Abstract not available.

Quantum dot infrared photodetectors QDIP) have shown promising performance in various applications such as surveillance, night vision and applications in space. QDIPs have many advantages compared to their counterpart quantum well infrared photodetectors QWIP). The advantages of the QDIP include: 1) sensitive to normal-incidence light which simplifies the configurations for imaging systems, 2) longer lifetime of carriers because of the decreased electron-phonon scattering which lead to a high photoconductive gain 3) lower dark current because of the three dimensional confinement of the carriers which enable higher operating temperature. Recent studies have shown that the QD structures and QD-based devices are much more resistant to irradiation than bulk semiconductors or quantum wells, which make QDIPs a promising candidate for space applications. The objective of this work is to design and fabricate quantum dot infrared photodetectors and investigate the influence of the proton irradiation on the fabricated photodetectors. The photodetectors in this work were grown by using molecular beam epitaxy MBE) technique. The final devices were fabricated by standard processing techniques including photolithography, chemical wet etching, evaporative metal deposition, lift-off and rapid thermal annealing. The dark current of the photodetectors was measured at temperatures of 77K. The photoresponse ranging from visible and near infrared NIR) band to middle infrared MIR) band was observed under different temperatures and bias voltages. Then, the photoresponse and I-V characteristics of the devices were investigated as a function of proton fluences in the range of 9.0×1010 — 5.0×1014 cm-2. The intensity of the photoresponse spectra was reduced by about two orders of magnitude after irradiating the device with a fluence of 5.0×1014 cm -2. On the other hand a partial recovery of the photoresponse is observed when the rapid thermal annealings were done on the device.

Problems often arise that require the use of MCNP to simulate a small response change caused by a variation in system parameters. The direct computation approach for such problems often leads to a too large statistical uncertainty for the predicted response change to be acceptable when the response change is small, unless extremely large number of particle histories is used in the MCNP simulations. Consequently, correlated sampling and the differential operator perturbation technique have been developed for MCNP to simulate small response changes with an acceptable uncertainty at reasonable number of particle histories. A thorough investigation is conducted on the theory, performance, and applicability of the two methods in this dissertation research and it consists of three components. First, the performance of correlated sampling is investigated. It is found that under certain conditions using the default output format of MCNP5 and the original practice of batch statistics developed for correlated sampling may overestimate the uncertainty of the change in system response. The cause of the overestimation is analyzed； correspondingly two new improved procedures are proposed for correlated sampling to ensure correct estimation of the uncertainty. The performances of the improved procedures of correlated sampling are also compared with that of direct, uncorrelated computation. Results show that the improved correlated sampling method may yield a standard deviation that is up to one magnitude smaller than that predicted by the direct simulation at the same number of particle histories. Secondly, the performances of improved correlated sampling method and the differential operator perturbation technique of MCNP are compared for different types of fixed source problems. In terms of precision of response changes, it is found that the MCNP perturbation technique significantly outperforms correlated sampling for one type of problems but performs comparably with or even underperforms correlated sampling for the other two types of problems. In terms of accuracy of changes in system response, it is found that the MCNP normal perturbation calculations may produce biased results for two types of problems. The magnitude of the bias is problem-dependent and can be quite significant even when response changes are very small. However, accurate results can be obtained for all the test problems if the MCNP perturbation calculations are done by the midpoint correction technique that takes into account the effect of second order cross-differential terms. Thirdly, the two methods are applied to the pin diversion analysis of PWR spent fuel assemblies, for which the small change in neutron flux needs to be estimated when some of the spent fuel pins are missing or replaced with dummy pins. Since the pin diversion analysis involves variation in particle source, the current MCNP perturbation technique cannot be directly applied to such problems. The correlate sampling method in theory can be applied to the pin diversion problem, but it cannot reduce the uncertainty of response change significantly. Some kind of source treatment must be introduced in order for the two methods to work effectively. Two different source treatment strategies are proposed for the pin diversion analysis in this work. It is demonstrated that when coupled with an appropriate source treatment strategy, the MCNP perturbation technique may yield inaccurate results, but the MCNP correlated sampling method works well for the pin diversion analysis. Key Words: MCNP； Small change； System response； Correlated sampling； Perturbation

