Edward T. Dugan

Optimization of an RSD x-ray backscatter system for detecting defects in the space shuttle external tank thermal foam insulation

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 defects 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.

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.

Development and field testing of a mobile backscatter x-ray lateral migration radiography land mine detection system

Lateral migration radiography (LMR), a new form of Compton backscatter x-ray imaging, is applied to the detection and identification of buried land mines. A mobile LMR land mine detection system was developed and field tested. Weight for this initial system was about 175 kg; weight for a prototype should be about 100 kg. X-ray generator power level was 750 watts; the power level requirement for a prototype should be about 300 watts. An innovative rotating collimator for the x-ray source beam was developed to provide rapid side-to-side scanning of the beam without having to move the x-ray generator in this direction. Acquisition of images of a 40 cm by 40 cm area takes from 30 to 60 seconds, depending on the desired resolution. The imaging capabilities of LMR make it well suited for use as a land mine detection confirmation sensor. This system was employed on the vehicular test lanes at Fort A.P. Hill in October, 2001. High quality images were obtained for a variety of buried land mines. The system was also used to scan 30 locations on one of the test lanes where GPR consistently yielded false alarms. In only two cases did the LMR image sets yield a signature that could be considered to possibly indicate a mine.

Lateral migration radiography: a new x-ray backscatter imaging technique

A new Compton backscatter imaging (CBI) technique, described as lateral migration radiography (LMR), has been developed and applied successfully to two difficult diagnostic problems: Detection of buried, plastic landmines, and detection of material flaws which lie close to, and parallel to, a surface, the method is based on image contrast generated by alteration of photon lateral migration relative to the illuminating beam direction. It is extraordinarily sensitive to density and/or atomic number variation along the photon lateral-direction travel paths. In LMR, relevant information-carrying photon detection efficiencies are two to three orders-of-magnitude greater than other CBI techniques such that the electric energy requirement for x-ray generation is only about one joule per acquired image pixel. The resulting small product of pixel illumination dwell time and x-ray generator electric power implies that current, easily accessible technology can be used to fabricate LMR systems with practical usage protocols. Three have been designed and built at the University of Florida: A laboratory device for perfecting buried landmine acquisition; a mobile system for field-demonstrating landmine detection; and, a laboratory system for detection of material defects in small structural parts. The LMR images, acquired in a laboratory landmine detection setting, are so definitive that identification of the mine-type, as well as presence, can be often accomplished. Results of a field test are near-perfect, both in determining buried landmine presence and in lack of false positive response. Images acquired in material flaw detection indicate ability to detect lateral cracks or delaminations with thickness less than 100 microns, as well as corrosion on surfaces between layers of structural sheets. These applications provide evidence of the viability of a new, one-sided x-ray radiography technique which images hidden structures of objects which have here-to-fore been difficult, or impossible, to detect with practical image aquisition times.

Computational Methods for Shape Restoration of Buried Objects in Compton Backscatter Imaging

Image restoration techniques are studied for Compton backscatter imaging as applied to identification of a land mine buried in soil. Mathematical methods are developed to restore images, which include artifacts due to photon noise, soil surface irregularity, and vertical motion of the imaging system. The image restoration is formulated as an inverse photon transport problem. The forward photon transport is modeled by using a two-collision response function. The inverse problem then is solved by applying an iterative minimization algorithm, resulting in an estimation of characteristic parameters of objects. Mathematical relations among detector responses are derived by experimentally analyzing the detector response characteristics when there are soil surface irregularity and vertical motion of the imaging system. These are used to remove the artifacts from the images. The method successfully restores the geometrical feature of the object under simulated battlefield imaging conditions.

ThO2–UO2 annular pins for high burnup fuels

Abstract The main purpose of this work is to investigate the use of annular fuel pins (particularly pins containing thorium dioxide) for high burnup fuel. The following parameters were evaluated and compared between postulated mixed thorium–uranium dioxide standard and annular (9% void fraction) type fuel assemblies, as a function of burnup: the infinite multiplication factor, the uranium and plutonium isotopic compositions, the fuel temperature coefficient of reactivity and the conversion ratio. We used the SCALE-4.3 code system. The calculation method consisted in obtaining actinide and fission product number densities as functions of assembly burnup, by means of a 1-D transport calculation combined with a 0-D burnup calculation. These number densities were then used in a 3-D Monte Carlo code for obtaining k∞ from two-dimensional-symmetry “snapshots”.

