Doctoral Dissertations

Date of Award

8-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Charles L. Melcher

Committee Members

Lars A. Eriksson, Mariya Zhuravleva, Jason P. Hayward

Abstract

In most scintillator applications, the energy resolution is an important scintillation property and is related to other scintillator properties. In order to observe how these properties relate to the energy resolution, a simulation was created to quantify most of these characteristics for a LSO:Ce scintillator. These results were validated with good agreement to experimental results. From the separable components of the simulation, an understanding of the contributions to the energy resolution broadening was developed. A thought to improve the energy resolution by improving the energy migration was tested by observing and modifying the scintillation kinetics of YSO:Ce. The scintillation kinetics in YSO:Ce are quite different from LSO:Ce even though the materials are similar in crystal lattice structure and the cerium activator dopant. The scintillation kinetics differences are observed when measuring the scintillation decay time with the results varying in decay times and different mathematical decay models. Using thermoluminescence, it was observed that YSO:Ce has more shallow traps with trap lifetimes at ~300K on the same order as the Ce3+ excited state lifetime. Using these same data, it was calculated that these shallow traps have lifetimes ~years when the sample is cooled to 40K. Re-measuring the decay time at 40K yields a decay time of 32ns and shows that the shallow traps in YSO:Ce are the cause of impeded energy migration to the luminescence centers. By using calcium co-doping during crystal growth, most of the trap structure was significantly suppressed. With these YSO:Ce:Ca samples, the scintillation decay times were decreased nearly to the cerium excited lifetimes. In order to measure any improvement in the non-proportional response, a new measurement technique was developed. The new method used angular based measurements using a PET scanner to calculate the energy of a Compton electron deposited in the sample. The results agreed with published data for NaI:Tl and LSO:Ce scintillators. Finally, it was demonstrated that the non-proportional response of YSO samples were the same with improvement in energy resolution without a large increase in light output. The conclusion was that the homogeneity of our YSO:Ce:Ca samples led to a 3% improvement in energy resolution.

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