CHAPTER 13 

 STANDARDIZATION OF RADIOACTIVE SAMPLES 



Cornelius A. Tobias 



13.1. Introduction. Precise methods for determining the absolute inten- 

 sity of radioactive samples are desirable for the uniformity and standardiza- 

 tion of measurements and are highly important in dose determinations. In 

 this chapter the most commonly used methods of measurement of alpha, 

 beta, and gamma radiations are described. 



The methods and units of absolute measurement have not yet been 

 developed to an entirely satisfactory stage, and the agreement between 

 measurements made in different laboratories is not quite perfect. Efforts 

 toward unification, however, have been undertaken by the National Research 

 Council which recently established a committee on radioactivity to recom- 

 mend standard procedures and units in the United States. 



In order to have a clear conception of the units used in radioactivity, the 

 terminology used in this field should be mentioned. Since, except for a few 

 units, there is no international agreement on the terminology and the mag- 

 nitude of the units used in the present chapter, they will be defined according 

 to present usage (see also Chap. 7). 



The rate of disintegration of a radioactive substance is the rate of change 

 in the number of atoms of the parent radioactive isotope per unit time. 

 The decay usually occurs by the spontaneous emission of a charged particle 

 and one or more quanta; the emission of two or more such radiations is, for 

 practical purposes, considered to be simultaneous. The rate of emission of 

 radiations from a quantity of radioactive substance is the total number of 

 quanta and particles emitted per unit time. For example, each time a radio- 

 active Na 24 atom disintegrates into Mg 24 , one beta particle and two gamma- 

 ray quanta are emitted. The rate of emission is therefore three times the 

 rate of disintegration of Na 24 atoms. The unit of radioactivity is usually 

 expressed in terms of disintegration rates, whereas dose calculations often 

 require the knowledge of emission rates. 



Conventionally the unit of activity or quantity of radioactive material 

 is the curie. One curie is the quantity of a radioisotope which decays at 

 the rate of 3.7 X 10 10 disintegrations per second. (One millicurie cor- 

 responds to 3.7 X 10 7 disintegrations per second; one microcurie, to 3.7 X 10 4 

 disintegrations per second.) The curie was originally used as a measure of 



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