Speaker
Description
Fast and efficient scintillators are required by a number of applications, namely in high energy physics and medical imaging (Positrom Emission Tomography – PET) where so call time-of-light (TOF) techniques are used and timing coincidence resolution (TCR) is of critical importance. Today, with LSO-type single crystals, TCR values closed to 100 ps have been demonstrated, but it seems improbable to reach values below 70 ps using a bulk scintillator of this kind. The goal of COST FAST project is to seek the concept(s) to advance TCR up to by one order of magnitude, down to 10 ps which would give an unprecedented improvement in the applications using TOF techniques, e.g. a direct image reconstruction could be used in TOF-PET imaging.
To reach such a goal sufficiently high number of scintillation photons must be detected within several tens of ps after the absorption of high energy photon (particle) by the scintillator. Such a scintillator should posses high light yield, as short as possible decay time and negligible rising components in scintillation response. Direct band gap semiconductors as ZnO, GaN, PbI2, CdSe or CsPbCl3 [1] could be used as their luminescence based on Wannier exciton shows the radiative lifetime below 1 ns and theoretical light yield could exceed 100 000 phot/MeV due to smaller value of band gap. However, small Stokes shift, which is a consequence of Wannier exciton nature, practically disable to use these materials in a bulk form due to huge reabsorption losess. Moreover, due to the fact that such excitation is not localized, nonradiative quenching e.g. at the surface or interface of these materials becomes a critical problem. The transparent composite materials, in which the scintillation nanophase is embedded in an optically transparent host, e.g. ZnO:Ga nanocrystals in a polystyrene matrix, became intensively studied [2]. Moreover, excitonic emission in GaN-GaInN multiple quantum wells shows also very promising timing characteristics in this respect [3]. Finally, other ultrafast phenomena as crossluminescence, Cherenkov and intraband luminescence are also considered for this purpose though their yield seems to be too low at the materials studied so far.
In the presentation we will discuss in the detail some of the above mentioned materials and phenomena and their practical potential for fast timing.
Acknowledgement. This work is based upon work from COST Action (TD1401, FAST), supported by COST (European Cooperation in Science and Technology).
- S.E.Derenzo et al, Nucl. Instr. Meth. Phys. Research A 486 (2002) 214.
- H. Buresova et al, Optics Express 24 (2016) 15289.
- A. Hospodkova et al, Nanotechnology 25 (2014) 455501.
