note4 - Med Phys 4R06/6R03 Radioisotopes and Radiation...

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Med Phys 4R06/6R03 Radioisotopes and Radiation Methodology Page 4-1 Chapter 4 Scintillation Detectors 4.1. Basic principle of the scintillator Scintillator Ionizing radiation Light (visible, UV) Light sensor Fig. 4.1. Principle of the scintillation detectors. Scintillates are one of the oldest types of radiation detector because measurements could be made with photographic film. Images could be collected or intensity measurements could be made. Measurements were also made with the human eye observing the brightness of frequency of flashes in the scintillator. Nowadays the light output is converted into voltage pulses that are processed in the same way as pulses from proportional counters, semiconductor detectors etc. The whole point of scintillation detectors is that we want to produce a large light output in the visible range. There are two commonly used types of scintillators, inorganic crystals and organic scintillators. The scintillation mechanism is different for these two types. 4.2. Inorganic Scintillators Conduction Valence Conduction Valence Conduction Valence Insulator Semiconductor Conductor Fig. 4.2. Band structure for electron energies in solids. The scintillation mechanism depends on the structure of the crystal lattice. In a pure inorganic crystal lattice such as NaI, electrons are only allowed to occupy selected energy bands. The forbidden band or band gap is the range of energies in which electrons can never be found in the pure crystal. In the pure crystal, absorption of energy can elevate electrons from the valence band to the conduction band leaving a gap in the valence band. However, the return of an electron to the
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Med Phys 4R06/6R03 Radioisotopes and Radiation Methodology Page 4-2 valence band with the emission of a photon is an inefficient process. Few photons are released per decay, the energy is emitted by other mechanisms. In addition, band gap widths in pure crystals are such that the resulting emitted photon is too high to lie within the visible range. Small amounts of impurities are therefore added to the crystal. Tl is added to NaI in trace amounts. The impurities are called activators , they create special sites in the lattice at which the band gap structure, the energy structure, is modified. The energy structure of the overall crystal is not changed, just the energy structure at the activator sites. (a) Pure crystal (b) Activated crystalline scintillator Fig. 4.3. Energy band structure of an inorganic scintillator. At the few activator sites within the sample, the energy structure is modified. Energy states are created within what would be the forbidden band in the pure crystal. The electron can de-excite through these levels back to the valence band. The energy levels created by the activator’s presence within the crystal are narrower than in the
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note4 - Med Phys 4R06/6R03 Radioisotopes and Radiation...

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