimate of counting efficiency for a given radioisotope can be 

 obtained from the data presented in Chap. 6. 



A listing of sensitivities has been compiled by Leddicotte and Reynolds 

 (5) for the conditions in the Oak Ridge National Laboratory graphite 

 reactor. Table 10-1 presents these values. The fisting is fimited to 

 elements producing a radioisotope with a half-fife greater than about 

 10 min and does not include elements of atomic number greater than 83. 

 The sensitivity values are based on & flux of 5 X lO^^ neutrons /cmy sec 

 and represent the micrograms of the element required to give an amount 

 of radioactivity measurable by usual counting methods after exposure to 

 saturation or to a 1-month bombardment. For example, sodium could be 

 measured by bombardment to saturation in any sample containing at 

 least 0.007 Mg of this element. These sensitivity values are valid only 

 when the sample does not contain large amounts of other elements that 

 become highly radioactive under irradiation, and when there are no sub- 

 stances present that interfere with the chemical separation procedures. 



SOURCES OF RADIATION 



The nuclear chain reactor will probably be most widely used because of 

 technical reasons and the availabifity of radioactivation-analysis service 

 to the pubfic. However, other sources of bombarding particles such as 

 the cyclotron, betatron, and linear accelerator may also be employed. 

 Table 10-2 presents a listing of such sources and estimates of the flux 



