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A new Compton x-ray backscatter imaging technique, backscatter radiography by selective detection (RSD), has been used for inspection of the spray-on-foam-insulation (SOFI) on the space shuttle external tank. RSD employs detection of selected backscatter field components, by using specially designed detectors with movable detector collimators, to achieve high image contrast. The optimization study utilized test panels with simulated and natural defects in the spray-on foam insulation. Some of the test panels include structural features, stiffener-stringers and connection flanges, which were bolted to an aluminum base plate representative of the external tank. The SOFI was then layed down over the base plate and structural components with thicknesses varying from a few tens of mm up to a few hundred mm. The simulated defects range in cross-sectional size from 6 × 6 mm to 50 × 50 mm. Natural defects including roll-over voids and knit-line delaminations have a wide range of sizes, geometries, and orientations with a minimum critical cross-sectional size of 6 mm. Imaging registration is currently obtained at 0.05 seconds per 2 mm pixel, or about 19 minutes per 0.093 m2(1 ft2). The current system is being evaluated to enhance the detection of natural defects of a minimal critical size. Monte Carlo (MC) simulations with MCNP5 are being used to determine the history and corresponding spectrum of the detected photons that are responsible for improving defect image contrast. The simulation results are used in combination with experimental data to select optimal detector configurations. Detector configurations are sensitive not only to the type of defect being detected, but also the defect's depth in SOFI, distance from aluminum substrate, and defect orientation. Additional parameters including detector type, detection mode, and x-ray illumination beam size were also evaluated. Both NaI and plastic (BC404) scintillation detectors in pulse and integral mode were used to determine their effect on image quality and defect detection sensitivity. The x-ray illumination beam geometry (round versus square) and beam spot size were varied to determine resolution and the effect on defect contrast. The current system using pulse mode NaI detectors, and a 2 mm round x-ray illumination beam can detect the presence of the smallest critical size defects at a scan rate of 0.05 seconds per 2 mm pixel.
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