WFU Physics Colloquium

TITLE: Ph. D. Defense Presentation: "Experimental and Computational Studies of Nonlinear Quenching in Materials Used as Radiation Detectors"

SPEAKER: Joel Grim,

TIME: Wednesday July 25, 2012 at 1:30 PM

PLACE: Room 101 Olin Physical Laboratory

The next generation of radiation detectors used in security scanning, medical imaging, and high energy physics applications depend on understanding the physical mechanisms that limit intrinsic energy resolution. Nonproportionality between electron energy response and the initial energy of a stopping electron is a significant cause of the degradation of energy resolution. The physical origin of nonproportional response is generally regarded to start with nonlinear quenching, proportional to 2nd or 3rd power of local excitation density. A large part of the present work is the experimental determination of the important physical parameters governing nonproportional response. My research demonstrates the importance of kinetic quenching order on scintillator light yield and proportionality. This work is the first experimental proof that heavier halides, as a material class, exhibit 3rd order quenching, implying free-carrier Auger recombination. In tandem with the experimental determination of relevant physical parameters (carrier mobilities, nonlinear quenching magnitude and order, and the electron track radius), we have calculated electron energy response using a model of diffusion and quenching and Monte Carlo electron cascade simulations using Geant4. This is the first time electron energy response has been calculated using only measured input parameters. Of the critical parameters, all except carrier mobilities were measured for the first time in this work. Calculated electron response for NaI:Tl and SrI2:Eu is shown to be in good agreement with experimental electron response determined from Compton coincidence and K-dip spectroscopy.