BASIC PHENOMENA IN GAS CHAMBERS 



radioautography, and under favourable conditions it is possible to achieve 

 a resolving power of the order of a few microns. Since this technique does 

 not directly involve the use of electronic apparatus it will not be discussed in 

 detail here, and the reader is referred for further information to the general 

 texts already mentioned and to reviews by Doniach, Howard and Pelc^, 

 and Boyd^. 



Scintillation counting 



Sir William Crookes observed in 1903 that the impact of a-particles on a 

 screen of zinc sulphide resulted in the emission of flashes of light. This 

 phenomenon depends on the formation of 'excited' atoms by the displace- 

 ment of electrons from atoms in a crystal lattice, and the subsequent emission 

 of radiation in the visible spectrum (and beyond it) when the excited atoms 

 revert to the unexcited ('ground') state. It formed the basis for the visual 

 counting techniques used by Rutherford and others in the early researches 

 on nuclear physics, but visual scintillation counting had severe limitations, 

 and was later abandoned in favour of methods depending on the use of gas 

 chambers (see below). In the last few years the simultaneous development 

 of improved methods for converting brief flashes of light into electrical pulses, 

 and of more convenient 'phosphors' (the name given to solids or liquids in 

 which ionizing radiation can be converted to visible or ultraviolet light), 

 has brought scintillation counting back into use, particularly for y-radiation. 

 The apparatus needed for scintillation counting is discussed in a later section 

 of this chapter. 



Gas chamber counting 



The majority of techniques used in the assay of radioactivity depend on the 

 ionization arising from the passage of radiation through gases. It is possible 

 either to observe the integrated effects of a number of ionizing particles or 

 quanta of radiation, as in ionization chambers, or to detect the emission of 

 single particles, as in proportional counters and Geiger-Mtiller counters. 

 The relationship between these three types of gas chamber and their special 

 features are considered in the following sections. 



BASIC PHENOMENA IN GAS CHAMBERS 



As has already been seen, when radiation interacts with the atoms in a gas, 

 pairs of ions are formed, each consisting of one negative electron and one 

 heavy positive ion. The charged a- and /5-particles ionize directly, while 

 y-radiation causes ionization indirectly by setting in motion secondary 

 charged particles. If the gas in which the ionization is occurring is contained 

 in a chamber provided with electrodes maintained at an appropriate potential 

 difference relative to one another, the electrons will tend to move towards 

 the anode and the positive ions will move towards the cathode. The amount 

 of charge collected varies with the applied potential in a characteristic way, 

 as shown in Figure 31.1. Here the charge collected at the anode is plotted 

 logarithmically against the anode voltage for two different situations: (1) for 

 an initial ionization consisting of only a few ion pairs, as might be produced 

 by the passage of a /3- or y-ray through the sensitive volume between the 



423 



