1.5.1 Ionising radiations

When radiations pass through matter they may react with the atoms they encounter. The reactions often lead to the ionisation of atoms and their displacement from their normal position in the structure. All or part of the energy of the incident radiation may be transferred to these disturbed atoms. In this way ra­diation may damage the structures through which they pass — in particular they may damage the structure

of living tissue. The effects of, and protection against, such damage are discussed in Chapter 4. The op­eration of radiation detecting instruments and the selection of shielding materials are also dependent on an understanding of the interaction of radiation with matter.

The following is confined to considering the ionisa­tion of atoms by the ionising radiations a, 0 and 7. It is worth recalling here that the nomenclature a, 0, 7 radiation dates back to the early workers on radioactivity who, unaware of what the radiations were, identified three types by their penetrating power and called them after the first three letters of the Greek alphabet.