3 Scintillation Detectors for Charged Particles and Photons
61
improved low energy background rejection. A first thin scintillator layer detects (and
rejects) the low energy background activity, whereas a thicker layer on the back will
be mainly sensitive to the 3–10 MeV range of interest. The two scintillators must
have different emission wavelength and/or decay time for a good identification of
the hit source.
3.1.4 Organic Material, Glass and Condensed Gases
There is a particular class of scintillators, which does not require a regular lattice to
produce scintillation light when excited by ionizing radiation. These are organic
solid and liquid materials, condensed gases as well as scintillating glasses. A
common feature of all these materials is that scintillation (also called fluorescence
in this case) results from a direct excitation of a molecule and does not involve the
transport of the excitation energy through the material. As the molecule is directly
excited and the coupling with the host material is minimal, the fluorescence decay
time is solely determined by the quantum numbers of the excited and ground states.
If properly chosen the molecule will emit between two singlet states giving rise to a
fast emission (usually not more than a few ns).
Different material combinations can be engineered, in particular in plastic scintillators, to meet specific requirements. The most popular one concerns wavelength
shifters. Binary or even ternary solutions of different fluors can be dissolved in a
plastic base containing aromatic molecules. After excitation by ionizing radiation,
these aromatic rings will relax the stored energy by emitting UV photons. Properly
chosen additional fluors can absorb these photons and reemit them at longer
wavelength, e.g. to better match the quantum efficiency of a photodetector. As there
are only energy transfer and no charge transfer mechanisms involved, the whole
process is very fast.
Plastic scintillators can be easily machined in any shape, including in the form
of fibres, one important advantage. However, these materials are intrinsically light
(density around 1–1.2 g/cm 3 ) and therefore are not suitable for homogeneous
calorimetry. They find a number of applications in sampling calorimetry and
tracking. More information can be found in ref. [15].
3.2 Scintillation and Quenching Mechanisms in Inorganic
Scintillators
3.2.1 The Five Steps in Scintillation Process
In contrast to luminescence (such as in lasers), where the excitation source is
tuned to the energy levels of the luminescent centres, scintillation is the result of a
61
improved low energy background rejection. A first thin scintillator layer detects (and
rejects) the low energy background activity, whereas a thicker layer on the back will
be mainly sensitive to the 3–10 MeV range of interest. The two scintillators must
have different emission wavelength and/or decay time for a good identification of
the hit source.
3.1.4 Organic Material, Glass and Condensed Gases
There is a particular class of scintillators, which does not require a regular lattice to
produce scintillation light when excited by ionizing radiation. These are organic
solid and liquid materials, condensed gases as well as scintillating glasses. A
common feature of all these materials is that scintillation (also called fluorescence
in this case) results from a direct excitation of a molecule and does not involve the
transport of the excitation energy through the material. As the molecule is directly
excited and the coupling with the host material is minimal, the fluorescence decay
time is solely determined by the quantum numbers of the excited and ground states.
If properly chosen the molecule will emit between two singlet states giving rise to a
fast emission (usually not more than a few ns).
Different material combinations can be engineered, in particular in plastic scintillators, to meet specific requirements. The most popular one concerns wavelength
shifters. Binary or even ternary solutions of different fluors can be dissolved in a
plastic base containing aromatic molecules. After excitation by ionizing radiation,
these aromatic rings will relax the stored energy by emitting UV photons. Properly
chosen additional fluors can absorb these photons and reemit them at longer
wavelength, e.g. to better match the quantum efficiency of a photodetector. As there
are only energy transfer and no charge transfer mechanisms involved, the whole
process is very fast.
Plastic scintillators can be easily machined in any shape, including in the form
of fibres, one important advantage. However, these materials are intrinsically light
(density around 1–1.2 g/cm 3 ) and therefore are not suitable for homogeneous
calorimetry. They find a number of applications in sampling calorimetry and
tracking. More information can be found in ref. [15].
3.2 Scintillation and Quenching Mechanisms in Inorganic
Scintillators
3.2.1 The Five Steps in Scintillation Process
In contrast to luminescence (such as in lasers), where the excitation source is
tuned to the energy levels of the luminescent centres, scintillation is the result of a