Transparent Ceramics Research Center, SICCAS
Key Laboratory of Transparent Opto-fuctional Inorganic Materials, CAS
Scintillator materials and phenomena studied for fast timing
Prof. Martin Nikl
Institute of Physics, AS CR, Prague, Czech Republic
Abstract: 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 possess 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 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. Moreover, excitonic emission in GaN-GaInN multiple quantum wells shows also very promising timing characteristics in this respect. 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.
Prof. Martin Nikl graduated in 1981 at Faculty of Nuclear Science and Physical Engineering, Czech Technical University in Prague, and obtained his PhD. in 1986 in Institute of Physics, CAS. Currently serves as the department head and vice-director for targeted research in Institute of Physics, AS CR. His research interests include luminescence and scintillation mechanism in wide band-gap solids, energy transfer processes and role of material defects in them. He is the coordinator and participant in about 30 domestic and 10 international projects so far in the field of scintillation materials. Author and co-author of more than 600 papers in the refereed impacted international journals, 48 papers in non-impacted journals, six chapters in books and 49 papers in conference proceedings. Author and co-author of 38 invited keynote and plenary lectures at International Conferences. The publications received more than 6800 citations (Scopus, auto-citations excluded), Hirsch factor, H=49.