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This study investigated the mobility and trapping of charges and the recombination of x-ray induced charges in a-Se. X-ray induced changes in these parameters were also examined. Prior to exposure to x-rays, the mobilities and deep trapping lifetimes of both holes and electrons were constant. Exposure to x-rays caused no change in the mobility of these charges. Immediately following exposure, the hole deep trapping lifetime would fall ~30% while the electron deep trapping lifetime would only change ~10%. The deep trapping lifetimes continued to change unpredictably several hours after the initial exposure. Following an extended rest period, the charge lifetimes within the a-Se film would return to a stable state, but not necessarily equal to the initial lifetime prior to the x-ray exposure. These changes were proposed to occur because of a relaxation or reordering of the atoms in a-Se, similar to accepted changes thought to occur resulting from exposure to visible light. Analysis of the experimental evidence suggests that intimate valence alternation pair (IVAP) charged defects are created by x-ray irradiation. These defects are relatively unstable, disappearing within two hours after irradiation. Since the hole and electron lifetimes continued to change for at least 12 hours, it was concluded that the traditional view of deep charge trapping into IVAP defects cannot be dominant. A new charge trapping theory, consistent with published optically induced effects, is proposed in this work to explain these observations. The energy required to create a free electron-hole pair in a-Se by exposure to x-rays was measured. This energy was found to exhibit a strong field and temperature dependence, with little dependence on the mean x-ray beam energy. These findings are consistent with the geminate recombination theory, generally agreed to be the dominant charge loss mechanism with optical photons in a-Se. The persistent x-ray photocurrent was found to be thermally activated below ~250 K with an activation energy of 0.16 eV. This does not correspond to the energy level of any known traps in a-Se, and lends further support to the charge trapping theory developed earlier.

Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 112-119). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Hasler, Paul; Committee Member: Anderson, David; Committee Member: Degertekin, F.; Committee Member: Ghovanloo, Maysam; Committee Member: Minch, Bradley. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis: M.S., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 1979 / Includes bibliographical references. / by Leslie Louis Szepesi, Jr. / M.S. / M.S. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science

Dark current is an unwanted source of noise in images produced by digital imagers, the de facto standard of imaging. The two most common types of digital imager architectures, Charged-Coupled Devices (CCDs) and Complementary Metal-Oxide-Semiconductor (CMOS), are both prone to this noise source. To accurately reflect the information from light signals this noise must be removed. This practice is especially vital for scientific purposes such as in astronomical observations. Presented in this dissertation are characterizations of dark current sources that present complications to the traditional methods of correction. In particular, it is observed that pixels in both CCDs and CMOS image sensors produce dark current that is affected by the presence of pre-illuminating the sensor and that these same pixels produce a nonlinear dark current with respect to exposure time. These two characteristics are not conventionally accounted for as it is assumed that the dark current produced will be unaffected by charge accumulated from either illumination or the dark current itself. Additionally, a model reproducing these dark current characteristics is presented. The model incorporates a moving edge of the depletion region, where charge is accumulated, as well as fixed recombination-generation locations. Recombination-generation sites in the form of heavy metal impurities, or lattice defects, are commonly the source of dark current especially in the highest producing pixels, commonly called "hot pixels." The model predicts that pixels with recombination-generation sites near the edge of an empty depletion region will produce less dark current after accumulation of charge, accurately modeling the behavior observed from empirical sources. Finally, it is shown that activation energy calculations will produce inconsistent results for pixels with the presence of recombination-generation sites near the edge of a moving depletion region. Activation energies, an energy associated with the temperature dependence of dark current, are often calculated to characterize aspects of the dark current including types of impurities and sources of dark current. The model is shown to generate data, including changing activation energy values, that correspond with changing activation energy calculations in those pixels observed to be affected by pre-illumination and that produce inconsistent dark current over long exposure times. Rather than only being a complication to dark current correction, the presence of such pixels, and the model explaining their behavior, presents an opportunity to obtain information, such as the depth of these recombination-generation sites, which will aid in refining manufacturing processes for digital imagers.

Thesis: M.S., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 1980 / Includes bibliographical references. / by Peter Maynard Osterberg. / M.S. / M.S. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science

The term“Digital X-ray Imaging" refers to a variety of technologies that electronically capture x-ray images. Once captured the images may be electronically processed, stored, displayed and communicated. Digital imaging has the potential to overcome weaknesses inherent in traditional screen-film imaging, with high detection efficiency, high dynamic range and the capability for contrast enhancement. Image processing also makes possible innovative techniques such as computer-aided diagnosis, tomosynthesis, dual-energy imaging, and digital subtraction imaging. Several different approaches to digital imaging are being studied, and in some cases, have been developed and are being marketed. Common to all these approaches are a number of technological and medical issues to be resolved. One of the technological issues is the optimal pixel size for any particular image sensor technology. In general, the spatial resolution of the digital image is limited by the pixel size. Unfortunately while reducing pixel size improves spatial resolution this comes at the expense of signal to noise ratio (SNR). In a scintillator-charge-coupled device (CCD) system, the signal can be increased by improving the efficiency of the scintillator or by reducing noise. This study used a very low noise CCD to determine if image quality, as indicated by the modulation transfer function (MTF), the noise power spectrum (NPS) and the detective quantum efficiency (DQE), could be maintained while reducing pixel size. Two scintillators, one a commonly used radiographic screen the other a thallium doped cesium iodide scintillator, were used and the results compared. The results of this study show that image quality can be maintained as pixel size is reduced and that high DQE can be attained and maintained over a wide range of spatial frequencies with a well designed scintillator.