414 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



investigation which might occur in processing the sample material; very- 

 little processing is needed before bombardment, so that the opportunity 

 for contamination is minimal. Thus the difficulties with large "blanks" 

 which are usually an obstacle in the conventional estimation of trace 

 amounts of elements are avoided. There is usually little trouble from 

 interfering substances except when samples contain large amounts of 

 neutron-absorbing material or of an extraneous element that will produce 

 gross amounts of radioactivity in the sample. Activation analysis offers 

 specificity in that the radioisotope to be assayed has a definite half-life 

 and radiation characteristics that can be identified or measured. Both 

 of these properties are never exactly duplicated in any two radioisotopes. 

 Difficult chemical separations are often avoided, and the procedure has 

 particular advantage for those elements for which there are no adequate 

 traditional methods of analysis. In some cases the sample is unchanged 

 after decay of the induced activity and can be used for other analyses. 

 As always, there are limitations. These result primarily from radiation 

 characteristics and are discussed in some detail later on. 



PRINCIPLES 



When a sample is exposed to a flux of bombarding particles, the 

 elements contained therein become activated to form radioactive species. 

 There are then two opposing processes in operation: the growth of radio- 

 activity, as governed by the flux of radiation and cross section of the ele- 

 ment; and the decay, as governed by the half-fife of the radioisotope 

 formed. The amount of radioactivity present at any given time during 

 the exposure can be expressed by the following equation, derived from the 

 usual growth and decay laws (1): 



^ ^ G(&.02 X 10-^^)/a(l - e-^')(e) ^^Q_j^ 



where A = disintegrations per second in radioisotope produced 



G = grams of element of natural isotopic composition in sample 

 / = flux of bombarding particles in units of particles/cm'/sec 

 a — cross section of nuclear reaction in units of cm-/target atom 

 X = radioactive-decay constant 

 t = time of bombardment 

 = fractional abundance of target isotope in naturally occurring 



element 

 M = atomic weight of element 

 It is theoretically possible to calculate (7, the grams of unknown ele- 

 ment, by substitution of measured and known values in Eq. (10-1). 

 However, it is much more practical to evaluate G by use of a comparison 

 sample, comprised of a known amount of the element to be determined, which 