Detective quantum efficiency DQE) is widely accepted as the golden rule to objectively evaluate the performance of x-ray imaging systems. It provides a comprehensive characterization of an x-ray imaging system, because it combines several important image-quality-related measurements such as contrast, resolution, and noise, and because it measures the efficiency of the utilization of x-ray in the imaging process. Despite its importance, the current DQE methodology is imperfect in general agreement. The focus of this dissertation is to investigate the DQE methodology for digital x-ray imaging systems, in an effort to clarify some confusing aspects of the current DQE methodology. Through a detailed theoretical derivation of the DQE methodology for digital x-ray imaging, a more clarified understanding of the DQE theory is provided. Besides the re-visited DQE theory, techniques to determine the constituent parts of DQE, including the photon fluence, Modulation Transfer Function MTF), and Noise Power Spectrum NPS) are also discussed in this dissertation. The dissertation is structured as follows. After a brief introduction of the current DQE theory in Chapter 1, the DQE theory for digital x-ray imaging systems is revisited in detail in Chapter 2, with experimental results for the demonstration purpose. In Chapter 3, DQE theory for the magnification radiography is provided, and the theory is supported by experimental results. In Chapter 4, the measurements of x-ray photon fluence and spectral composition are discussed in detail, and uncertainty analysis is conducted to investigate the impact of the calibration uncertainty on the two measurements. In Chapter 5, an innovative alignment procedure that was designed to reduce the error in the spectral measurements and imaging experiments is introduced. MTF measurement techniques are covered in Chapter 6, and NPS measurement techniques are discussed in Chapter 7. As an example application of the DQE methodology, a study about the impact of additive noise on the imaging performance of a CCD based x-ray system is also reported in Chapter 7. In Chapter 8, a DQE analysis on an innovative dual detector x-ray imaging system is detailed, as another example application of DQE. Finally, a summary of this dissertation is provided in Chapter 9.

This project has examined one specific issue facing human explorers at Mars — radiation dose from solar particle events SPEs). While the issue is specific, the work involved in carrying out this study has been truly multidisciplinary. From space physics to modeling the Martian atmosphere to assessing radiation risk to humans, a lot of ground is covered. At each point along the way, efforts were made to clarify the science and assumptions used to derive quantitative estimates and draw meaningful conclusions. The data and analyses provided in this project represent the most comprehensive study of SPEs at a planetary body other than Earth. The analysis includes dose estimates from galactic cosmic radiation. The specific aims of this project are to: 1) Establish a methodology for estimating the fluence of energetic particles at Mars from SPEs； 2) Establish a predictive model to estimate the frequency of SPE occurrence and cumulative SPE fluence received at Mars, as a function of the Solar Cycle； 3) Develop a model to estimate the fluence of energetic particles from SPEs on the Martian surface； 4) Estimate and assess the radiation dose on the Martian surface from SPEs, as compared to galactic cosmic radiation GCR)； and 5) Assess the viability of various types of shielding and mission profiles to mitigate the radiation risk to human explorers on the Martian surface. Some of the conclusions drawn from this study include: 1) The MCP detector onboard MGS ER and presumably other spacecraft) can serve as an effective detector for high-energy ionizing radiation. 2) Though more work is necessary, it is possible to estimate future solar activity at Earth and Mars based on current solar observations. 3) Spectral information from SPEs at Mars is required to thoroughly assess the radiation environment on the Martian surface. 4) An unshielded astronaut on the Martian surface may face long-term and acute radiation risk from SPEs, depending on the likelihood of their occurrence. 5) Various mitigation strategies can be employed to minimize the radiation risk to human explorers on Mars, though shielding is limited for astronauts outside of a fixed habitat.

This dissertation explores the radiomodulatory affects of the Nrf2/Antioxidant Response Element Nrf2/ARE) pathway. Exposure to ionizing radiation produces a large amount of reactive oxygen species ROS) that are a major source of DNA damage. Activation of the Nrf2/ARE pathway increases production of antioxidant enzymes that might be expected to be cytoprotective. However, initial studies with several Nrf2-inducing agents found treatment prior to ionizing radiation failed to protect lymphocytes against radiation-induced apoptosis and several other cell types against cell death in longer term, clonogenic assays. Nrf2-deficient mouse models were then used to test the impact of the Nrf2/ARE pathway on intrinsic cellular radiosensitivity. Mouse embryo fibroblasts with loss of Nrf2 had decreased expression of heme oxygenase 1 HO-1) protein expression and glutathione activity, as expected, and after exposure to ionizing radiation dramatically increased ROS formation relative to wild type cells. Although no differences in initial DNA double strand breaks was observed, Nrf2 deficient cells were more radiosensitive under both aerobic and hypoxic conditions. Furthermore, Nrf2-deficient mice were sensitive to whole body irradiation with doses that were sublethal for wild type and heterozygous mice, supporting a radioprotective role of the Nrf2/ARE pathway. Studies finally focused on the ability of repeated damage to activate the Nrf2/ARE pathway over time, using a gene reporter system. No change was observed 1 to 24 hours after single dose radiation exposure, but by day 5 there was a dose-dependent increase in ARE-reporter activity above 2Gy. This correlated with expression of HO-1 in several cell lines. Similar results were obtained with five daily fractions of I to 4Gy given to cells in vitro and in spleens from irradiated mice. These data suggest that ARE-activation of cytoprotective enzymes is due to persistent oxidative stress after ionizing radiation exposure. These studies are the first to report on the impact of the Nrf2/ARE pathway on intrinsic cellular radiosensitivity and activation by ionizing radiation.

