372 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 13 



thickness can all be changed and the validity of the Bragg-Gray conditions 

 tested. 



In spite of the care required for absolute determination of disintegration 

 rates with ionization chambers, their use has much value in routine secondary 

 standardization procedures. They may be made in very rugged form and 

 retain their sensitivity through periods of several years without appreciable 

 change. One form, the Lauritsen electroscope, has been used in biological- 

 tracer studies for many . years. Improved high-sensitivity ionization 

 chambers and electrometers for standardization are available in several 

 laboratories. 



13.10. Standardization of Beta-counter Geometry. Much of the beta 

 counting in practice is carried out using end-window type Geiger-Miiller 



TO AIR 



TO PUMP 



Fig. 103. Bell-jar counter and vacuum-chamber assembly used for beta-particle stan- 

 dardization. The principal components of the assembly are A, counter wall (cathode); 

 B, counter anode; C, mica window; D, aluminum vacuum chamber lined with lucite; E, 

 lucite rods for supporting sample; F, sample, point source mounted on thin supporting foil; 

 G, diaphragm, absorbers are placed on top of diaphragm. The vacuum-tight door of the 

 chamber is not shown. [H. Anger and C. A. Tobias, unpublished.] 



counters with thin mica windows. In principle, the usual procedure con- 

 sists in counting a standard sample which is placed in a standard position 

 underthe counter tube after calibration by one of the independent methods. 

 In this way the efficiency of a particular counter with a fixed geometry is 



