Effect of repeated x-ray exposure on the resolution of amorphous selenium based x-ray imagers

Journal Article · Mon Mar 15 00:00:00 EDT 2010 · Medical Physics

DOI:https://doi.org/10.1118/1.3326947· OSTI ID:22096653

Kabir, M. Z.; Chowdhury, L.; DeCrescenzo, G.; Tousignant, O.; Kasap, S. O.; Rowlands, J. A. ^[1]

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1. Department of Electrical and Computer Engineering, Concordia University, 1455 de Maisonneuve W., Montreal, Quebec H3G 1M8 (Canada)

Purpose: A numerical model and the experimental methods to study the x-ray exposure dependent change in the modulation transfer function (MTF) of amorphous selenium (a-Se) based active matrix flat panel imagers (AMFPIs) are described. The physical mechanisms responsible for the x-ray exposure dependent change in MTF are also investigated. Methods: A numerical model for describing the x-ray exposure dependent MTF of a-Se based AMFPIs has been developed. The x-ray sensitivity and MTF of an a-Se AMFPI have been measured as a function of exposure. The instantaneous electric field and free and trapped carrier distributions in the photoconductor layer are obtained by numerically solving the Poisson's equation, continuity equations, and trapping rate equations using the backward Euler finite difference method. From the trapped carrier distributions, a method for calculating the MTF due to incomplete charge collection is proposed. Results: The model developed in this work and the experimental data show a reasonably good agreement. The model is able to simultaneously predict the dependence of the sensitivity and MTF on accumulated exposure at different applied fields and bias polarities, with the same charge transport parameters that are typical of the particular a-Se photoconductive layer that is used in these AMFPIs. Under negative bias, the MTF actually improves with the accumulated x-ray exposure while the sensitivity decreases. The MTF enhancement with exposure decreases with increasing applied field. Conclusions: The most prevalent processes that control the MTF under negative bias are the recombination of drifting holes with previously trapped electrons (electrons remain in deep traps due to their long release times compared with the time scale of the experiments) and the deep trapping of drifting holes and electrons.

• OSTI ID: 22096653
• Journal Information: Medical Physics, Vol. 37, Issue 3; Other Information: (c) 2010 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
AMORPHOUS STATE
CHARGE COLLECTION
CHARGE TRANSPORT
CONTINUITY EQUATIONS
FINITE DIFFERENCE METHOD
HOLE MOBILITY
HOLES
IMAGES
LAYERS
MODULATION
NUMERICAL ANALYSIS
POISSON EQUATION
REACTION KINETICS
RECOMBINATION
SELENIUM
SENSITIVITY
TRANSFER FUNCTIONS
TRAPPED ELECTRONS
X RADIATION