WFU Physics Colloquium

TITLE: Coupled rate and transport equations modeling light yield, pulse shape and their proportionality to energy in electron tracks: a study of CsI and CsI:Tl scintillators

SPEAKER: Xinfu Lu

TIME: Monday December 5, 2016 at 2:00 PM

PLACE: Room 107 Olin Physical Laboratory

This dissertation reports on development and testing of a scintillation response model of progressive comprehensiveness that computes emission intensity over time and space in electron tracks by solving coupled rate and transport equations describing both the movement and the linear and nonlinear interactions of the charge carriers deposited along the ionization track. The tracks are initially very narrow before hot and thermalized carrier diffusion takes effect. This suggests that an adequate and computationally manageable representation may be obtained by modeling diffusion in one dimension, the radius. The initial track resulting from Monte Carlo simulations by GEANT4 or NWEGRIM codes is numerically chopped into cells small enough to approximate their ionization density as constant, and these form the individual parts of a finite element model. The initial ionization density values vary from cell to cell along the length of the track with the variation in dE/dx and we calculate the light yield for each local value of dE/dx. This intermediate quantity that we call local light yield as a function of dE/dx cannot itself be directly measured by experiments. The local light yields must be multiplied by the number of times the associated ionization density occurs in repeated Monte Carlo simulations for the given initial electron energy, and then the yields are summed to report the total light yield. When this calculation is carried out over a range of energies the results give the predicted electron energy response or proportionality curve as a function of initial electron energy, for comparison to Compton-coincidence and K-dip experiments. This has been done in the present work for CsI at 295 K and 100K, and for CsI:Tl at 295 K.

Relatively recent experiments on the scintillation response of CsI:Tl have found that there are three main decay times of about 730 ns, 3 us, and 16 us, i.e. one more principal decay component than had been previously reported; that the pulse shape depends on gamma ray energy; and that the proportionality curves of each decay component are different, with the energy dependent light yield of the 16 us component appearing to be anticorrelated with that of the 0.73 us component at room temperature. These observations can be explained by the described model of carrier transport and recombination in a particle track. It takes into account processes of hot and thermalized carrier diffusion, electric field transport, trapping, nonlinear quenching, and radiative recombination.