ABSTRACT
Inorganic scintillators are widely used for fast timing applications in high-energy physics (HEP) experiments, time-of-flight positron emission tomography and time tagging of soft and hard x-ray photons at advanced light sources. As the best coincidence time resolution (CTR) achievable is proportional to the square root of the scintillation decay time it is worth studying fast cross-luminescence, for example in BaF2which has an intrinsic yield of about 1400 photons/MeV. However, emission bands in BaF2are located in the deep-UV at 195 nm and 220 nm, which sets severe constraints on photodetector selection. Recent developments in dark matter and neutrinoless double beta decay searches have led to silicon photomultipliers (SiPMs) with photon detection efficiencies of 20%-25% at wavelengths of 200 nm. We tested state-of-the-art devices from Fondazione Bruno Kessler and measured a best CTR of 51 ± 5 ps full width at half maximum when coupling 2 mm × 2 mm × 3 mm BaF2crystals excited by 511 keV electron-positron annihilation gammas. Using these vacuum ultraviolet SiPMs we recorded the scintillation kinetics of samples from Epic Crystal under 511 keV excitation, confirming a fast decay time of 855 ps with 12.2% relative light yield and 805 ns with 84.0% abundance, together with a smaller rise time of 4 ps beyond the resolution of our setup. The total intrinsic light yield was determined to be 8500 photons/MeV. We also revealed a faster component with 136 ps decay time and 3.7% light yield contribution, which is extremely interesting for the fastest timing applications. Timing characteristics and CTR results on BaF2samples from different producers and with different dopants (yttrium, cadmium and lanthanum) are given, and clearly show that the the slow 800 ns emission can be effectively suppressed. Such results ultimately pave the way for high-rate ultrafast timing applications in medical diagnosis, range monitoring in proton or heavy ion therapy and HEP.
Subject(s)
Luminescence , Scintillation Counting , Photons , Positron-Emission Tomography , VacuumABSTRACT
The formation of radiation defects in calcium and strontium fluoride single crystals doped with cadmium or zinc has been investigated by luminescence and absorption spectroscopy, as well as by electron spin resonance spectroscopy. It was found that x-irradiation could convert divalent impurity ions located at essentially cubic sites into the univalent state. Three types of Cd(+) or Zn(+) centers differing by local environment with point symmetries O(h), C(3v) and C(2v) are identified in the crystals. The formation of the last two results from the interaction between reduced impurities in the cubic environment and anion vacancies. The latter are intrinsic radiation defects and are not created by x-irradiation in undoped crystals. We also discuss the possible implications of the electric field of the charge impurity defects on separation of the intrinsic radiation defects in these crystals.
ABSTRACT
The emission and excitation spectra as well as decay of emissions of alkaline-earth fluoride crystals doped with CdF2 were investigated in the 2-24 eV range at temperatures in the range 8-300 K. Emission bands at 4.2 and 3.5 eV, respectively, were found under excitation into the Cd absorption region in CaF2-Cd and SrF2-Cd crystals at low temperatures. Both emission bands have slow luminescence decay times of microsecond timescale. No Cd-related emission was found in BaF2-Cd crystals. The calculations of the geometrical configurations of excited triplet Cd2+ centres and the Cd-related electron transitions were carried out by using the ab initio Hartree-Fock method. The results of experiments and calculations lead us to the conclusion that the observed Cd2+ emission bands are due to the triplet-singlet transitions from the Cd s-states to the nearest fluorine ions. The calculated energies are in good agreement with experimentally observed ones.
