GENERAL PROCEDURES FOR RADIOASSAY 



179 



(5-3) 



If a set of self-absorption standards is prepared usiiij>; a constant amount 

 of activity for each sample, the resultant self-absorption curve may be 

 used to correct the observed activity to the activity that would be meas- 

 ured if the samples were weightless. In this method the reference point 

 (activity of a weightless sample) is well defined and not artificial. This 



4000 - 



3000 



I 2000 



<3 











40 



10 20 30 



Tc, mg/cm2 BaS04 (constant specific activity) 



Fig. 5-7. Effect of self-absorption on measured activity of S^* in barium sulfate of 

 constant specific activity. [Frovi Walter L. Graf, C. L. Comar, and Ira B. Whilney, 

 Relative Sensitivities of Windowless and End-window Counters, Nucleonics, 9: 22-27 

 (1951).] 



widely used procedure, designated as the constant-activity method, is as 

 follows: A series of samples is prepared, each containing the same amount 

 of radioactivity but with different masses. It will be necessary that one 

 sample be essentially weightless so that the counting rate with zero loss 

 due to mass can be determined. In the other samples the radioactivity 

 should be uniformly distributed throughout the mass of the sample. If 

 the measurements are usually to be made on a given compound, then this 

 material should be used to supply the mass. 



Some typical data for Ca^^ are listed in Table 5-3, and Fig. 5-8 shows 

 typical self-absorption curves for Ca^^ in calcium oxalate as measured in 

 two different types of counters (42). The fraction counted N/Nq is 

 plotted against the number of milligrams per square centimeter in the 

 sample (.Vo = true activity of sample as measured with zero mass, and 

 A^ = measured activity of sample). 



