XVII. 



K A D 1 O A C T I V E TRACERS 



(329 



plotted vs. sample weight (since the area is the same in all the samples. 

 the weight and thickness are proportional) and this curve is used to 

 obtain the corrections, using the weights of the various samples 

 measured under these conditions. Correction can be made to zero 

 sample weight (thickness) or to some chosen value. 



Another approacli to the problem, which applies <jnly to nuclides 

 that are solely (S emitters, is to approximate an infinitely thick source. 

 As the weight of material of a given specific activity mounted on a 

 constant area is increased, the measured activity will first increase 

 and then level off approaching a constant value when the total thick- 

 ness (including air and counter window) is equivalent to the range of 

 the |8 particle. For C^^, with a range of 25 mg. per square centimeter, 



5000 



SAMPLE THICKNESS, mg./cm.' 



Fig. 1 1 . Increase in measured activity with increasing sample thickness 

 for material of constant specific activity (4^). 



^^^ndow thickness of 2.5 mg. per square centimeter, and 1.1 cm. (1.5 

 mg./cm.^) of air between source and counter, a sample of thickness of 

 21 mg. per square centimeter would be an "infinitely thick" source. 

 The relative measured activity of a number of such thick sources 

 ^vill, it can be seen, be proportional to the specific activities of the 

 samples and not to their total activities. One technique suggested 

 for handling such cases when the weight of active material is large is 

 to have a number of uniform dishes or sample pans and to fill them 

 to the top with the active material for counting. In this way the 

 specific activity will be determined directly in the counting operation. 

 This will only be a relative method, but very little biological tracer 

 research will involve absolute measurements. If in using this method 



