Ionizing radiation counting techniques

The second type of detection system uses the delay wire technique (61), and a schematic diagram of this detector is shown in Figure 2. The beta radiation (fast electrons) emitted from the radioactivity source on the plate ionizes the counting gas, which has been specifically chosen so that this process can freely take place. This is the primary mode of ionization and the resultant charged particles, free electrons and positive ions, are then accelerated towards the anode wire and cathode, respectively. In this primary mode of ionization, the free electrons are accelerated to such an extent that they themselves cause ionization of the counting gas, producing further free electrons and ions. This is the secondary ionization mode. This continues causing an avalanche of ionization from the primary point of ionization towards the anode wire. 

These techniques are based on the principle that ionizing radiation causes certain materials to emit flashes of light or scintillations. There are two different forms of scintillation counting ... 

Another example of a teclmique for detecting absorption of laser radiation in gaseous samples is to use multiphoton ionization with mtense pulses of light. Once a molecule has been electronically excited, the excited state may absorb one or more additional photons until it is ionized. The electrons can be measured as a current generated across the cell, or can be counted individually by an electron multiplier this can be a very sensitive technique for detecting a small number of molecules excited. 

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