Creation of Electron-Hole Pairs in Inorganic Scintillators
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The inorganic scintillation detector is the most widely used radiation sensor. Yet in many applications the scintillator requirements for an optimal operation are not met. Often either the light yield is too low or the response time is too slow, or both. Much research is in progress to improve this situation. The scintillation process can be divided in three stages: a) the primary interaction and thermalization of the resulting electrons and holes to e-h pair energies roughly equal to the band-gap energy Eg, b) further relaxation, formation of excitonic states and energy transport to the luminescence centra, and c) luminescence. It is well known that crystals having a high light output under photoexcitation can give a rather low light emission under excitation by high-energy charged particles, X-rays or gamma rays [1]. This is primarily a consequence of energy losses in stages a and b. In this paper we will focus on the mechanisms of stage a. The primary interaction of an X-ray or a gamma ray will result in the production of one or more energetic electrons, depending on the energy and the interaction mechanism (photoelectric effect, Compton effect, pair formation). X-rays produced by recombination of electrons with holes in core levels (from photoelectric effect) will also produce electrons by the above mentioned mechanisms. Heavy charged particles and low-energy electrons transfer their energy primarily by ionization, thus producing electrons and holes. High-energy electrons transfer their energy by production of bremsstrahlung which in turn gives pair formation. Therefore the main mechanism of stage a to be studied is the slowing down of electrons. Their energy is absorbed by the production of a number of secondary excitations: electrons and holes, and plasmons. The energy loss, i.e. the energy not available for further ionization and thus not available for luminescence, is characterized by the average energy E,, needed for the creation of a single electron-hole pair at the band edge. The determination of the parameter E& is rather difficult due to the complexity of the processes of energy dissipation in a scintillator. Various physical models have been used for the description of these complicated processes of energy dissipation: simple phenomenological [2,3], "crazy-carpentry" [4], plasmon [5], and polaron [6]. These models consider only a single mechanism and do not take into account the whole variety of processes taking place. At present a general conception of energy loss in a scintillation crystal does not exist. In this paper we undertake an attempt to critically analyse and generalize the known theories and models. Some corrections and additions were made to the models and a comparison of theoretical and experimental data was undertaken. 'This study has been supported by The Netherlands Technology Foundation (STW) 379 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
BASIC FORMULAS AND CONCEPTIONS The scintillation efficiency is determined by the number of photons NPh emitted upon in
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