Date of Award

12-2017

Degree Type

Thesis

Degree Name

Master of Science

Major

Nuclear Engineering

Major Professor

Charles L. Melcher

Committee Members

Mariya Zhuravleva, Jason Hayward

Abstract

CsCaI3:Eu [cesium calcium iodide doped with europium] is a promising scintillator material that can be grown from the melt, but undergoes a tetragonal to orthorhombic phase transition upon cooling at 255 °C [degrees Celsius], causing twinning and cloudiness. The purpose of this work is to suppress this solid to solid phase transition in the CsCaI3:Eu scintillator, which has a light yield of ~40000 ph/Mev and energy resolution at 662keV of ~4%, by halide replacement to form the compound CsCaBrxI3-x:Eu [cesium calcium bromo-iodide doped with europium]. Crystals 8 cm3 [cubic centimeters] in volume were grown using the vertical Bridgman method with varying bromine replacement from x=0.2 to x=1, resulting in improved transparency for crystals with bromine content x > 0.6. Powder X-ray diffraction data coupled with differential scanning calorimetry and radioluminescence measurements were used to investigate structural modifications, melting point dependence and spectral emission dependence on the bromine/iodine ratio. Partial replacement of iodine by bromine improves optical quality and scintillation properties by stabilizing the structure, rendering it useful for isotope identification for national security applications. The composition CsCaBr0.8I2.2:Eu [cesium calcium bromo (0.8) iodide (2.2) doped with europium] was determined to be the best combination of improved structure and performance, and larger 22 and 38 mm Ø [diameter[ crystals were grown for further evaluation. Large size slabs showed good crystal quality and improved performance over CsCaI3Eu with 8.4% and 9.5% energy resolution at 662 keV, respectively. The improved performance this composition over CsCaI3:Eu introduces the possibility of its use for gamma spectroscopy in national security applications.

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