In order to evaluate the radiation dose to an indi­vidual, or the radiation and contamination levels of an area, there must be an effective means of mea­suring and detecting radiation.

Radiation detection

The detection of radiation, and ultimately its mea­surement, utilises the principles of the interaction of radiation with matter, and this usually means ion­isation. Ionisation is simply the removal of electrons from the outer shells of atoms due to the interaction between radiation and the atoms. If an electric field is present, then the electrons will flow and a current is produced which can be amplified and displayed. This principle gives rise to the most common radiation devices known as gas-filled detectors which include ionisation chambers, proportional counters and Geiger- Muller counters.

A solid state equivalent of the gas ionisation de­tectors are the semiconductor detectors, usually ger­manium-lithium, silicon-lithium or cadmium sulphide. These types of detector are usually used when it is important to know the energy of the radiation.

With scintillation detectors the radiation interacts with a scintillant, e. g., zinc sulphide, and produces flashes of light which may then be used to produce an electrical output. Scintillation detectors are very often used for measuring alpha and beta radiation.

A device which combines the properties of solid state detectors and those using the scintillation principle is the thermoluminescence detector (TLD). When ir­radiated, the TLD ‘stores’ the energy produced by the radiation until it is heated. When heated this energy is released in the form of light, the amount being proportional to the initial amount of radiation. Com­mon substances used for TLD material are lithium fluoride, calcium sulphate and lithium borate. TLD detectors are mainly used for personal monitoring and are particularly useful for extremity monitoring, e. g., fingertips, when it is used in the powder form sealed into a plastic sachet.

In addition to physical changes, radiation may also produce chemical or physio-chemical changes in a material. The most important device using this prin­ciple is the film badge dosemeter, which works on the same principle as the photographic process. The radiation interacts with the film emulsion, which when developed shows a darkening proportional to the amount of radiation exposure.

Radiation and contamination monitoring The choice of technique for the measurement of an individual’s dose depends on the nature of the work to be performed, the type and level of radiation en­countered and whether contamination is present. In most operational circumstances the film badge or TLD device is used. These can discriminate, depending on the design, between thJivarious types of radiation. If the job to be performed entails handling radioactive material, or if any other body part is irradiated to a different extent, then additional devices are worn on these body parts. It is important to recognise that the film badge and TLD cannot be directly read, they must be processed in a laboratory to determine the dose received. If high radiation doses are involved then electronic dosemeters are issued, based on the — ionisation chamber principle. These have an electronic display of the accumulated dose and may also be coupled to audible alarm circuits, which set off at preselected doses or dose rates.

If there is likely to be an airborne contribution to the dose, or if it is required to demonstrate that there is no airborne hazard, then a personal air sam­pler may be issued. This is simply a filter paper con­nected to a pump which draws through air at the worker’s breathing zone. Subsequent measurement determines whether or not there has been airborne contamination present.

In addition to personnel being monitored to deter­mine their dose, areas of plant are also monitored. This, as already described, may be in the form of radiation and contamination surveys to determine the classification of the zone. In addition certain areas may have fixed instrumentation to detect the radia­tion. These may be gamma detectors, for the mea­surement of direct radiation, or fixed air samplers, where airborne contamination is present. These may be connected to alarm circuits to warn of potential or actual hazards and may even be interlocked to

such things as cranes, to prevent raising or lowering high dose rate objects, or door locks to prevent entry to (but not exit from) high dose rate areas.