Dr. Richard Williams
Reynolds Professor of Physics,
Department of Physics,
Wake Forest University,
Friday, November 1
Room 622, Min Kao Building
"Using Ultrafast Laser Pulses and Numerical Modeling to Understand Nonproportionality in Scintillator Detectors of High-Energy Radiation"
Under high-energy electron or gamma-ray excitation, light yield is a complicated function of carrier diffusion and cooling in the electron track along with kinetic rate terms depending on local excitation density. Extracting the fundamental rate constants from such data would require solving the diffusion and cooling problems in complex track structures first.
Using the extracted rate constants and other data in a predictive model of scintillation requires modeling once again the diffusion and cooling, which threatens to be a circular approach. Laser interband photon density response (PDR) allows measuring the nonlinearity and the population fraction interacting in each kinetic order without the complexity of electron tracks.
The experimental method, analysis, and conclusions on the nonlinearity of luminescence versus excitation density and excitation energy in scintillator materials will be discussed. Then Dr. Williams will look at how to add track structures, diffusion, and hot electron cooling into a rate model making predictions directly comparable with electron energy response (proportionality and total light yield) measured by methods such as Compton-coincidence and K-dip. The result that is sought is a predictive model of scintillator proportionality based on independently measured material parameters.
Dr. Richard Williams received his PhD in physics from Princeton University in 1973 following undergraduate education at Wake Forest University, and worked from 1973 to 1985 at the US Naval Research Laboratory in Washington, DC. He studied exciton self-trapping and time-resolved defect formation in insulators, starting with nanosecond electron pulse methods and then making the first use of picosecond laser interband excitation and pump-probe spectroscopy to study defect formation in real time.
Williams became head of the Ultraviolet Technology Section of the NRL Optical Probes Branch, and headed a group responsible for instrumenting and using the NRL portion of beam line X-27 at the National Synchrotron Light Source (Brookhaven). Returning to his alma mater of Wake Forest in 1985 as Reynolds Professor of Physics, he used the then-developing femtosecond spectroscopy methods to study excitons and defects, as well as connecting with the scintillator field when CERN (specifically Paul Lecoq and Marv Weber) started the serious search in 1990 for new classes of fast scintillators to handle the anticipated LHC experiments.
Williams hosted SCINT2007 as well as EXCON2006 and ICDIM1996 at Wake Forest and during a sabbatical visit to Lawrence Berkeley Lab in 2007 with Bill Moses, Greg Bizarri, Andrey Vasil'ev, and Jai Singh, became very interested in what fast uv lasers might be able to do toward understanding nonproportionality. The rest is the topic of the talk.
Besides scintillators, Williams is interested in renewable energy and serves as associate director of the Wake Forest Center for Energy, Environment, and Sustainability. He is lead investigator of the Four Universities Solar Consortium in the Piedmont Triad NC area.