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Electromagnetic radiation can interact with matter via
The last effect only occurs at energies above 1.02 MeV. In practice, all effects have a chance to occur, this chance being proportional to the energy of the radiation and the atomic number (Z-value) of the absorber (the scintillation material). In the photo(electric) effect, all energy of the radiation is converted into light. This effect is important when determining the actual energy of the impinging X-ray or gamma-ray photons. The lower the energy and the higher the Z-value, the larger the chance on photo effect. Fig. 2.1 shows a typical pulse height spectrum measured with a 76 mm diameter, 76 mm high NaI(Tl) crystal in which the radiation emitted by a 137Cs source is detected. The photopeak, Compton maximum and backscatter peak are indicated. The lines around 30 keV are Ba X-rays emitted by the source. |
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The total detection efficiency (counting efficiency) of a scintillator depends on the size, thickness and density of the scintillation material. However, the photopeak counting efficiency, important for e.g. gamma-ray spectroscopy, increases with the Z 4-5 of the scintillator. At energies below 100 keV, electromagnetic interactions are dominated by the photoelectric effect. The absorption can be calculated from the attenuation coefficient for a certain scintillator (or absorber). Consult the SCIONIX leaflet "Attenuation coefficients" for data on the most common materials. Electrons (e.g. b-particles) can be backscattered from a material which implies that no energy is lost in the interaction process and the particle is not detected at all. The backscattering fraction is proportional to the Z of the material. For NaI(Tl) the backscatter fraction can be as high as 30%! This implies that for efficient detection of electrons, low Z materials such as plastic scintillators or e.g. CaF 2 (Eu) are preferred. The window material is also of importance. |
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Select a link for more information: Scintillation Response to g-rays Scintillator Interaction with Charged Particles; a- and b -particle detection
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