Sec. 13.10] STANDARDIZATION OF RADIOACTIVE SAMPLES 



375 



polated back to zero thickness. Actually such a procedure is only per- 

 missible if the extrapolation is very small. Using soft beta emitters, for 

 example, the absorption curve with the absorber close to the counter window 

 is usually very different from that with the absorber right over the sample 

 [41]. 



The angular distribution of beta particles from a point source of small 

 mass is isotropic. A sample support of finite thickness, or an extended 



130 



120 



2 

 UJ 



o 



or 



UJ 



a. 



HO 



100 



20 40 60 80 100 120 140 160 180 



MG PER CM 2 of ALUMINUM REFLECTOR 

 Fig. 105. Backscattering of P 32 and Co 60 beta particles (E ma % = 1.72 and 0.3 mev, respec- 

 tively) as a function of sample support thickness. The counting rate is given in per cent 

 of the rate obtained with a sample support of negligible thickness. The samples were 

 point sources, and the counting geometry is indicated in the diagram showing the position 

 of sample, counter window and sensitive region of counter (broken line). [Unpublished 

 data of H. Anger.] 



source with finite thickness, changes the isotropic distribution into one 

 preferring a direction perpendicular to the plane of the sample. The first 

 effect is usually called backscattering. The magnitude of this depends on 

 the beta-particle spectrum and the atomic number and thickness of the 

 sample support. Using an aluminum sample support, the effect of different 

 thicknesses on the counting rate of Co 60 and P 32 is plotted on Fig. 105. It is 

 apparent that if the support thickness is greater than one-half the amount 

 of absorber necessary to stop all beta particles the counting rate does not 

 increase with support thickness. This condition is often called "saturation 

 backscattering." The dependence of saturation backscattering on atomic 

 number of sample support is shown in Figs. 106 and 107. Some beta- 

 particle standards are mounted on supports that give saturation backscatter- 



