William L. Dunn

Corrosion detection in aircraft by X-ray backscatter methods

A limited-scan backscatter technique for detecting hidden corrosion and other flaws in aircraft has been demonstrated. Both simulation and experimental results indicate that as little as 5% material loss at depths up to 0.508 cm (0.200 in.) and 10% material loss at depths up to 0.635 cm (0.250 in.) beneath aircraft skin surfaces can be realiably detected. The technology involves analysis of backscattered photon data to yield a figure-of-merit whose value as a function of position indicates with high probability of detection whether or not flaws--such as corrosion, disbonding, macro-cracks, or voids--exist within scanned samples.

A modular straw drift tube tracking system for the Solenoidal Detector Collaboration experiment Part I. Design

Abstract We have developed the baseline design for a straw drift tube tracking system for the Solenoidal Detector Collaboration (SDC) detector. The system was designed to operate in the high-rate environment of a high luminosity hadron collider. We present an overview of the tracking system and the requirements it was expected to fulfill. We describe the construction and properties of the straw drift tubes. We discuss the design of the carbon-fiber foam-laminate shell, which supported the wire tension and held the straws in alignment. We also present descriptions of the designs of the front-end and digitization electronics as well as the electronics associated with the level 1 track trigger.

Signature-based radiation scanning using radiation interrogation to detect explosives.

Improvised explosive devices (IEDs) pose a serious threat to society. The signature-based radiation scanning technology has been developed to counter this threat. This technology can be automated, requires minimal operator involvement, and in principle can rapidly identify IEDs from standoff distances. Preliminary research using neutron interrogation indicates that explosive samples of about 7 kg or greater hidden in various targets can be detected from standoff distances of more than a meter, with high sensitivity and high specificity.

Detection of chemical explosives using multiple photon signatures.

A template-matching procedure is being investigated for rapid detection of improvised explosive devices at standoff distances. Photon-scattered and photon-induced positron annihilation radiation responses are being studied as a part of a signature-based radiation scanning approach. Back-streaming radiation responses, called signatures, are compared to templates, which are collections of the same signatures if the interrogated volume contained a significant amount of explosive. Experiments have been conducted that show that explosive surrogates (fertilizers) can be distinguished from several inert materials.

Wireless neutron and gamma ray detector modules for dosimetry and remote monitoring

Compact neutron detectors are being designed and tested for use as wireless low-power real-time personnel dosimeters and for remote neutron sensing. The neutron detectors are pin diodes that are mass produced from high-purity Si wafers. Each detector has thousands of perforations etched vertically into the device. The perforations are backfilled with 6LiF to make the pin diodes sensitive to thermal neutrons. The first prototype devices delivered over 3.8% thermal-neutron detection efficiency while operating at 15 volts. Recent high-efficiency perforated devices have delivered over 20% thermal-neutron detection efficiency. Compact packages with wireless communication capability have been constructed and are under test. The compact packages record counts every second and transmit ten data points every ten seconds. The platforms have been designed to incorporate an additional fast neutron detector and a Frisch- collar CdZnTe gamma-ray spectrometer. The overall goal is to manufacture a compact package capable of remote and wireless neutron dosimetry and gamma ray spectroscopy/dosimetry.

Radiation Interrogation Using Signature Analysis for Detection of Chemical Explosives

A signature-based radiation technique for detecting chemical explosive is described. Radiation techniques offer the advantage that they can operate at standoff, although rapid detection of explosives at safe standoff distances remains problematic. The technique we describe differs from the major nuclear approaches, which involve imaging (using either photon or neutron interrogation) and/or quantitative analysis (for instance by use of prompt-and inelastic-scatter gamma-ray production based on neutron interrogation). Our technique utilizes both photon and neutron interrogation but avoids imaging or quantitative analysis. The technique, which we call signature-based radiation scanning (SBRS), limits itself to detecting whether a target contains an explosive and does not attempt to characterize the internals of a target further. A template-matching technique is employed, which provides a single figure-of-merit whose value is used to distinguish between safe targets and those containing explosives. Both simulation and experiment have been used to verify the validity of SBRS.

Neutron/photon damage assessment of commercial plastic scintillating fibers

Abstract Five types of plastic scintillating fibers from three manufacturers were analyzed before and after mixedfield (neutron/photon) exposures in the PULSTAR reactor at North Carolina State University. The fibers received fast neutron fluences between 2.5×10 12 and 2.0×10 14 cm −2 , which are comparable to multi-year fluences expected within detectors for the Superconducting Super Collider (SSC). The gamma-ray doses delivered were between about 7 and 560 krad, which are generally below the doses considered to be required to induce significant permanent damage in the types of fibers tested. Thus, we were able to make preliminary assessments of neutron effects on light output and attenuation length. The results indicate that, although there is some variability depending on fiber type and manufacturer, significant long-term optical degradation is induced in fibers exposed to fast neutron fluences above approximately 2×10 13 cm −2 .

Characterization of the High-Efficiency Neutron Detector Array (HENDA)

Two new pixellated neutron detectors developed at the Semiconductor Materials and Radiological Technologies Laboratory (SMART Lab) at Kansas State University, for eventual use at Oak Ridge National Laboratory Spallation Neutron Source, were tested for resolution, count rate and efficiency.

A LIMITED-SCAN BACKSCATTER TECHNIQUE FOR DETECTION OF HIDDEN CORROSION

Abstract A photon backscatter technique to detect corrosion hidden beneath an obstructing surface layer was investigated. The technique is sensitive to both composition and density changes resulting from corrosion in metals; further, the sensitivity of some system variables to corrosion-like conditions affords opportunity for design optimization and corrosion characterization. The primary application of the technology is in non-destructive evaluation of critical structures, such as aircraft, for detecting hidden and inaccessible corrosion.

Applications of the inverse Monte Carlo method in photon beam physics

Abstract Photon beams are used in a variety of applications, for instance to measure properties of materials and to produce or induce changes within materials. The measurement process typically requires that inverse methods be employed to obtain the desired properties from radiation responses, whereas the use of photon beams to activate or deposit energy in samples often requires system design optimization. We report here on the use of the Inverse Monte Carlo method to solve inverse and optimization problems involving photon beams. This method benefits from being able to be applied in complex situations (e.g., multidimensional geometries) and from being relatively computationally efficient, since the simulation is noniterative.