Detection of defects in foam thermal insulation using lateral migration backscatter x-ray radiography

A new Compton x-ray backscatter imaging technique called lateral migration radiography (LMR) has been successfully applied to the detection of voids and delaminations in the foam thermal insulation used on the shuttle external tank. LMR employs detection of selected scatter field velocity components, by using specially designed detectors and detector collimators, to achieve high image contrast. LMR is based on image contrast generated by migration of probe x-ray radiation in directions transverse to the illumination radiation beam. Because LMR is sensitive to electron density variations in these directions, thin, but large density variations, such as cracks and delaminations, generate signal-to-background ratios sufficient to produce images of features which are not even detectable in the usually interrogated thin dimension. The examined foam thermal insulation test panels consist of aluminum plates onto which the sprayed-on foam insulation (SOFI) is applied. Some of the test panels include structural features bolted to the base plate. The SOFI was layed down over the base plate and structure with a thickness varying from a few tens of mm up to a few hundred mm. The test panels included voids and simulated delaminations in the SOFI ranging in cross-sectional size from 6 x 6 mm to 50 x 50 mm. High quality images were acquired using pixels of 2 to 3 mm and irradiation times as low as 0.05 s per pixel.