Light-induced reorientation and birefringence in polymeric dispersions of nano-sized crystals

Nanocrystals (50-250 nm) of a Palladium complex within a polyisobutylmethacrylate matrix were prepared by a phase separation method. In these dispersions, a light-induced birefringence with Deltan approximately 10(-3) was induced, without the application of an electric field. This effect was related to the photoconducting properties of the dispersion. Evidence for charge photogeneration without any applied field was obtained. The photorefractive behaviour of the material confirmed that the nanocrystals reorientation is a consequence of photoconducting properties. A light-generated electric field approximaely E 3 V/microm was estimated. These results illustrate the potential of materials with a nano-crystalline dispersion morphology as light-responsive media

Photoconductivity and phototropy in non-crystalline solids

Photoconductivity and phototropy in noncrystalline solid

Well-posedness for weak and strong solutions of non-homogeneous initial boundary value problems for fractional diffusion equations

We study the well-posedness for initial boundary value problems associated with time fractional diffusion equations with non-homogenous boundary and initial values. We consider both weak and strong solutions for the problems. For weak solutions, we introduce a new definition of solutions which allows to prove the existence of solution to the initial boundary value problems with non-zero initial and boundary values and non-homogeneous terms lying in some arbitrary negative-order Sobolev spaces. For strong solutions, we introduce an optimal compatibility condition and prove the existence of the solutions. We introduce also some sharp conditions guaranteeing the existence of solutions with more regularity in time and space

Design and Characterization of an 8x8 Lateral Detector Array for Digital X-Ray Imaging

X-ray imaging has become one of the most pervasive and effective means of diagnosis in medical clinics today. As more imaging systems transition to digital modes of capture and storage, new applications of x-ray imaging, such as tomosynthesis, become feasible. These new imaging modalities have the potential to expose patients to large amounts of radiation so the necessity to use sensitive imagers that reduce dose and increase contrast is essential. An experimental design that utilizes laterally oriented detectors and amorphous semiconductors on crystalline silicon substrates has been undertaken in this study. Emphasis on fabricating a device suitable for medical x-ray imaging is the key principle throughout the design process. This study investigates the feasibility and efficiency of a new type of x-ray imager that combines the high speed, low noise, and potential complexity of CMOS circuit design with the high responsivity, large area uniformity, and flexibility of amorphous semiconductors. Results show that the design tradeoffs made in order to create a low cost, high fill factor, and high speed imager are realistic. The device exhibits good responsively to optical light, possesses a sufficient capacitive well, and maintains CMOS characteristics. This study demonstrates that with sufficient optimization it may be possible to design and deploy real time x-ray system on chip imagers similar to those used in optical imaging

Global uniqueness in an inverse problem for time fractional diffusion equations

International audienceGiven $(M,g)$, a compact connected Riemannian manifold of dimension $d \geq 2$, with boundary $\partial M$, we consider an initial boundary value problem for a fractional diffusion equation on $(0,T) \times M$, $T>0$, with time-fractional Caputo derivative of order $\alpha \in (0,1) \cup (1,2)$. We prove uniqueness in the inverse problem of determining the smooth manifold $(M,g)$ (up to an isometry), and various time-independent smooth coefficients appearing in this equation, from measurements of the solution on a subset of $\partial M$ at fixed time. In the ``flat" case where $M$ is a compact subset of $\mathbb R^d$, two out the three coefficients $\rho$ (weight), $a$ (conductivity) and $q$ (potential) appearing in the equation $\rho \partial_t^\alpha u-\textrm{div}(a \nabla u)+ q u=0$ on $(0,T)\times \Omega$ are recovered simultaneously

Spectral stability analysis of the Dirichlet-to-Neumann map for fractional diffusion equations with a reaction coefficient

This paper focused on the stability analysis of the Dirichlet-to-Neumann (DN) map for the fractional diffusion equation with a reaction coefficient $q$. The main result provided a Hölder-type stability estimate for the map, which was formulated in terms of the Dirichlet eigenvalues and normal derivatives of eigenfunctions of the operator $A_q : = -\Delta + q$

Dark Current modeling and characterization of amorphous lead oxide-based x-ray photoconductive devices for applications in medical imaging

