48
P. Lecoq
for their relaxation on luminescent centres through an energy or a charge transfer
mechanism.
For a material to be a scintillator it must contain luminescent centres. They are
either extrinsic, generally doping ions, or intrinsic i.e. molecular systems of the
lattice or of defects of the lattice, which possess a radiative transition between
an excited and a lower energy state. Moreover, the energy levels involved in the
radiative transition must be smaller than the forbidden energy bandgap, in order to
avoid re-absorption of the emitted light or photo-ionization of the centre.
In a way, a scintillator can be considered as a wavelength shifter. It converts the
energy (or wavelength) of an incident particle or energetic photon (UV, X-ray or
gamma-ray) into a number of photons of much lower energy (or longer wavelength)
in the visible or near visible range, which can be detected by photomultipliers,
photodiodes or avalanche photodiodes.
3.1.2 Important Scintillator Properties
Scintillators are among the most popular ionizing radiation detectors.
There are two main classes of scintillators: inorganic and organic. For the
inorganic systems (generally ionic crystals), scintillation arises from thermalized
electrons and holes, moved to the bottom of the conduction band or the top of
the valence band respectively, by scattering from the initially produced fast charge
carriers. For the organic systems, scintillation arises upon transition between an
excited molecular level and the corresponding electronic ground state. Inorganic
scintillators are generally brighter but with a slower decay time than organic ones.
However no “ideal” material exists and the choice of a scintillator depends on
the application, as it is generally driven by a trade-off between a number of
physico-chemical and optical parameters such as density, scintillation properties and
radiation hardness. The production and processing cost is also an important issue
taking into consideration the very large volumes required for some applications.
3.1.2.1 Physico-chemical Properties
Physico-chemical properties are related to the material composition, structure and
density, as well as to its chemical stability when exposed to different environmental
conditions: air, humidity, ionizing radiation.
Frequently the density and hence the compactness of the detector is essential
in order to reduce the detector volume and cost. This is achieved by using high
stopping power and therefore high density materials. This reduces the size of the
shower for high energy γ’s and electrons as well as the range of Compton scattered
photons for lower energy γ-rays. A dense material also reduces the lateral spread of
the shower, which is particularly important for the majority of High Energy Physics
detectors.
P. Lecoq
for their relaxation on luminescent centres through an energy or a charge transfer
mechanism.
For a material to be a scintillator it must contain luminescent centres. They are
either extrinsic, generally doping ions, or intrinsic i.e. molecular systems of the
lattice or of defects of the lattice, which possess a radiative transition between
an excited and a lower energy state. Moreover, the energy levels involved in the
radiative transition must be smaller than the forbidden energy bandgap, in order to
avoid re-absorption of the emitted light or photo-ionization of the centre.
In a way, a scintillator can be considered as a wavelength shifter. It converts the
energy (or wavelength) of an incident particle or energetic photon (UV, X-ray or
gamma-ray) into a number of photons of much lower energy (or longer wavelength)
in the visible or near visible range, which can be detected by photomultipliers,
photodiodes or avalanche photodiodes.
3.1.2 Important Scintillator Properties
Scintillators are among the most popular ionizing radiation detectors.
There are two main classes of scintillators: inorganic and organic. For the
inorganic systems (generally ionic crystals), scintillation arises from thermalized
electrons and holes, moved to the bottom of the conduction band or the top of
the valence band respectively, by scattering from the initially produced fast charge
carriers. For the organic systems, scintillation arises upon transition between an
excited molecular level and the corresponding electronic ground state. Inorganic
scintillators are generally brighter but with a slower decay time than organic ones.
However no “ideal” material exists and the choice of a scintillator depends on
the application, as it is generally driven by a trade-off between a number of
physico-chemical and optical parameters such as density, scintillation properties and
radiation hardness. The production and processing cost is also an important issue
taking into consideration the very large volumes required for some applications.
3.1.2.1 Physico-chemical Properties
Physico-chemical properties are related to the material composition, structure and
density, as well as to its chemical stability when exposed to different environmental
conditions: air, humidity, ionizing radiation.
Frequently the density and hence the compactness of the detector is essential
in order to reduce the detector volume and cost. This is achieved by using high
stopping power and therefore high density materials. This reduces the size of the
shower for high energy γ’s and electrons as well as the range of Compton scattered
photons for lower energy γ-rays. A dense material also reduces the lateral spread of
the shower, which is particularly important for the majority of High Energy Physics
detectors.