Browsing by Author "Mohamed Bourham, Committee Member"

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A Gd Based Gaseous Electron Multiplier Detector for Neutron Scattering Applications
(2008-05-15) Di Julio, Douglas David II; Ayman Hawari, Committee Chair; Pierre Gremaud, Committee Member; Mohamed Bourham, Committee Member; Ronald Berliner, Committee Member
The Gaseous Electron Multiplier (GEM) neutron detector is one of several new concepts that are being designed for the next generation neutron sources. The detector contains multiple modules where each module contains a central neutron converter, several cascaded GEM foils, and readout plates on both sides of the cathode. The device employs a Gd/CsI converter sandwich to convert the neutron to an electric signal. Upon neutron absorption in the Gd layer, conversion electrons are emitted with a probability of 86.5%. Primary electrons travel into the CsI and multiply, inducing the emission of low energy electrons from the converter surface. Several cascaded GEM foils placed on both sides of the converter amplify the signal. The position of the signal is then time stamped and detected by position sensitive anodes and localization electronics. Extensive Monte Carlo simulations, using the PENELOPE code package, have been completed in order to calculate integral and differential characteristics (i. e. the average number of secondary electrons emitted from the converter and the secondary electron escape probability distribution) of various converter thicknesses. The performance of the detector can be assessed by estimating the secondary electron (SE) leakage from the converter foil. Simulations to calculate the SE leakage have been performed for both the single and several multiple module designs. The multiple module design allows for a greater SE leakage while maintaining high detector efficiency and divides the total count rate among several sets of decoding electronics. Tests of several different detector prototypes were completed at the NCSU PULSTAR reactor. Results of the tests of the prototype detectors revealed the essential elements of detector operation. Detector efficiencies, pulse height spectra, and 2-D position spectra have been measured with each of the detector prototypes.
A High Resolution Study of the 52Cr(p,p0) and 52Cr(p,p1) Reactions
(2005-07-05) McLean, Lance; Gary E. Mitchell, Committee Co-Chair; John F. Shriner, Jr., Committee Co-Chair; D. Ronald Tilley, Committee Member; Mohamed Bourham, Committee Member
High resolution measurements of the 52Cr(p,p0) and 52Cr(p,p1) cross sections were performed at the High Resolution Laboratory (HRL) of the Triangle Universities Nuclear Laboratory (TUNL). Results were obtained for the (p,p₀) reaction over the energy range E[subscript p] = 2.1039-3.4711 MeV and for the (p,p₁) reaction over the energy range E[subscript p] = 2.6101-3.4711 MeV. The primary goal was the improvement of the purity and completeness of the level sequences in ⁵³Mn. Excitation functions were generated at five angles for both reaction channels. Fits to the observed cross sections were obtained using the multi-channel, multi-level R-Matrix code, MULTI6. Jπ assignments have been given to 263 resonances. Many resonances were observed for the first time, and many previously observed levels were given new quantum number assignments. Level sequences of specific Jπ were evaluated for purity and completeness using statistical tests based on random matrix theory. Several Jπ assignments were revised after application of the statistical tests. The resulting nearest-neighbor spacing distributions were compared to the Wigner distribution, and the reduced width distributions were compared to the Porter-Thomas distribution. Estimates of the observed fraction of levels for each sequence were obtained using both the spacing and width distributions. Strength functions and level densities were determined for each J[superscript π] sequence. The data for J=Ω allow a test of whether the level density depends on parity; no parity dependence was observed.
