SCINTILLATION COUNTERS 



which presents particular difficulty because of its exceedingly weak radiation, 

 the isotope may be added to the filling gas either as methane or as molecular 

 hydrogen. ^''C may similarly be counted as carbon dioxide or acetylene. 



One of the respects in which proportional counters differ from Geiger 

 counters is, or course, that the output pulses are proportional in size to the 

 energy of the /3-particles that gave rise to them. By suitably analysing the 

 pulses with the help of discriminator circuits it is therefore possible both to 

 sort out the radiations emitted by mixtures of isotopes in double-labelling 



Figure 31.9 



The basic elements of a negative feedback amplifier with a 

 gain of the order of 100. The main section of A.E.R.E. Pulse Amplifier Type 

 1008, designed for use with proportional counter tubes, consists of two circuits 



of this type in cascade, separated by an attenuator 



experiments, and to achieve very low background counts by rejecting all 

 pulses outside the range characteristic of the isotope being used. 



SCINTILLATION COUNTERS 



As has already been mentioned, scintillation counting was first used over 50 

 years ago for the determination of a-particles, but as soon as gas chambers 

 had been developed the technique fell into disuse. It is now coming into 

 steadily increasing favour once more, since it not only provides a means for 

 detecting y-rays with much higher efficiency (up to about 50 per cent) than 

 any gas chamber method, but has also been applied successfully to the 

 determination of low-energy /3-particles. The initial re-introduction of 

 scintillation counting resulted from the availability of highly sensitive 

 photomultiplier tubes (see Chapter 28) capable of converting the minute 

 flashes of light produced by y-rays in suitable crystals into electric pulses 

 which could be counted in the same way as the pulses generated by Geiger 

 tubes. The later developments have followed from further improvements in 

 photomultiplier sensitivity, together with the discovery of additional sub- 

 stances, including various liquids, which can be employed as phosphors. 



The discharge mechanism in proportional and Geiger counters depends 

 fundamentally on the separation of electrons and positive ions formed from 



439 



