62
P. Lecoq
complex chain of processes, each of them characterized by a specific time constant
and efficiency factors [16]. This is summarized in Fig. 3.8, where the valence and
conduction bands of an insulator with a bandgap width E g (forbidden band) are
represented. The upper level core band (energy E c and bandwidth E c ) is also
shown.
The sequence of processes is shown as a function of time and can be qualitatively
divided into five main phases:
• The first one is the energy conversion phase and the subsequent production
of primary excitations by interaction of ionizing particles with the material.
For an incident particle energy in the keV range or higher, the excitations are
essentially deep holes h created in inner core bands and hot electrons e in the
conduction band. Subsequently, on a very short time scale (10 −16 –10 −14 s), a
large number of secondary electronic excitations are produced through inelastic
electron-electron scattering and Auger processes with creation of electrons in the
conduction band and holes in core and valence bands. At the end of this stage,
the multiplication of excitations stops. All electrons in the conduction band have
an energy smaller than 2E g (e-e scattering threshold) and all holes occupy the
valence band if there is no core band lying above the Auger process threshold
(general case).
• The second stage is the thermalization of electronic excitations through a phonon
coupling mechanism with the crystal lattice, leading to low kinetic energy
electrons in the bottom of the conduction band and of holes in the top of the
valence band. This thermalization phase takes place in the sub-picosecond range,
typically between 10 −14 and 10 −12 s.
• The next stage, between 10 −12 and 10 −10 s, is characterized by the localization
of the excitations through their interaction with stable defects and impurities of
the material. For example, electrons and holes can be captured by different traps
or self-trapped in the crystal lattice. Excitons, self-trapped excitons, self-trapped
holes (V K centers) can be formed with emission of phonons. Localization of
excitations can be sometimes accompanied by displacements of atoms (defect
creation, photo-stimulated desorption).
• The transfer of excitations to the luminescent centres through the sequential
capture of charge carriers or different energy transfer mechanisms takes place
during the following 10 −10 and 10 −8 s.
• Finally, the radiative relaxation of the excited luminescent enters produces the
light signal with an efficiency and time structure, which is given by the quantum
selection rules of the transition. Parity allowed transitions with more than 3 eV
energy gaps are generally preferred as they give rise to fast luminescence.
However, smaller energy gaps (2–3 eV) are likely to favour higher light yield,
as discussed in Sect. 3.1.2.2.
The scheme depicted in Fig. 3.8 describes the scintillation mechanisms in the
case of ionic crystals with simple energy structures. However, important groups of
scintillators exhibit a more complicated band structure.
P. Lecoq
complex chain of processes, each of them characterized by a specific time constant
and efficiency factors [16]. This is summarized in Fig. 3.8, where the valence and
conduction bands of an insulator with a bandgap width E g (forbidden band) are
represented. The upper level core band (energy E c and bandwidth E c ) is also
shown.
The sequence of processes is shown as a function of time and can be qualitatively
divided into five main phases:
• The first one is the energy conversion phase and the subsequent production
of primary excitations by interaction of ionizing particles with the material.
For an incident particle energy in the keV range or higher, the excitations are
essentially deep holes h created in inner core bands and hot electrons e in the
conduction band. Subsequently, on a very short time scale (10 −16 –10 −14 s), a
large number of secondary electronic excitations are produced through inelastic
electron-electron scattering and Auger processes with creation of electrons in the
conduction band and holes in core and valence bands. At the end of this stage,
the multiplication of excitations stops. All electrons in the conduction band have
an energy smaller than 2E g (e-e scattering threshold) and all holes occupy the
valence band if there is no core band lying above the Auger process threshold
(general case).
• The second stage is the thermalization of electronic excitations through a phonon
coupling mechanism with the crystal lattice, leading to low kinetic energy
electrons in the bottom of the conduction band and of holes in the top of the
valence band. This thermalization phase takes place in the sub-picosecond range,
typically between 10 −14 and 10 −12 s.
• The next stage, between 10 −12 and 10 −10 s, is characterized by the localization
of the excitations through their interaction with stable defects and impurities of
the material. For example, electrons and holes can be captured by different traps
or self-trapped in the crystal lattice. Excitons, self-trapped excitons, self-trapped
holes (V K centers) can be formed with emission of phonons. Localization of
excitations can be sometimes accompanied by displacements of atoms (defect
creation, photo-stimulated desorption).
• The transfer of excitations to the luminescent centres through the sequential
capture of charge carriers or different energy transfer mechanisms takes place
during the following 10 −10 and 10 −8 s.
• Finally, the radiative relaxation of the excited luminescent enters produces the
light signal with an efficiency and time structure, which is given by the quantum
selection rules of the transition. Parity allowed transitions with more than 3 eV
energy gaps are generally preferred as they give rise to fast luminescence.
However, smaller energy gaps (2–3 eV) are likely to favour higher light yield,
as discussed in Sect. 3.1.2.2.
The scheme depicted in Fig. 3.8 describes the scintillation mechanisms in the
case of ionic crystals with simple energy structures. However, important groups of
scintillators exhibit a more complicated band structure.