Sec. 13.3] STANDARDIZATION OF RADIOACTIVE SAMPLES 359 



the quantity of radon gas in equilibrium with one gram of radium. Later 

 this definition of the curie was extended to include other naturally radio- 

 active decay products of radium [1]. Finally, by general use, the unit was 

 extended to all radioactive isotopes. Recent determinations do not give the 

 rate of disintegration for one gram of radium as 3.7 X 10 10 disintegrations 

 per second. The best value is somewhat lower: (3.608 + 0.028) X 10'" per 

 sec. This was obtained by Kohman et al. [2] and corresponds to 0.652 cc; 

 of radon gas at 0°C and 760 mm Hg. There is thus a small discrepancy 

 between the number of disintegrations adopted by international agreement 

 for one curie and the actual number. To avoid ambiguities that may arise 

 if the curie is used in these two different connections, Curtis and Condon 

 recently proposed a new unit for radioactivity which they called the ruther- 

 ford [3]. As defined by them, one rutherford is the quantity of radioisotope 

 decaying at the rate of 10 6 disintegrations per second. This unit has certain 

 merits inasmuch as it allows the use of the term curie for the purpose origi- 

 nally designated, namely, to denote quantities of radium decay products. 

 So far, however, the unit has not found general acceptance. 



13.2. General Methods. In most measurements one is interested in the 

 specific activity or the quantity of radioactive isotope per gram of sample 

 substance expressed in curies per gram. If the entire quantity, i.e., every 

 atom, of a particular element in the sample is radioactive, it is called a carrier- 

 free (C.F.) sample. The rate of disintegration R is related to the number 

 of parent atoms TV by 



R = \N 



where X is the decay constant. If R is the unknown, it is necessary to 

 evaluate both X and N, or if N is the unknown, one must measure X and R. 

 Most of the methods of primary standardization depend on measuring these 

 quantities. The number A 7 can be determined either gravimetrically, as in 

 the preparation of some primary standards of uranium, or by mass spectro- 

 graphic deposition of a known amount of carrier-free isotope. R is obtained 

 if the absolute number of particles and radiations emitted can be measured 

 or if the quantity of the accumulated decay products is measured over a 

 certain time interval. 



13.3. Primary Alpha-particle Standards. Because of the accuracy with 

 which the disintegration constants are known, the most satisfactory alpha 

 standards are prepared from several of the long-life natural alpha-particle 

 emitters. Uranium and Ra 226 have been used more than others. With 

 weighed amounts of these substances it is possible, with appropriate cor- 

 rections, to count all alpha particles emitted from them in alpha-particle 

 ionization chambers. Since the vast majority of alpha particles follow 

 straight trajectories, have a well-defined range, and produce relatively 



