Time Domain Terahertz Axial Computed Tomography Non Destructive Evaluation

In this Phase 2 project, we propose to develop, construct, and deliver to NASA a computed axial tomography time-domain terahertz (CT TD-THz) non destructive evaluation (NDE) system which will provide true three dimensional images of aerospace polymer, ceramic, and composite structures. Traditional time domain terahertz reflection tomographic imaging captures only a single view of an object, generating images of laminar structure similar to an ultrasound "B-Scan". This reflection tomographic imaging is limited, however, in revealing only the laminar structure which presents a clear specular reflection from each interface. Furthermore, traditional time domain terahertz reflection tomographic imaging has substantial difficulty in determining the layer index of refraction an absorption properties without ambiguity. In Phase 1 we demonstrated the feasibility TD-THz axial computed tomography to generate cross-sectional slices of aerospace materials. This method acquires not one view, but many radial axial views, generating a sinogram which can be used to reconstruct images using a derivative of standard X-Ray CT filtered back-projection. The sinogram can be generated by the transmission absorbance, transmission time of flight, and, in principle, reflection measurements. The reconstructed TD-THz CT images are 3D maps of the absorption coefficients and/or the index of refraction of the subsurface material.