Browsing by Author "Dr. Ayman I. Hawari, Committee Chair"

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Design and Implementation of a Digital Positron Annihilation Lifetime Spectrometer for Measurements in Graphite
(2009-05-19) Bodnaruk, Ethan Wesley; Dr. Ayman I. Hawari, Committee Chair; Dr. Man-Sung Yim, Committee Member; Dr. John Blondin, Committee Member
A digital Positron Annihilation Lifetime Spectrometer was designed and used to perform measurements on three graphite materials, reactor grade graphite, pyrolytic graphite, and a foam graphite developed by Oak Ridge National Laboratory. Positrons are a useful probe of the microstructural features of matter since they are attracted to open-volume pores and defects where the electron density is lower than in other parts of the material. Various types of graphite were studied because of their importance in nuclear technology, including as a moderator in nuclear reactor cores. A lifetime spectrometer consists of a scintillation detector, photomultiplier tube, and equipment to perform timing analysis on the detected radiation. This equipment can either consist of analog pulse shaping and timing electronics or a system that digitizes and processes the radiation pulses. A new type of scintillation material, Lanthanum Bromide [LaBr3(Ce)], was tested and compared to the scintillator usually used for lifetime experiments, Barium Fluoride. The Lanthanum Bromide was expected to perform somewhat better than BaF2 based on its scintillation properties, and this was confirmed. The digital system was tested and its performance optimized. The digital lifetime spectrometer shared some similar equipment with a standard analog spectrometer and as such, both could be used simultaneously to take measurements. The digital spectrometer showed improvement in its timing resolution over the analog system. The measurements on graphite were more conclusive for the digital system than for the analog system, as results from the former matched published data well and were more consistent in general. This was due to the greater flexibility in timing methods and opportunity for optimization afforded by the digital system. Measurements on the graphites supported other work in the literature showing a lifetime of approximately 200 ps in the reactor grade and pyrolytic graphites with a second lifetime on the order of 410 to 425 ps. In the reactor grade graphite the second lifetime was about 410 ps while for the pyrolytic graphite it was about 425 ps. The second lifetime is higher in the pyrolytic graphite and it is attributed to its greater disorder. The 200 ps lifetime is explained as the lifetime of positrons in a perfect graphite crystalline structure while the 400 ps lifetime is explained as the lifetime of positrons at grain boundaries or defects between different regions in the graphite. Data fits involving a third lifetime for the materials were not satisfactory in terms of fitting statistics or results related to physical phenomena. Measurements on the foam graphite yielded two lifetimes of approximately 125 ps and 340 ps. The first value matches the lifetime of para-positronium, a bound state of an electron and positron known to form in porous materials. The theoretical lifetime of this type of positronium is 129.3 ps. The 340 ps lifetime is most likely the result of positron annihilation in the graphite structure of the foam reflecting positron annihilation in both the perfect crystal structure of the graphite and in grain boundaries. This lifetime is close to the 334 ps mean lifetime found in the reactor grade graphite. Further work and improvements in the experimental technique and equipment could provide more insight into the measurements on graphite.
Development of a Thermal Neutron Imaging Facility at the N.C. State University PULSTAR reactor
(2005-10-11) Mishra, Kaushal Kishor; Dr. Man-Sung Yim, Committee Member; Dr. Bibhuti Bhattacharyya, Committee Member; Dr. Ayman I. Hawari, Committee Chair
A Thermal Neutron Imaging facility is being set up at the PULSTAR reactor at North Carolina State University. The PULSTAR is an open pool type light water moderated research reactor with a full power of 1-MWth and fuel that is enriched to 4% in U-235. It is equipped with 6 Beam Tubes (BT) to extract the radiation out of the reactor core. BT #5 is being used for the neutron imaging facility. Neutron imaging has expanded rapidly as a means of Non-Destructive Testing of materials. It offers some very explicit advantages over the usual γ-ray (or x-ray) imaging. Neutron cross-sections, being almost independent of the atomic number (Z) of the material, result in neutron imaging being capable of discerning materials of similar Z and/or low Z materials even when they are present inside high Z surroundings. Also, hydrogen, which is a very important element in determining the properties of materials, can be imaged even if present in minute quantities due to its significant neutron scattering and absorption cross-sections. Neutrons also offer the advantage of being capable to differentiate between isotopes of an element. Furthermore, radioactive materials which cannot be imaged using photons due to fogging of the detector can be imaged with neutrons using the transfer technique. The facility at the PULSTAR is intended to have both radiographic and tomographic capabilities. The radiography capabilities include using conventional film, digital image plate systems, as well as a real-time radiography system. In the present work the design of the facility is being presented. The collimator constitutes the major part of the imaging facility. The collimator design and its performance were simulated using MCNP. The designed collimator has a poly-crystal bismuth filter that is 4-inches in length, and a single crystal sapphire filter that is 6-inches in length. To aid in the design process, the bismuth and sapphire thermal neutron scattering cross-sections were calculated and implemented as libraries that can be used in MCNP calculations. The L/D of the system ranges from 100 to 150. The filter length can be changed to vary the estimated neutron flux from 1.8x10⁶ to 7x10⁶ n/cm².sec at full power with a sub-cadmium neutron content >98% as estimated by the MCNP simulations. Using the designed collimator, the beam divergence angle is 2° which translates to a beam size of 35-cm at 6-m from the aperture. Radiography and tomography simulations were also performed using MCNP and the effect of scattering was observed in the image. In addition, the Point Spread Function (PSF) for different detection systems was simulated and the corresponding resolution defined by the FWHM for film, image plate and real time detection systems was obtained and found to be between 33 to 50-μm, 106 to 118-μm and 113 to 118-μm respectively. The results obtained were in good agreement with the measurement performed using a 25-μm thick gadolinium foil. The designed beam was evaluated using the standards of the American Society of Testing and Materials (ASTM) and it was found that the designed beam achieves quality I[superscript A] ranking. Initial radiographs using the facility have been taken and are presented. The real-time radiography and tomography system will be setup in the near future.