Growth of penetrating radiation mirrors broadening application field
This year's program on Penetrating Radiation at the SPIE Annual Meeting (Denver, CO) features a wide range of systems for diffraction, imaging, and spectroscopy. An outgrowth of the conference on Radiation Detector Physics, the Penetrating Radiation program focuses on instrumentation for intractable radiation spectra and particles, such as x-ray, gamma ray, and neutron spectroscopy, diffraction, and imaging.
Penetrating Radiation program chairman F. Patrick Doty of Sandia National Laboratory (Albuquerque, NM) describes the program as an interdisciplinary meeting place for engineers interested in nondestructive testing, medical imaging, environmental monitoring, nuclear nonproliferation, neutron scattering science, and many other application areas. These seemingly diverse technologies share a common thread: At the heart of their systems is radiation that stubbornly refuses to easily interact with bulk matter.
Researchers and system developers have recently begun to split their efforts between components and systems development. The number of presentations dealing with commercial applications indicates their efforts to mold these technologies into user-friendly commercial systems have obviously met with some success.
Doty expects the program to include more than 200 papers and attract more than 400 engineers from diverse fields. He and others have selected a handful of these presentations that illustrate, through the success of the authors' systems, the utility of penetrating radiation.
ARACOR (Sunnyvale, CA) is among the few companies in the U.S. providing high-power x-ray sources for commercial industrial applications. At the SPIE Annual Meeting, ARACOR President Bob Armistead will discuss several advanced x-ray NDT systems used for reverse engineering and contraband detection.
"Our systems make heavy use of high-energy sources," Armistead said. "We do have some systems that use tube x-ray sources, but the majority are linear accelerators from 2 to 16 MeV."
ARACOR's products are split into two main categories: high-resolution 3D computer tomography and 2D contraband detection systems. The 3D tomography systems use a high-power pencil beam x-ray source to image an object around 180 degrees of its central axis, similar to a medical CAT scan. The individual images are deconvolved and their Fourier transforms are combined to create a 3D density map with 5- to 10-µm resolution. One of these system has been used by the Air Force to inspect Peacekeeper missile engines for cracks in the rocket motor ring. The system can adapt to the size of the object, working just as well on 128-ft Peacekeeper missiles as it does on automobile power trains.
According to Armistead, automobile manufacturers have used the NDT system to search for engineering flaws in prototype power trains, eliminating the need to tear the transmission apart and potentially ruin crucial data. The Air Force has used the system in similar reverse engineering applications where 3D density maps of older replacement parts can provide computer-aided design (CAD) data for new manufacturing runs.
"Because of budget cuts," Armistead said, "the armed forces are having to stretch the lifetime of their planes and other vehicles. Many times the original manufacturing specs aren't available. Using this system, we can recreate that data."
ARACOR recently delivered the first 2D mobile high-power x-ray inspection system large-container contraband control to the Department of Customs testing center in Tucson, AZ. Mounted on a special carrier similar to those that load and unload steel containers from ships and railcars, the system can image a 44-ft container's contents in 30 sec. Previously, these systems were housed in concrete, fixed buildings at border crossings and large ports. According to Armistead, this system costs one-fifth the price of fixed facilities, while offering the flexibility and speed of a mobile system. He anticipates the system will have wide use at ports, national borders, embassies and other high-security facilities because it can limit insurance fraud by confirming a container's contents as well as detect weapons, explosives, or other dangerous materials.
High-power portable x-ray sources will also be one of the main topics addressed by David Fry of Los Alamos National Laboratory (Los Alamos, NM). "I'll be doing a kind of overview of radiography at Los Alamos, including all types of x-ray sources and imaging systems from about 3 KeV to 20 MeV," Fry said.
Among the diverse sources will be several portable x-ray and imaging systems used for field work and emergency response with energies up to 6 MeV. X-ray facilities, such as a 6- to 20-MeV microtron and microfocus x-ray source with 5-µm focal spot and energies to 360 KeV, will also be presented.
Perhaps some of the most interesting original work at Los Alamos involves x-ray imaging systems. Fry will discuss high-speed x-ray systems at Los Alamos, such as the Kodak (San Diego, CA) high-speed 4540 and 1000HR cameras. These cameras work in concert with x-ray imagers to provide the capability of imaging dynamic events up to 40,000 fps. Based on early experience with EG&G flat-panel amorphous silicon x-ray detectors, Los Alamos is now working with dpiX (Palo Alto, CA), creating a variety of x-ray and neutron single-frame, real-time, and computed-tomography (CT) imaging systems. A program to convert the CT data to CAD data is underway. The program also features other digital x-ray imaging systems under development.
