Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
delay of 10 ‐8 s or less. The emission of energy through phosphorescence and delayed
fluorescence should be minimal. In addition, it is necessary that scintillators be
transparent to their own fluorescent radiation and that the photons emitted have energies,
known as a spectral distribution, consistent with the response of available collecting
apparatus usually photocathodes. (Photocathodes play a role for scintillators similar to
that of anodes which are the collectors for gas‐filled detectors. However, there is no
analogous attraction of the photons to the photocathode, but rather an optical coupling is
required to channel the light.)
Principle of Operation
The process whereby a substance absorbs energy and then re‐emits it as photons in the
visible or near‐visible energy range, is termed luminescence. The initial excitation energy
for this process has many origins such as light chemical reactions, mechanical strains,
heat, and ionizing radiation. The last named causes a scintillation because of the
excitation or ionization produced in the substance. Only scintillants that de‐excite
promptly, i.e., through prompt fluorescence in 10 ‐8 s or less, are useful as radiation
detectors. However, most energy deposited in the substance by ionizing radiation is
quickly degraded into heat. In choosing a scintillation material, therefore, it is important
to determine the efficiency with which the kinetic energy of charged particles is
converted into visible light and also to determine that this conversion is linear, i.e., that
the light yield is proportional to deposited energy over as wide a range as possible.
2.4.2.2. Semiconductor Detectors
Another type of solid state detector, known as a “semiconductor” detector, also takes
advantage of the properties of particular crystals. In this case, the crystal is a very good
insulator which under certain conditions, can be made to conduct electricity. The typical
semiconductor detector can be thought of as a sandwich with the crystal in the center and
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