Lateral migration radiography application to land mine detection, confirmation and classification

Lateral migration radiography (LMR) employs scattered photons to acquire detailed images of covered objects. images of plastic encased real mines buried in soil using LMR have shown dramatic differences compared to images generated using simulated mines. The major characteristic that enables the discernibility of land mines to the degree of actual type identification is the presence of voids (air volumes) required for the operation of the fuse assembly or for blast direction control. Air volumes greatly modify the detected field of both once and multiple-scattered photons. The LMR system consists of an x-ray generator and two uncollimated detectors positioned to detect once- scattered photons and two collimated detectors designed to detect primarily multiple-scattered photons. The x-ray generator is located in the gap between symmetrically arranged detectors; the collimated x-ray beam typically has a spot size of 1.5x 1.5 cm with perpendicular incidence on the soil surface. The optimal x-ray spectra for land mine detection with the LMR system range from 130 to 180 kVp with mean x-ray energies of from around 40 to 60 keV. Air volumes modify both exit paths and the position of first-scatter events; they also modify the migration paths of multiple-scattered photons, thus producing different images in the two detector types. The burial mode (below surface or laid on the surface) of the land mine can also be discerned by LMR due to a shadowing effect seen for surfaced-laid land mines. The presence of even a minute amount of metal in the land mine also aids in discerning the mine, because metal produces a signal decrease in both types of detectors. Monte Carlo calculations are performed with the MCNp code to obtain an understanding of the details of the photon lateral migration process. Images generated from these Monte Carlo calculations are in agreement with the experimental measurements. The real mine images confirm that LMR is capable not only of mine detection, but also of mine identification.

Detection of flaws and defects using lateral migration x-ray radiography

A new Compton X-ray backscatter imaging (CBI) technique called lateral migration radiography (LMR) is applied to detecting a class of sub-surface defects in materials and structures of industrial importance. Examples are delamination in layered composite structures, defects in deposited coatings on metal surfaces such as in aircraft jet engine components and geometrical structural/composition changes (e.g. due to corrosion)on the inside of shell-like components with only outside surface area access. LMR scans on aircraft samples showed intensity decreases of up to 25% in corroded areas relative to intensities in clean areas. Especially significant were scans of samples that were performed with the clean or uncorroded side facing up. The corrosion on the opposite side of these 2 mm thick samples, where there was contact between the frame member and the aircraft skin, was clearly visible. Scans of other samples showed that LMR is capable of detecting small flaws on the inside of shell-like components with only outside surface area access. Cracks around a fastener hole that were ~ 15 mm in length and no more than 0.25 mm in width were seen through the aircraft skin. Scans of an aluminum honeycomb structure demonstrated that LMR is also capable of picking up internal defects that include crushed core and debonding zones.

Analytical studies of a backscatter x-ray imaging landmine detection system

The Compton Backscatter Imaging (CBI) technique has been applied successfully to detect buried plastic anti-tank landmines. The images acquired by a CBI system are often cluttered by surface features. Additionally, some buried objects give the same response as the plastic landmines. The landmine detection can be successful only when the detection system is capable of distinguishing between surface features and the mine-like objects. This can be accomplished by designing detectors that differentiate between the surface features and the buried objects. An understanding of the physical phenomena underlining the CB image formation helps us to design these detectors. To study the physics of the Compton backscattering, the photon transport in a CBI system is simulated using Monte-Carlo calculations with the generalized particle transport program MCNP. The photon tracks are graphically displayed using a visualization program SABRINA. On the basis of the results from these Monte-Carlo analyses, a four-detector system has been designed. This detector design utilizes the unique nature of various collision components of the scattered photons to generate separate images of buried objects and surface features. The success of this detector design is demonstrated through a series of analytically generated images. The results of the experimental measurements that validate these analytical predictions are brought out in a separate paper to be presented in this conference.

Image restoration techniques using Compton backscatter imaging for the detection of buried land mines

Earlier landmine imaging systems used two collimated detectors to image objects. These systems had difficulty in distinguishing between surface features and buried features. Using a combination of collimated and uncollimated detectors in a Compton backscatter imaging (CBI) system, allows the identification of surface and buried features. Images created from the collimated detectors contain information about the surface and the buried features, while the uncollimated detectors respond (approximately 80%) to features on the surface. The analysis of surface features are performed first, then these features can be removed and the buried features can be identified. Separation of the surface and buried features permits the use of a globbing algorithm to define regions of interest that can then be quantified [area, Y dimension, X dimension, and center location (xo, yo)]. Mine composition analysis is also possible because of the properties of the four detector system. Distinguishing between a pothole and a mine, that was previously very difficult, can now be easily accomplished.