RADIATION DETECTION

RADIATION DETECTION

Radiations cannot be perceived by our normal senses, such as sight,feeling and smell. Hence, a suitable device is required, to detect and measure the amount and energy of the radiation. Generally, these devices consist of a detector in which interaction takes place and a measuring device to record the interaction. Some important effects on which the detection of radiation is based are: ionization, luminescence, photographic effect, thermoluminescence, chemical effect, and biological effect.

IONIZATION

This effect consists of removing electrons from originally neutral atoms, or molecules, thereby giving rise to positive ions and negative ions.These are known as ion pairs, which can be collected by applying an electric field, to give rise to current or pulses. Gaseous and solid media are used for ionization. The ionization chambers, proportional counters, Geiger-Muller (GM) counters and semiconductor detectorsfall under this category.

LUMINESCENCE

This is the process in which radiation excites the atoms of a material and its energy is converted into visible light flash. Using light sensitive photomultipliers, the light flashes are converted into electrical pulses, which can be detected and registered with the help of electronic circuit.Detectors making use of this effect for radiation detection are known as scintillators. A scintillator coupled to a photomultiplier forms a scintillation detector.

PHOTOGRAPHIC EFFECT

Similar to light, radiation also affects X-ray film. If the film is exposed by X or gamma rays, it will form a latent image, with black metallic silver. The degree of blackness can be measured by means of opticaldensity, which is proportional to radiation exposure.

THERMOLUMINESCENCE

Radiation can impart energies to certain crystalline materials (lithiumfluoride, calcium sulfate) or certain glasses, which can store these energies for long time. The energy thus stored can be later released in the form of light or luminescence by heating these materials. The quantity of light released can be measured and correlated to radiation dose. The devices based on these effects are called TL dosimeters.

CHEMICAL EFFECTS

Ionizing radiation can cause chemical changes, e.g. oxidation of ferrous sulfate to ferric sulfate. These effects can be quantitatively measured to correlate to the radiation dose. Some of these chemical systems may be used in conjunction with indicators to measure high personnel exposures. Radiation can also cause change of coloration in certainplastics. Such color changes can also be measured and correlated to radiation doses.

BIOLOGICAL EFFECTS

Radiation exposure to the body can be measured bybiological methods,e.g. analysis of blood for chromosomal aberrations in persons exposed to radiation doses of range 10–1000 rem. This is perhaps the only method of dosimetry, when there is no other information is available to assess the radiation exposure.