ASSAY OF RADIOACTIVITY 



quenching pulse, and the straightforward probe circuit illustrated in Figure 

 31.6, which either inverts and slightly amplifies the pulses or can be used as 

 a cathode follower, has the merits of greater simplicity and reliability. 



Various types of scaling circuit are described in Chapter 41. Until recently, 

 most workers used hard- valve scalers incorporating several scales of eight or. 



iEHT+ 



Anode 

 of Geiger 



tube 



HT+ 

 300 V 



^Output to 

 scaler 



Fipire 31.6 A simple probe circuit {A.E.R.E. Probe type 200) 



more conveniently, often, arranged in cascade, driving a mechanical register: 

 but the development of Dekatron counting tubes seems likely to supplant the 

 hard-valve circuits, except for the first stages of scalers designed to operate at 

 very high counting rates. The sensitivity of the scaler, taking into account 

 some amplification of the pulses by the probe unit, is normally such that a 



0* 



c 



c 

 o 



Operating 

 potential 



Starting 

 potential 



_J 



500 1,000 1,500 



Anode voltage 

 Figure 31.7 Characteristic curve for an argon-alcohol filled Geiger tube 



few pulses begin to be recorded when the applied potential is just below the 

 Geiger-Muller region {Figure 31.1). The voltage at which counting begins 

 is called the starting potential, and the curve obtained by plotting the count- 

 ing rate for a fixed source against applied potential is the characteristic curve 

 of the counter. A typical curve for an argon-alcohol filled tube is shown in 

 Figure 31.7. 



430 



