PhD Defense: Applying Novel Material Characterization Techniques Using Ultrafast Laser Excitation and Neutron Diffraction in Radiation Detector Crystal

Drew Onken, PhD Candidate
Public Presentation in ZSR Library Auditorium, Room 404
Monday, July 23, 2018, at 2:00 PM
Richard Williams, PhD, Advisor

The defense will follow.


To address outstanding issues in the growth and performance of crystals for radiation detection, I develop and employ several material characterization techniques not previously implemented in the field. Two main, interconnected issues are addressed: the cracking of certain crystals during the growth process and spatial inhomogeneity in the defect distributions of radiation detector crystals. Although the purpose of these materials is to detect high-energy gamma rays, no gamma rays were used in these studies. Instead, much can be learned from the more precise interactions of low-energy photon and neutron beams.


The growth of single-crystal boules of certain semiconductors and scintillators can be plagued by cracking during post-growth cool-down. I have conducted proof-of-concept studies on a survey of laser techniques both to map thermal and chemical non-uniformity in situ as well as to perform laser ablation to separate material, to produce cleaner cuts, and to drive dislocation multiplication. In addition, neutron diffraction at high temperatures approaching the melting point can characterize the thermal and chemical stresses which can be exacerbated to the point of cracking by thermal gradients in the furnace.


After fabrication, asymmetries in the growth process can result in an inhomogeneous distribution of defects and dopants in the crystal. These non-uniformities in the crystal can have a significant impact on the energy resolution and degradation of the radiation detector. I construct a two-photon “multi-scope” and implement position-sensitive spectroscopic techniques to map the inhomogeneity of a crystal’s response, considering the origin of the inhomogeneity and the impact it has on detector performance. Finally, with an understanding of how spatial and energy density non-uniformity of response can affect energy resolution, digitized scintillation pulses are analyzed in an effort to extract extra information from the pulse shape in hopes of improving non-proportionality and energy resolution.