chAPter | 29 Nuclear Instrumentation Technology 525 29.1.2.7 Thermoluminescent Detectors For many years it was known that if one heated some sub- stances, particularly fluorites and ceramics, they could be made to emit light photons and, in the case of ceram- ics, could be made incandescent. When ionizing radiation is absorbed in matter, most of the absorbed energy goes into heat, whereas a small fraction is used to break chemi- cal bonds. in some materials a very minute fraction of the energy is stored in metastable energy states. Some of the energy thus stored can be recovered later as visible light photons if the material is heated, a phenomenon known as thermoluminescence (TL). in 1950 daniels proposed that this phenomenon could be used as a measurement of radiation dose, and in fact it was used to measure radiation after an atom-bomb test. Since then interest in TL as a radiation dosimeter has progressed to the stage that it could now well replace photographic film as the approved personnel radiation badge carried by people who may be involved with radioactive materials or radiation. thermoluminescent powers and, if left for a long period in a constant radioactive background, accumulates an amount of TL proportional to the time it has lain undisturbed in that environment. 29.1.3 health and Safety Anyone who works with radioactive materials must under- stand clearly the kinds of hazards involved and their mag- nitude. Because radioactivity is not directly observable by the body's senses, it requires suitable measuring equipment and handling techniques to ensure that any exposure is mini- mized; because of this, suitable legislation governs the han- dling and use of all radioactive material. in Part 4 an outline of the regulations is given as well as advice on contacting the local factory inspector before the use of any radioactive source is contemplated. Because everyone in the world already receives steady radiation (from the natural radiopotassium in the human body and from the general background radiation to which all are subjected), the average human body acquires a dose of about 300 micro-grays (nGy) (equivalent to 30 mil- lirads) per year. Hence, though it is almost impossible to reduce radiation exposure to zero, it is important to ensure that using a radioactive source does not increase the dose to 29.1.2.8 Materials for TL Dosimetry The most popular phosphor for dosimetric purposes is