Exploring Fast Neutron Computed Tomography For Non Destructive Evaluation Of Additive Manufactured Parts PDF Download

Unlike X-Rays that interact strongly with the electron cloud surrounding atoms, neutrons interact very weakly with these electrons, but quite strongly with the nucleus of the atom. X-Rays follow a nearly linear trend of decreasing transmission as the atomic number of the element increases. Neutrons follow no such behavior and certain isotopes of elements such as lithium, boron, cadmium, and gadolinium interact very strongly with neutrons, unlike dense elements such as lead or tungsten. This gives neutrons a unique probing capability where X-Rays do not suffice. Neutron imaging is widely used in the commercial sector for munitions and critical aerospace component inspection as well as in the research realm for biological forensics, cultural heritage providence studies, and capturing repetitive processes such as cyclical fluid motion in rotating motors. Current neutron sources are typically nuclear reactors and spallation sources that provide incredibly high neutron yields and flux for the images, leading to reduced image times and/or high spatial resolution as well as new imaging techniques such as computed tomography, polarized neutron imaging, and phase contrast imaging. However, such sources are very expensive, require thorough regulatory oversight, and pose biological hazards from spent nuclear materials. Smaller sources such as sealed tube deuterium-deuterium or deuterium-tritium sources can allow for in-field inspection at a fraction of the cost of a reactor, but don't necessarily provide fast image acquisitions or adequate image quality. This thesis explores a system that is intended to fill the space between weak portable devices and strong, costly, immobile ones. An accelerator-based neutron generator has been designed and manufactured by Phoenix LLC in Madison, WI. This system operates on the principle of deuterium-deuterium fusion, which releases a free 2.45MeV (nominal) neutron in the fusion process. The system can be installed and commissioned in different locations throughout its lifetime and provides for in-field or commercial inspection of components. It has a neutron yield several orders of magnitude higher than sealed-tube neutron sources. The goal herein was to: enhance the performance of the neutron generator through careful beamline design from the source to the target to obtain the maximum neutron yield, optimize the geometry of the neutron moderator and collimator to maximize the imaging metrics of a thermalized and collimated neutron beam free of background gamma and fast neutron radiation contamination, and find efficient neutron detector solutions that can capture high-quality neutron images. Monte Carlo simulations were extensively employed for these optimization efforts. The system has been shown to produce high quality neutron radiographs in terms of contrast, noise, and resolution, using many different techniques for imaging. Many materials were explored for moderation and shielding, and many neutron detectors were researched and experimented with to demonstrate advantages and disadvantages for their use on the Phoenix neutron imaging system. Sample images will be shown and discussed including conventional radiography using medical, industrial, and photographic film, 2-dimension digital imaging using pixelated cameras such as Charge Coupled Devices (CCD), Complementary Metal Oxide Semiconductors (CMOS), amorphous Silicon (aSi), and Computed Radiography (CR). Neutron Computed Tomography (nCT) is demonstrated as a 3-dimensional imaging technique as well. Case studies will be discussed and options for further optimization for collimator, moderator, and detector designs will be outlined. Recommendations for future research will be discussed in the final chapter.