CARBON 



225 



istics, the C^''/mass ratio in the sample to be measured, and the amount of 

 sample available. In general, the ordinary thin-mica-window tube for 

 counting BaCOs or solid samples is preferred if sufficiently sensitive, on 

 account of simplicity and trouble-free operation and because the same 

 equipment can be used for various measurements. If additional sensitiv- 

 ity is required, the internal counter can be used at the cost of slightly 

 increased operating difficulties. The ionization chamber gives greatest 

 sensitivity but requires specialized equipment and experience in handling 

 the gas phase. 



Table 6-1 illustrates the role of self-absorption and shows how the ratio 

 of activity to total mass of the sample to be measured may govern the 



T.\BLE G-l. Effect of Self- absorption and Size of Sample Available on 

 Relative Sensitivities of Counting Techniques for C''" 



" Values are given as ratio of counting rate observed to background of instrument. 



choice of counting procedure. In case 1, assume that a sample contains 

 10~* fjLC C^* in essentially zero mass. The internal counter and the ion 

 chamber will be about five times as sensitive as the thin-mica-window 

 tube. However, there would be no advantage in going to the ion- 

 chamber measurement. Since C^^ in a biological system becomes diluted 

 with stable carbon, one would seldom encounter a situation as in case 1. 

 In case 2, assume that the 10~^ fxc C^* is contained in 1 mg carbon, which 

 would be converted to about 16.4 mg BaCOs- The self-absorption now 

 causes a decrease in the sensitivity of the solid measurements, but the ion 

 chamber still offers an advantage of only 2 over the internal counter. If 

 the activity were contained in 5 mg carbon, as in case 3, this would give a 

 solid sample approaching infinite thickness, and the advantage of the ion 

 chamber over the internal counter becomes a factor of 6.5. If there were 

 large amounts of a sample available, such as 10~^ /xc C^'* in 120 mg carbon, 

 as in case 4, there would be no advantage in using more than 5 mg carbon 



