3 Scintillation Detectors for Charged Particles and Photons
69
Fig. 3.11 Probability of binding or separation of an e-h pair as a function of energy (courtesy A.
Vasiliev)
Two competing recombination processes can take place, both being intrinsically
non-linear with energy as shown on Fig. 3.11, and inducing therefore a non-linear
energy response of the scintillator. The first one is the self-trapping of the exciton
in the vicinity of a luminescent centre which decreases rapidly with the e-h pair
energy. The second one is the direct capture of the separate electron and/or hole by
defects or luminescent centres and increases with the kinetic energy of the electron
and hole.
The energy threshold between these two mechanisms is related to the correlation
distance R 0 between the electron and hole, which is temperature dependant. As a
result, the energy dependence of the scintillator response to thermalized e-h pars
is strongly non-linear as shown on Fig. 3.10, which also shows the influence of
the defects (crystal quality) on the excitation transfer efficiency to the luminescent
centres.
The quantitative link between the low energy non-linearity of the scintillator
response and the deviation of its energy resolution from the predicted counting
statistics is far from being fully understood. It has however its seed in the fact that for
the same total amount of deposited energy both photons and electrons release this
energy in a number of quanta over a large energy range and that the light response for
each of these quanta has different proportionality constants as a function of energy.
The event-to-event variation of this cascade process induces a spread in the energy
response, which deteriorates the energy resolution.
This is obvious in the case of Compton scattering. In a detector of a finite size,
the events in the photopeak result from the sum of true photoelectric events and
of events having undergone single or multiple Compton scattering interactions all
69
Fig. 3.11 Probability of binding or separation of an e-h pair as a function of energy (courtesy A.
Vasiliev)
Two competing recombination processes can take place, both being intrinsically
non-linear with energy as shown on Fig. 3.11, and inducing therefore a non-linear
energy response of the scintillator. The first one is the self-trapping of the exciton
in the vicinity of a luminescent centre which decreases rapidly with the e-h pair
energy. The second one is the direct capture of the separate electron and/or hole by
defects or luminescent centres and increases with the kinetic energy of the electron
and hole.
The energy threshold between these two mechanisms is related to the correlation
distance R 0 between the electron and hole, which is temperature dependant. As a
result, the energy dependence of the scintillator response to thermalized e-h pars
is strongly non-linear as shown on Fig. 3.10, which also shows the influence of
the defects (crystal quality) on the excitation transfer efficiency to the luminescent
centres.
The quantitative link between the low energy non-linearity of the scintillator
response and the deviation of its energy resolution from the predicted counting
statistics is far from being fully understood. It has however its seed in the fact that for
the same total amount of deposited energy both photons and electrons release this
energy in a number of quanta over a large energy range and that the light response for
each of these quanta has different proportionality constants as a function of energy.
The event-to-event variation of this cascade process induces a spread in the energy
response, which deteriorates the energy resolution.
This is obvious in the case of Compton scattering. In a detector of a finite size,
the events in the photopeak result from the sum of true photoelectric events and
of events having undergone single or multiple Compton scattering interactions all