Doctoral Dissertations

Date of Award

8-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Charles L. Melcher

Committee Members

Mariya Zhuravleva, Claudia J. Rawn, Jason Hayward

Abstract

Ce doped Gd3Ga3Al2O12 [gadolinium gallium aluminium oxides] is considered as a promising candidate for the next generation Positron Emission Tomography material due to its high light yield in theory. This dissertation is focused on studying the Gd3Ga3Al2O12:Ce crystals by codoping, aiming to improve the light yield and decay time experimentally and understand the underlying mechanism.

The work starts from prescreening appropriate codopants for Gd3Ga3Al2O12:Ce crystals. A cost-effective method is developed to predict the performance of the single crystals by characterizing the radioluminescence intensity and photoluminescence decay of the small polycrystalline pellets. This method is demonstrated by showing that the results from pellets and crystals are sufficiently similar. Based on the prescreening, crystals codoped with B, Ba and Ca are selected for growth and further study on the scintillation properties, optical properties, and charge traps. B and Ba codoping increase the light yield from 47,000 to ~ 53,000 photons per megaelectron volt, whereas Ca codoping reduces the scintillation decay time from 51 to 43 nanoseconds, and suppresses the shallow traps hence improves the afterglow.

The properties of Ca codoped crystals show strong dependence on the concentration of Ca. The relationship between Ca concentration and the optical/scintillation properties is explored. The Ce valence state and F+ [F plus] center are first studied by annealing in the Ca codoped crystals. As Ca concentration increases, both light yield and decay time decrease, which can be understood by considering a Ce4+ [tetravalent cerium] emission model. Ca promotes the transition of Ce valence state from Ce3+ [trivalent cerium] to Ce4+ and introduces an F+ center, both of which can be affected by annealing. A redox mechanism and a charge compensation process are proposed to explain the change in Ce valence state and F+ center.

An innovative method was invented to create an intrinsic self-reflective layer serving as an alternative to the traditional external reflector used in radiation detectors. The intrinsic self-reflector is a white layer formed on Gd3Ga3Al2O12 crystals after annealing in a reducing atmosphere, and shows excellent performance in terms of maximizing photon collection thanks to its high reflectivity (92%).

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