378 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 13 



reproduced for C 14 in Fig. 109. When samples of finite thickness are used, 

 nonuniformity of thickness results in a decreased counting rate as a con- 

 sequence of variable self-absorption. The precise methods for obtaining 

 calibrations for self-absorption have been discussed by Yankwich etal. [31]. 



Fig. 109. Fraction of maximum observable activity of C 14 in a BaC0 3 sample as a function 

 of sample thickness. The broken line indicates the activity which would be observed in 

 the absence of self-absorption. [Redrawn from P. E. Yankwich, T. H. N orris, and J. Hus- 

 ton, Anal. Chem., 19, 439 (1947).] 



13.11. Secondary Beta-particle and Gamma-ray Standards. The Bureau 

 of Standards has made available a number of secondary standards. Among 

 these are gamma-ray standards of Ra and of Co 60 (the latter standardized by 

 the coincidence method) and RaE beta-particle standards. In the near 

 future secondary beta-particle standards of Co 60 , I 131 , P 32 , and C 14 will also be 

 added. In the meanwhile further work is going on in the search for a con- 

 venient long-lived beta standard with higher maximum energy than Co. 60 

 Some laboratories and commercial enterprises have also prepared UX 2 beta- 

 particle standards. 



13.12. Standardization of Neutrons and Protons. Detailed processes for 

 standardization of neutrons are beyond the scope of this chapter, although 

 some of the more important considerations of the problems that arise can be 

 briefly stated. The number of fast neutrons is usually determined by 

 measuring the radioactivity induced in some substance for which the inter- 

 action cross section is known accurately. From the point of view of biology, 

 the ionization produced by fast neutrons is more important than the number 

 of such neutrons. For some purposes, therefore, the ionization is often 

 measured by a thimble ionization chamber. As in the case of gamma-ray 