Imaging properties of a rotation-free, arrayed-source micro-computed tomography system
(2009-04-22) Quan, Enzhuo; David Lalush, Committee Chair; Caterina Gallippi, Committee Member; Mohamed Bourham, Committee Member; Jianping Lu, Committee Member; Wesley Snyder, Committee Member
We study the three-dimensional reconstructions and imaging properties of a proposed rotation-free micro-computed tomography (CT) system. The system uses linear arrays of the carbon nano-tube (CNT)-based X-ray sources which have ultra-short switch time and are individually addressable. With such sources, the micro-CT system is able to achieve ultrahigh temporal resolution, reduce dose and facilitate gated imaging. A square and a hexagonal geometry have been proposed for the system. In the square geometry, two linear source arrays and two area detectors form a square; whereas in the hexagonal geometry, three linear source arrays and three area detectors form a hexagon. The tomographic angular sampling for both of the geometries requires no motion of the sources or subject. Based on the sinogram maps, the hexagonal geometry has improved angular coverage than the square geometry. The ordered-subset convex iterative algorithm is implemented in both geometries for reconstructions from cone-beam projection data. Mean squared errors at the uniform regions in the reconstructed images are calculated to quantify the artifact level. Point spread functions are examined for point objects located at different axial and transverse positions throughout the FOV. Variance images are generated from 100 reconstructions with simulated Poisson noise and the mean variance are calculated for different regions of interest. The effect of gaps between the source arrays and the detectors is also studied. The reconstructed images from both geometries are generally consistent with the phantom, although some streaking artifacts due to the limited-angle nature of the geometries are observed. The gapfree hexagonal geometry produces lower mean squared error in the reconstructed images, lower FWHM of the point spread functions and lower variance. However, in more realistic situations where gaps appear between the source arrays and the detectors, the angular coverage of the hexagonal geometry degrades faster, resulting in an increase in artifacts, so that the square geometry becomes superior in this case.
The Impact of Input Energy, Fiber Properties, and Forming Wires on the Performance of Hydroentangled Fabrics
(2003-10-29) Zheng, Huabing; Mohamed Bourham, Committee Member; Andrey Kuznetsov, Committee Member; Behnam Pourdeyhimi, Committee Member; Donald Shiffler, Committee Co-Chair; Abdelfattah Seyam, Committee Chair
Extensive critical literature review of the development of hydroentangled technology and research regarding fabric performance in terms of fiber and process parameters was conducted. The review revealed that hydroentanglement is the fastest growing nonwoven bonding technology with an annual growth rate of about 20%. The review also indicated that the research in public domain regarding fabric performance as related to forming wire geometry and fiber properties is not thoroughly covered. The research areas in process and fabric geometry modeling have not been considered by previous researchers. A model describing the force and energy required to form fabric aperture was derived by developing hydroentangled fabric geometry and calculating the energies required to achieve the geometry. Three energy components were considered, namely fiber bending, fiber-to-fiber friction, and fiber stress-strain. The model predicts the three energies in terms of fiber properties and forming wire geometry. Numerical examples illustrating the use of model to calculate the three energies for range of forming wires a fiber are given. The numerical solution shows that the calculated energies from the model that is required to form fabrics is extremely very small as compared to the water jet energy. This indicates that most of the energy is lost. Experimental trials were conducted using different fibers with range of properties, forming wires, and water jet pressure. Fabric tensile strength is used as an indicator of degree of hydroentanglement to assess the fabric performance. The results show that the hydroentangled fabric tensile strength is significantly influenced by forming wire type, fiber properties, and jet pressure. Three force mechanisms (flexural rigidity, friction force, and strain force) were analyzed to reveal which force is more significance in governing fabric strength.
Magnetic Focusing of Ion Beams and Deposition of Hydroxyapatite Films With Graded Crystallinity
(2006-07-27) Thomas, Brent; Jerome Cuomo, Committee Chair; Mohamed Bourham, Committee Member; John Hren, Committee Member
The purpose of the first part of this work has been to use an electromagnet to improve the performance of the Kaufman ion source. A solenoid was fabricated and placed around the substrate platform in a vacuum chamber equipped with a 3cm Kaufman ion source. The source was run under set conditions and the solenoid current was varied. Langmuir probe and Faraday cup measurements were taken under various solenoid currents. SRIM calculations were done to predict the effect of focusing the ion beam on etching performance. The purpose of the second part of this work has been to deposit hydroxyapatite films with graded crystallinity. An ion beam assisted deposition system was used to sputter hydroxyapatie films onto silicon samples. The temperature of the samples was lowered during the deposition in order to obtain samples with graded crystallinity. The samples were analyzed using X-ray diffraction and transmission electron microscopy to verify their crystal structure. The samples had a graded crystal structure with nanocrystalline hydroxyapatite with an orientation of (113) transitioning into amorphous hydroxyapatite.
