Experimental and computational studies of nonlinear quenching in materials used as radiation detectors

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. To that end, both the magnitude and kinetic order of nonlinear quenching rates have been determined using an above-gap z-scan technique in materials ranging from halides such as CsI:Tl and SrI2:Eu, to oxides and semiconductors. It is shown that the kinetic order of nonlinear quenching has direct consequences on scintillator light yield and proportionality. Furthermore, 2nd order quenching indicates the carrier population is dominated by bound electrons and holes (excitons or self-trapped excitons), and 3rd order quenching implies free carriers. A population of free carriers or bound electrons and holes has consequences beyond the difference in light yield and proportionality that result from pure 2nd or 3rd order quenching. For example, in halides with self-trapped holes, the mobile electrons can move away from the dense core of holes thus escaping nonlinear quenching. Coupled with calculations of hot electron group velocity, we have used this information to explain why NaI:Tl and SrI2:Eu have better proportionality and light yield than oxides and flourides, as well as why SrI2:Eu has better light yield and proportionality than NaI:Tl.;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. 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. This agreement is a confirmation of the validity of the model for predicting scintillator nonproportional response.