principle of radiation measurement

ionization

Radiation creates positive and negative ions in a gas or matter, this process is called Ionization. This process is caused by the release of electrons from ions or molecules. These ions which are two in number are called Ion Pair, and can be collected by applying an electric field, which is found in the form of curent or puslses. The total charge collected is proportional to radiation intensity. Generally gaseous or solid media is used for Ionization. Ionization can be used in Bq measurement and dose measurement. Ionization Chamber, Proportional Counter, Geiger-Muller (GM) Counter and Semiconductor Detector etc. fall under this category.

farmer chamber

Luminescence

There are some compounds that have the characteristic of emitting visible light when radiation falls. In this process, the energy of radiation is converted into visible light. This light flash is detected by a Photomultiplier tube (PMT) or Photo Diode, which generates electrical signals. These electrical signals are proportional to the intensity of radiation occurring. Detectors built on this principle are called scintillators e.g. NaI | Scintillation detectors are divided into organic compounds and inorganic crystals. 

Thermo-luminescence

Some materials have the ability to take energy from radiation and store it for long periods such as LiF and CaSO4. After heating these materials, storred energy is released as light or luminescence. The amount of light emitted is proportional to the radiation intensity incident on it. The devices based on this effect are called thermo luminescent dosimeters. 

Photographic Effect

Ionization Radiation affects the photographic film and forms a latent image in the form of density patterns. On processing this film, the optical density of the image is measured. The amount of blackness on the film is proportional to the amount of incident radiation. This film blackness is measured as optical density. 

  OD = Log10 I0 / It 

This principle is used in film dosimetery. E.g. Film badge.

Chemical Effect

Ionization Radiation can cause chemical changes in some chemicals. E.g. They oxidize ferrous sulfate to ferric sulfate. You can measure radiation dose by measuring concentration of ferric ion. Radiation also changes the color of some plastics, also measured radiation dose. These principles are useful in chemical dosimetry. 

Biological Effect

Radiation exposure can cause changes in lymphocytes and chromosomes. Exposed individual blood samples are analyzed for chromosomal aberration such as Dicentric and Ring Formation. This aberration score is a measure of radiation exposure and is called biological dosimetry. Chromosomal dicentric analysis is performed by blood samples that measure radiation doses in the range 100mSv to 6 Sv. This dosimetry is done during accidental exposure when there is no other radiation exposure information.