STUDIES ON PULSE SHAPE DISCRIMINATION AND EFFICIENCY OF GGAG:Ce SCINTILLATORS

Abstract

Among garnet based scintillators, Ce doped GGAG:Ce (Gadolinium Gallium Aluminium Garnet) scintillator proved to be a promising candidate for charged particle detection due to its high light yield and fast decay time combined with its non-hygroscopic nature. In the present thesis work, extensive studies were made on the pulse shape discrimination (PSD) properties of GGAG:Ce single crystals and on improving its PSD ability with co-doping and with phoswich combination. Owing to its high effective Z (55) and high density (6.7 g/cm3), the present work also highlights GGAG:Ce as a strong contender for gamma spectroscopy where detection efficiency is of paramount importance. The work starts with making a comparison of PSD abilities of GGAG:Ce,B and CsI:Tl scintillator coupled to PMT and SiPM. The studies were made employing digital charge integration and analog zero-crossing technique. The studies have shown that GGAG:Ce,B coupled to a PMT and CsI:Tl coupled to SiPM have shown better PSD abilities. For the first time, the opposite behavior of scintillation decay times corresponding to alpha and gamma radiations for GGAG:Ce and CsI:Tl scintillators has been reported. The detailed realistic Monte Carlo simulations of absolute efficiencies (both total detection and photo-peak) for gamma rays up to energy of 5 MeV and for different values of source-to-detector separation have clearly suggested that the efficiency of GGAG:Ce scintillator is highest in comparison with LaBr3:Ce, NaI:Tl, CsI:Tl, BaF2 and SrI2:Eu. The simulations have shown that the percentage difference in TDE of GGAG:Ce and NaI:Tl for 662 keV is 40%. Whereas, the percentage difference in PE of GGAG:Ce and NaI:Tl for 662 keV is 61%. The simulations wer4 validated by making experimental measurements. Simulated and measured efficiency values of GGAG:Ce scintillator having dimensions 18 mm × 18 mm × 10 mm and 25.4 mm ×10 mm for different values of source-to-detector separation are in good agreement. A study on the effect of co-dopants on the scintillation properties of GGAG:Ce single crystals by investigating their PSD abilities is carried out. B co-doped crystals exhibited the highest PSD while those with Ca co-doping showed no discrimination in spite of having significant effect on the scintillation kinetics and a strong quenching of the light yield by alpha radiations that resulted in a minimum α/γ ratio. A new approach of optically stimulated luminescence (OSL) is used to correlate PSD ii properties with the defect structure of GGAG:Ce single crystal. OSL studies with infrared, blue and green light provided an insight into the role of defect centers in the relaxation mechanism of the scintillation kinetics of GGAG:Ce crystals which subsequently affects their ability of discriminating different kinds of radiation. A novel design of phoswich detector is proposed for the discrimination of various types of nuclear radiation such as protons, heavy ions, neutrons and gamma rays. Its novelty lies in the use of two non-hygroscopic scintillators having similar light yield, peak emission wavelength and refractive index. Due to their different scintillation decay times and opposite behavior for alpha and gamma radiations, an increment of 100% in FOM is found compared to that observed for any other individual crystal.