3 Scintillation Detectors for Charged Particles and Photons
79
Fig. 3.16 Absorption spectra for different hydrolysed BaF 2 (ref. [28])
3.3.3 Radiation Damage
The exposure of crystals to ionizing or neutron radiation can induce a number of
modifications of the crystal lattice with potential consequences for the scintillation
efficiency and the light transport. These modifications can be related to pre-existing
crystal defects, when exposed to a high density of charge carriers that are easily
trapped producing colour centres with radiation-induced absorption bands. They can
also be associated to the production of new defects by elastic or knock-on collisions
of incident particles with the lattice ions resulting in a local modification of the
lattice structure. Finally, heavy energetic charged particles or neutrons may produce
dramatic events, such as heavily ionizing fission fragments. This last phenomenon is
usually of little concern in the majority of applications, even for the new generation
of high luminosity particle physics colliders, as it requires an enormous integral
fluence (10 17 –10 18 cm −2 ) to become significant. Indeed, it requires the formation
of about 10 17 cm −3 such defects to reach a 1 ppm contamination in the majority of
scintillator materials. However, such defects are by nature irrecoverable and their
progressive accumulation may affect parts of detectors highly exposed for very long
periods of time.
The situation is different for the majority of other cases (charge trapping or
ion displacement), for which relaxation processes play a fundamental role in the
kinetics of damage build-up. These defects introduce a local perturbation in the
crystal and do not change the main structure parameters and particularly the spatial
79
Fig. 3.16 Absorption spectra for different hydrolysed BaF 2 (ref. [28])
3.3.3 Radiation Damage
The exposure of crystals to ionizing or neutron radiation can induce a number of
modifications of the crystal lattice with potential consequences for the scintillation
efficiency and the light transport. These modifications can be related to pre-existing
crystal defects, when exposed to a high density of charge carriers that are easily
trapped producing colour centres with radiation-induced absorption bands. They can
also be associated to the production of new defects by elastic or knock-on collisions
of incident particles with the lattice ions resulting in a local modification of the
lattice structure. Finally, heavy energetic charged particles or neutrons may produce
dramatic events, such as heavily ionizing fission fragments. This last phenomenon is
usually of little concern in the majority of applications, even for the new generation
of high luminosity particle physics colliders, as it requires an enormous integral
fluence (10 17 –10 18 cm −2 ) to become significant. Indeed, it requires the formation
of about 10 17 cm −3 such defects to reach a 1 ppm contamination in the majority of
scintillator materials. However, such defects are by nature irrecoverable and their
progressive accumulation may affect parts of detectors highly exposed for very long
periods of time.
The situation is different for the majority of other cases (charge trapping or
ion displacement), for which relaxation processes play a fundamental role in the
kinetics of damage build-up. These defects introduce a local perturbation in the
crystal and do not change the main structure parameters and particularly the spatial