RADIO ACTIVATION ANALYSIS 415 



is simultaneously exposed to the same flux and treated in the identical way as 

 the unknown. The physical significance of the various terms in Eq. (10-1) 

 will be further explained in the later discussion of sensitivity. 



After the irradiation the sample may contain several radioactive 

 species. There is then the problem of measuring the particular radio- 

 isotope produced by the element for which the analysis is made. In sim- 

 ple cases the methods described in Chap. 5 for the radioassay of mixtures 

 can be employed. For example, radioactivation analysis of manganese 

 in aluminum was accomplished readily, since the aluminum radioisotope 

 has a half-life of 2.3 min, whereas that of manganese has a half-life of 

 2.59 hr. After irradiation the sample was allowed to stand at least 

 23 min before counting, so that the aluminum radioactivity was reduced 

 to an insignificant level and the counting measurements reflected only 

 the level of manganese present (6). Biological material, however, is 

 usually complex, and it becomes necessary to make chemical separations 

 so as to isolate the element being determined from extraneous activities 

 associated with other elements. This is usually done by the addition of 

 a known amount of the stable element, as an isotopic carrier, to the test 

 sample and to an aliquot of the comparison sample after irradiation. The 

 test sample and comparator are then chemically processed to isolate the 

 element being determined for radioassay. Since the separation yields are 

 seldom quantitative, it is often necessary to make a correction for the 

 actual yield obtained. This can be done by chemical determination of 

 the recovery of the known amount of added carrier. 



The general procedure may then be summarized as follows: (a) A 

 known weight of the sample to be analyzed is irradiated, together with a 

 comparative sample containing a known weight of the element to be deter- 

 mined, (b) After irradiation a known weight of the element to be deter- 

 mined is added as a carrier to the solutions of both the test and com- 

 parator samples, (c) Each solution is chemically processed to separate 

 the desired element and its radioisotope(s) from extraneous radioactiv- 

 ities, (d) The chemical yield of the carrier is determined, (e) The radio- 

 activities in the test and comparator samples are measured under similar 

 conditions. The purity of the separated radioactivity can be checked by 

 a measurement of decay and radiation-energy characteristics. 



SENSITIVITY 



Equation (10-1) is useful in explanation of the factors that govern the 

 sensitivity of the determination. To give a maximum value of produced 

 radioactivity (A), it is clear that 



1. The neutron flux / should be large, since the sensitivity is directly 

 proportional thereto. 



