Can Transient Phenomena Help Improving Time Resolution in Scintillators?

The time resolution of a scintillator-based detector is directly driven by the density of photoelectrons generated in the photodetector at the detection threshold. At the scintillator level it is related to the intrinsic light yield, the pulse shape (rise time and decay time) and the light transport from the gamma-ray conversion point to the photodetector. When aiming at 10 ps time resolution, fluctuations in the thermalization and relaxation time of hot electrons and holes generated by the interaction of ionization radiation with the crystal become important. These processes last for up to a few tens of ps and are followed by a complex trapping-detrapping process, Poole-Frenkel effect, Auger ionization of traps and electron-hole recombination, which can last for a few ns with very large fluctuations. This paper will review the different processes at work and evaluate if some of the transient phenomena taking place during the fast thermalization phase can be exploited to extract a time tag with a precision in the few ps range. A very interesting part of the sequence is when the hot electrons and holes pass below the limit of the ionization threshold. The only way to relax their energy is then through collisions with the lattice resulting in the production of optical and acoustic phonons with relatively high energy (up to several tens of meV) near the ionization threshold. As the rate of such electron/phonon exchange is about 100 events/ps/electron or hole and as the number of electrons/holes generated after mutiplication in a high light yield scintillator like LSO can be as high as 100,000 or more, we end up with an energy deposition rate of about 100 KeV/ps. This energy deposition rate contributes to many fast processes with a characteristic time in the ps range such as band-to-band luminescence, hot intraband luminescence, acoustic shock wave generation, fast local variation of index of refraction, etc. We will discuss if the part of the total energy which is rel- ased this way, and which can represent between 50% and 90% of the energy of the incoming ionization radiation, can be efficiently exploited to improve the time resolution of scintillators, presently limited to the 100 ps range.