The Modification of Cellulosic Surface with Fatty Acids via Plasma Mediated Reactions.
(2010-11-04) Nada, Ahmed; Samuel Hudson, Committee Chair; Peter Hauser, Committee Chair; Abdel-fattah Seyam, Committee Chair; Mohamed Bourham, Committee Member
Pulsed DC reactive magnetron sputtering of aluminum nitride thin films
(2003-08-29) Cho, Jung Won; Dennis Maher, Committee Member; Jagdish Narayan, Committee Member; Jerome J. Cuomo, Committee Chair; Mohamed Bourham, Committee Member
Aluminum nitride thin films have been deposited by pulsed DC reactive magnetron sputtering. The pulsed DC power provides arc-free deposition of insulating films. Two types of pulsed DC (unipolar and asymmetric bipolar) were studied with respect to characteristics and properties of resultant films. The unipolar power supply generates a series of 75 kHz DC pulses modulated with 2.5 kHz frequency. The frequency of asymmetric power supply can be varied from 50 kHz to 250 kHz. The duty cycle, which is a ratio of negative pulse time to total time, can be varied from 60% to 98%. Very fast oscillation and overshoot were observed when the polarity of the target voltage was changed. The control of crystal orientation of deposited film is important since the properties of AlN film is related with the orientation. For example, the acoustic velocity is high along the c-axis. The electromechanical coupling coefficient is large in a-axis direction. The crystal orientation and microstructure of the AlN films were strongly affected by the deposition conditions such as sputtering power, growth temperature, sputtering gas pressure and frequency/duty cycle. The crystal orientation of AlN films was closely related with the energy of sputtered atoms and mobility of adatoms on substrate. The c-axis oriented films were obtained when the target power and growth temperature were high. This provided higher energy of sputtered atoms and mobility of adatoms. The deposited AlN films have a columnar structure. The crystal orientation of the AlN films was changed from (101) to (002) by applying an RF bias was applied to the substrate in unipolar pulsed DC sputtering. The columnar structure disappeared when the RF bias was applied to the substrate. Applying bias was thought to increase mobility of adatoms by ion bombardment. MIM (aluminum-AlN-aluminum or molybdenum) structure was fabricated to measure electric properties of AlN films. Dielectric constants of 8.5 to 11.5 were obtained at 100 kHz. The relation of the current density and electric field of the AlN film followed ohmic conduction. The broad photoluminescence spectrum in the range of 300 nm and 650 nm was observed for the aluminum nitride thin films regardless of the deposition conditions. This spectrum might be due to the defects related with oxygen. The thermal conductivity of AlN films was evaluated by the 3omega method. The values in the range of 12 and 30 W/mK was obtained. Those values are very low compared to those of bulk. The reason might be attributed to the oxygen incorporation as well as the unique microstructure of the sputtered films. The plasma parameters such as electron temperature and charge density were obtained by the Lagmuir probe. The electron temperature in argon and nitrogen plasma increased from 2.7 to 5.3 eV when the frequency increased from 75 kHz to 250 kHz in asymmetric bipolar pulsed DC sputtering. The electron and ion densities were found to increase with frequency. The stochastic heating due to the fast oscillation in the target voltage waveform may be attributed to the increased electron temperature and electron/ion densities. The measured plasma characteristics were correlated with properties of the AlN film. The ion and energy flux were believed to increased as the frequency increased. The intensity of the (002) peaks was found to increase with the increase in the ion flux and energy flux.