XVII. RADIOACTIVE TRACERS 



627 



increases with increasing thickness of mount, but this effect Soon 

 reaches a maximum vahie. For absohite measurements it is impor- 

 tant to avoid back-scattering completelj'^, but since biological tracer 

 measurements are always relative the easier alternative of constant 

 back-scattering will suffice. 



Other precautions that should be taken to avoid errors are that 

 samples should cover the same area and that the counter arrangement 

 should include a method for obtaining reproducible geometry. Since 

 holders for GM tubes that include reproducible position sample 

 mounts are now commercially available, the latter points should cause 

 no difficulty. 



3. Self- Absorption Error 



One source of error that cannot ahvaj^s be eliminated is that due 

 to varying sample thickness. Since the energy of the jS radiation 



lij 



H 

 =) 



5 



a: 

 in 

 a. 



to 



10 



20 

 BaCOj.mg. 



30 



Fig. 10. Decrease in measured activity of C" sample 

 due to self-absorption (3). 



from several of the most important tracer isotopes is quite low, the 

 absorption of this radiation in the sample itself, called self-absorption, 

 will cause trouble {33,4^; 3, p. 104). With such low energies the 



