560 Tracer Techniques 30/ : 2 



y particles (photons) and the maximum energy of the /3 particles are 

 characteristic of the particular isotope. Usually, the energies of both 

 types of particles are measured in terms of the energy an electron would 

 acquire in being accelerated by a given potential difference. These 

 units of energy include electron-volts (ev), or kiloelectron-volts (Kev). 

 The size of the unit most convenient for description of radioactive 

 tracers is the megaelectron-volt, Mev, that is, the energy an electron 

 acquires in "falling through" 10 6 volts. The different energy ranges 

 are compared in Figure 1. 



The radioactively emitted particles produce a response in a detector. 

 The responses may then be counted by some type of electronic circuit. 

 In the past, the most widely used type of detector was the Geiger- 

 Mueller (G-M) tube, also called Geiger counter. As shown in Figure 2, 

 it consists of a wire in the center of a gas-filled cylindrical chamber. 

 The central wire is insulated from the outer cylinder and maintained 

 at a high potential relative to it. Ionizing particles entering the cylinder 

 produce a chain of ionization ending in a pulse of current, provided the 

 tube potential is sufficient. Figure 3 shows the dependence of the size 

 of the current on the potential between wire and cylindrical wall. In 

 region A, the height of the pulse is proportional to the energy of the 

 incoming particle, whereas in region B, it is independent. Finally, at 

 C the tube tends to conduct current continuously once one particle 

 starts it. 



Geiger counters are used in the plateau region B. The gas in the 

 tube, the exact geometry, and the purity of the wall material all affect 

 the operation of the tube. The pulses from the Geiger tube are counted 

 by electronic circuits. Eventually, one reads a count on a bank of 

 lights, or a dial, or a paper tape. Other electronic circuits permit a 

 direct measure or recording of pulse rate. 



Pulse counters can be designed to discriminate against all but pulses 

 within a certain height range. In this fashion, a proportional counter 

 similar to a Geiger tube, but designed to operate in region A of Figure 3, 

 can be used to detect one type of radioactive isotope in a mixture. 

 This is possible because each isotope emits particles of characteristic 

 energy, which in turn give rise to pulses of characteristic heights from the 

 proportional counter. 



Another type of detector with numerous advantages is the scintillation 

 counter. This uses the tiny bursts of light produced when ionizing 

 radiations fall on many types of crystals and liquids. The light occurs 

 at a wavelength characteristic of the scintillator. The size of the light 

 burst (scintillation) is proportional to the energy lost by the particle. 

 The scintillations are detected, in turn, with a photomultiplier, whose 

 output is fed through a pulse-height selector to a counter. In this 



