XVII. RADIO A CT I V E TRACERS 621 



thereby, scales of 64 (i.e., 2^) . The pulses up to 64 are recorded on the 

 neon Ughts and the sixty-fourth count clears the lights and is recorded 

 on the mechanical register. Decade scaling circuits are also avail- 

 able, but have not been used as widely (90,91). 



Most of the scaling circuits commercially made for use in this 

 field include, in addition, a regulated high voltage supply for the 

 GM tube. Regulation is essential since the counting rate varies with 

 changing voltage even in the region of the plateau. High counting 

 rates cause a drain on the high voltage supply, which may cause dif- 

 ficulty if the latter is not well regulated against this. A particularly 

 annoying source of trouble is an unsteady house supply of electricity. 

 During periods of overloading, the actual a.c. voltage delivered to the 

 set may be as much as 20 v. below the rated value. If this situation 

 exists some form of stabilizer should be added, since the circuit cannot 

 be expected to operate satisfactorily under such conditions. Elec- 

 tronic stabilizers now on the market appear to be quite ade(iuate for 

 both voltage and frequency find nations (96). A small motor genera- 

 tor set will take care of line voltage changes but will introduce fluctua- 

 tions in voltage if the frequency of the lin(> voltage is not constant. 



Scalers including high voltage supplies and employing the so- 

 called Higinbotham circuit (32), which is new and yet thoroughly 

 tested through Atomic Energy Commission use, are manufactured 

 by several companies and are at present available, with a built-in 

 mechanical register, for about $400 {84,88,89). If the research 

 budget can afford these, the problem of counter circuit is nearly 

 solved — nearly, because these are not perfect and they do require 

 adjustment and repairs. The latter are usually not serious, but 

 some one conversant with electronics should be available. 



4. Special Measuring Techniques 



In some cases f)f particular importance the /3 radiation of the 

 radioisotope is of such low energy and penetrating power that even 

 the thin window of the end window counter absorbs a large part of 

 the radiation. If it is necessary to measure samples of appreciable 

 weight, the self -absorption of the rays by the sample can be quite 

 serious. In order to circumvent these difficulties for the three impor- 

 tant tracers H^, C^* and S^^, various techniques have been developed. 

 In the case of tritium, H^, difficulties in its production and measure- 

 ment as compared to the ready availability, low cost, and easy 