High atomic number (Z) polycrystalline and amorphous photoconductors are currently being investigated to extend direct conversion X-ray detectors to real-time and high-energy lowdose applications. Amorphous lead oxide (a-PbO) is one of the most promising photoconductor candidates because of its negligible signal lag and high theoretical X-ray conversion efficiency. However, a-PbO layers are still experimental; PbO technology has been developed to the point where material science and engineering approaches must be applied to make a-PbO detector prototypes suitable for low-dose X-ray imaging. This includes determining the most appropriate a-PbO multilayer detector structures with specially designed blocking layers that will withstand the high electric fields needed for efficient (i.e., complete) collection of X-ray generated charge while maintaining an acceptable dark current (DC) level. DC is a source of noise in the detector structure that degrades the signal-to-noise ratio (SNR) of the detector system in low-exposure applications. Here we investigate the use of polyimide (PI) as a hole-blocking layer. PI blocking layers were proven successful in the only commercially used direct conversion detectors, which are based on layers of photoconductive amorphous selenium (a-Se). Experimentally, PI was shown to have the most suitable electrical and physical properties for our a-PbO technology. In addition, PI has a straightforward application process of spin coating. Therefore, PI was chosen as a hole blocking layer to decrease DC to tolerable levels in an a-PbO-based detector. [...

Modeling and Characterization of X-ray Image Detectors

The flat-panel image detectors capture an X-ray image electronically, and enable a smooth clinical transition to digital radiography by replacing traditional film/cassette based system. They provide excellent X-ray images and have been commercialized for different X-ray imaging modalities. However, there still remain significant scientific challenges in these detectors associated with dark current and ghosting which constitute critical performance requirements for modalities such as digital fluoroscopy. This doctoral dissertation involves both experimental characterization and physics-based theoretical modelling of time and bias dependent dark current behaviour and X-ray induced change in sensitivity (ghosting) in X-ray imaging detectors. The theoretical investigations are based on the physics of the individual phenomenon and a systematic solution of physical equations in the photoconductor layer; (i) semiconductor continuity equations (ii) Poisson’s equation, and (iii) trapping rate equations. The theoretical model has been validated with the measured and published experimental results. The developed dark current model has been applied to a-Se and poly-HgI2 based detectors (direct conversion detectors), and a-Si:H p-i-n photodiode (indirect conversion detectors). The validation of the model with the experimental results determines the physical mechanisms responsible for the dark current in X-ray imaging detectors. The dark current analysis also unveils the important material parameters such as trap center concentrations in the blocking layers, trap depths, and effective barrier heights for injecting carriers. The analysis is important for optimization of the dark current consistent with having good transport properties which can ultimately improve the dynamic range of the detector. The physical mechanisms of sensitivity reduction (ghosting) and its recovery has been investigated by exposing a-Se detector at high dose and then monitoring the recovery process under (i) resting the samples (natural recovery), (ii) reversing the bias polarity and magnitude, and (iii) shining light. The continuous monitoring of the sensitivity as a function of exposure and time reveals the ghosting mechanisms in a-Se mammography detectors. This research finds a faster sensitivity recovery by reversing the bias during the natural recovery process. The sensitivity recovery mechanisms (e.g., recombination between trapped and oppositely charged free carrier, trapping of oppositely charged free carriers, or relaxation of trap centers) have been qualitatively investigated by validating the simulation results with the experimental data. The ghost removal mechanisms and techniques are important to improve the image quality which can ultimately lead to the reduction of the patient exposure consistent with better diagnosis for different X-ray imaging modalities

Design, Fabrication and Characterization of a Unipolar Charge Sensing Amorphous Selenium X-ray Detector

Amorphous Selenium is a direct conversion photoconductor that has been widely used in X-ray imaging applications. Due to its high spatial resolution A-Se plays an important role in breast cancer screening and diagnostics, allowing for the detection of small and subtle lesions. However, a-Se has poor collection efficiency due to low carrier mobility and charge trapping resulting from its amorphous structure. The trapped charges can cause memory artifacts, including photocurrent lag, which can persist for several seconds after the X-ray pulse has ended. As a result, a-Se is a challenging material for dynamic imaging applications that require high spatial resolution. The research discussed in this thesis aims to investigate and address the temporal behavior of a-Se photoconductors, specifically the issue of lag, which can lead to image artifacts and degradation of image quality in dynamic imaging applications. The research involves the design of unipolar charge sensing detectors with pixel sizes of 20, 40, 80 and 150 microns to improve energy resolution and the temporal response compared to conventional a-Se detectors. Theoretical analysis and simulations are presented for the unipolar charge sensing detector including weighting potential, charge collection efficiency, pulse height spectroscopy and energy resolution which range from 5% to 2% . The work further discusses the fabrication process of the designed detector in the G2N lab at the university of Waterloo. It discusses the experimental results obtained and the challenges that were faced while fabricating the detector and how they can be overcome in the futur