Basic Materials Studies of Lanthanide Halide Scintillators
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1038-O01-03
Basic Materials Studies of Lanthanide Halide Scintillators F. P. Doty1, Douglas McGregor2, Mark Harrison1,2, Kip Findley3, Raulf Polichar4, and Pin Yang5 1 Engineered Materials Dept., Sandia National Labs, Livermore, CA, 94550 2 Dept. of Mechanical and Nuclear Engineering, Kansas State University, Manhattan, KS, 66506 3 School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, 99164 4 SAIC, San Diego, CA, 92127 5 Ceramic and Glass Dept., Sandia National Labs, Albuquerque, NM, 87185 ABSTRACT Cerium and lanthanum tribromides and trichlorides form isomorphous alloys with the hexagonal UCl3 type structure, and have been shown to exhibit high luminosity and proportional response, making them attractive alternatives for room temperature gamma ray spectroscopy. However the fundamental physical and chemical properties of this system introduce challenges for material processing, scale-up, and detector fabrication. In particular, low fracture stress and perfect cleavage along prismatic planes cause profuse cracking during and after crystal growth, impeding efforts to scale this system for production of low cost, large diameter spectrometers. We have reported progress on basic materials science of the lanthanide halides. Studies to date have included thermomechanical and thermogravimetric analyses, hygroscopicity, yield strength, and fracture toughness. The observed mechanical properties pose challenging problems for material production and post processing; therefore, understanding mechanical behavior is key to fabricating large single crystals, and engineering of robust detectors and systems. Analysis of the symmetry and crystal structure of this system, including identification of densely-packed and electrically neutral planes with slip and cleavage, and comparison of relative formation and propagation energies for proposed slip systems, suggest possible mechanisms for deformation and crack initiation under stress. The low c/a ratio and low symmetry relative to traditional scintillators indicate limited and highly anisotropic plasticity cause redistribution of residual process stress to cleavage planes, initiating fracture. Ongoing work to develop fracture resistant lanthanide halides is presented. INTRODUCTION Lanthanum halide scintillators have enabled scintillating gamma ray spectrometers competitive with room temperature semiconductors1,2, providing similar energy resolution with larger active volumes than available CdZnTe detectors, making such applications as hand held radioisotope spectrometers practical 3. However, increasing the active volume to larger sizes needed for applications in nuclear nonproliferation and homeland security has proven difficult due to profuse cracking during crystal growth and subsequent processing. Therefore basic studies of the materials science of the lanthanide halide system are needed to determine the causes of cracking, and develop strategies to scale the crystal growth.
RESULTS AND DISCUSSION Cleavage and slip Results of materials property
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