Management of lung tumor motion is a challenging and important problem for modern, highly conformal radiotherapy. Poorly managed tumor motion can lead to imaging artifacts, poor target coverage, and unnecessarily high dose to normal tissues. The goals of this dissertation are to develop a real-time localization algorithm that is applicable to rotational cone-beam projections acquired during regular &sim；60 seconds) cone-beam computed tomography CBCT) scans, and to use these tracking results to reconstruct a tumors trajectory, shape and size immediately prior to treatment. Direct tumor tracking is performed via a multiple template matching algorithm where templates are derived from digitally reconstructed radiographs DRRs) generated from four-dimensional computed tomography 4DCT). Three-dimensional 3D) tumor trajectories are reconstructed by binning twodimensional 2D) tracking results according to their corresponding respiratory phases. Within each phase bin a point is calculated approximating the 3D tumor position, resulting in a 3D phase-binned trajectory. These 3D trajectories are used to construct motion blurring functions which are in turn used to remove motion blurring artifacts from reconstructed CBCT volumes with a deconvolution algorithm. Finally, the initial direct tracking algorithm is combined with diaphragm-based tracking to develop a more robust “combined” tracking algorithm. Respiratory motion phantoms digital and physical), and example patient cases were used to test each technique. Direct tumor tracking performed well for both phantom cases, with sub-millimeter root mean square error e rms) in the axial and tangential imager dimensions. In patient studies the algorithm performed well for many angles, but exhibited large errors for some projections. Accurate 3D trajectories were successfully reconstructed for patients and phantoms. Errors in reconstructed trajectories were smaller than the errors in the direct tracking results in all cases. The deblurring algorithm performed excellently in phantom studies. Deblurring was also effective on an example patient case, though the benefits were less stark. Finally, the combined tracking algorithm performed equally to or better than direct tumor tracking in the phantom and patient cases examined. While the preliminary results for each technique are promising, the algorithms must be tested on a larger data set with well defined ground truth to investigate potential clinical applications.

Abstract not available.

Development and performance of the NSTAR “Neutron Sandwich Transmuter/Activation-gamma Radiator”) are discussed, a neutron detector based on a new approach to Gd-loaded scintillators. This detector has high detection efficiency for neutrons, from thermal up to multi-MeV energies with practically zero energy threshold. The NSTAR operating principle is similar to that of Gd-loaded liquid scintillation detectors but avoids many of their disadvantages and hazards. The NSTAR scintillator has the dual function, both to thermalize fast neutrons and to generate responses to the dissipated neutron energy and the emission of associated delayed Gd neutron capture gamma-rays. Consequently, the NSTAR features a time dependent two-component response to neutrons, which consists of a prompt, energy dependent light flash followed by a delayed, energy independent signal. This characteristic response allows one to “tag” neutrons, distinguish them from gamma-rays, and to obtain neutron multiplicity information for multiple-neutron bursts. The detector modules consist of stacks of plastic scintillator slabs Saint Gobain BC-408) alternating with thin Gd-loaded 0.5 wt.%) converter films PDMS-SYLGARD 184). The stacks are viewed by fast photomultipliers Philips XP2041) on one or both ends. The detector design combines high light output collection efficiency with large active volume. The NSTAR modules have been tested with neutrons produced by radioactive sources and a pulsed-beam neutron generator. Tests reveal an effective discrimination against gamma-rays, even in a high intensity background environment, when measurements are made relative to a reference signal. The NSTAR is capable of counting neutrons at rates of R &le； 7 x 104 n/s with losses below 1% and can measure event by event two moments of the neutron multiplicity distribution. A detection efficiency of epsilon ＝ 26 ＋/- 3)% was measured for DD-neutrons at an electronic threshold of Eth ＝ 0.2 MeVee. The average neutron capture time, a parameter determining the detector speed, turned out to be &lt；tc＞ ＝ 21.7 ＋/- 0.2)micros. The experimental values are in good agreement with theoretical estimates based on simulation calculations obtained with a modified version of a neutron transport code DENISE)).