In other research, Iowa State Univ. (Ames, IA) has developed x-ray beam propagation modeling software, called XRSIM. It is being used at Los Alamos for detector characterization, CT modeling, part inspectability, and training.
Detecting gamma rays -- whether they come from naturally occurring radiation or are induced by neutron beams -- is a large part of the Penetrating Radiation Program. Jim Morgan of Lawrence Livermore National Laboratory (Livermore, CA) will relate the performance characteristics of a gamma ray camera jointly developed by Livermore and the Research Institute of Pulse Techniques (Moscow, Russia).
"This is a new design, capable of being fielded in trying circumstances, such as in the field at weapon storage areas," Morgan said. "It's more for arms control applications where we have to determine if a warhead has been dismantled and confirm whether the material is actually plutonium or highly enriched uranium -- all without revealing classified design information."
The camera is essentially an Anger design, which is used in medical imaging and employs a honeycomb lead collimator followed by a sodium iodide scintillator. According to Morgan, the design has excellent energy conversion efficiency and offers the capability of differentiating between high-atomic number materials.
Western Kentucky Univ. is striving to expand the utility of neutron-beam-induced gamma ray detection to detect illegal drugs and explosives as well as characterize coal and cement. Western Kentucky's Phillip Womble and his associates use pulsed fast-thermal neutron analysis to evaluate different elements such as carbon, oxygen, chlorine, sulfur, calcium, sodium, and silicon, among others.
In drug enforcement, a small deuterium-tritium neutron generator with extension probe can be placed in liquids, such as those in tanker trucks, railcars, or any other container. Neutrons of 14 MeV with pulse frequencies of a few kilohertz are shot into the target area and a bismuth germanate (BGO) detector collects the resulting gamma radiation.
Detection occurs in several phases. The first phase keys on the fast-reaction gamma rays emitted from carbon and oxygen. Gamma rays with energies of 4.43 MeV for carbon and 6.13 MeV for oxygen measure the content of these two elements, and therefore their ratio. The second phase occurs during neutron emission as the radiation moderates to thermal energies. Again, gamma ray emission measures amounts of chlorine, sulfur, and calcium. In a separate setup that samples the coal, neutron activation spectra measure the amounts of sodium, silicon, and oxygen.
Womble's group is building a prototype of the mobile drug probe. The neutron generator and detector elements have already been tested on drug simulants.
A larger system using the same neutron source with multiple BGO detectors is also under development for coal characterization in coal burning plants. According to project leader Michael Belbot, the government requirements dictate close monitoring of sulfur emissions under the U.S. Clean Air Act. Knowing the quantities of other elements in coal, such as carbon, oxygen, hydrogen, and sulfur determine the grade of the coal based on its calorific value. Determining quantities of other elements such as sodium and chlorine also is important because these elements can foul boilers through the production of excess slag.
The system sits in an 8 X 8.5 X 48-ft trailer. Half of the trailer contains a coal chute, neutron generator, detectors, and associated electronics. The other half, separated and shielded by a maze of water, acts as the control room. According to the researchers, the precision of the measurements of quantities such as BTU content, sulfur, and sodium weight percentages, etc., are within the limits acceptable by the coal industry for the online coal bulk analysis.
Determining the exact amounts of minerals and nutrients in the human body is also crucial to maintaining a high quality of life. For the past several years scientists around the world have been using neutron-generated gamma rays to determine the amount of carbon, oxygen, nitrogen, and phosphorous.
Joseph Kehayias at the U.S. Department of Agriculture/Human Nutrition Research Center on Aging at Tufts University is using neutron inelastic scattering to determine the amount of carbon and oxygen in vivo to support his department's study on the aging process.
According to Kehayias, lean or muscle tissue contains high amounts of oxygen while fat, conversely, contains a large amount of carbon. Kehayias' technique, performed on adult volunteers, uses 0.03 mSv of radiation, which is approximately one-tenth of that used by a normal x ray. Neutrons from a deuterium-tritium neutron generator are directed at a patient at repetition rates of 4 to 8 kHz. These fast neutrons create gamma rays at 4.44 and 6.13 MeV in carbon and oxygen, respectively. By taking into account the amounts of body protein and the minimal amounts of carbon found in white bone ash and carbohydrates, the Tufts system can determine the amount of body fat to within ± 2 percent.
"This is important now because for the first time we have pharmaceuticals that are designed to alter the composition of the body," Kehayias said. "We had [drugs] that changed the body composition as a side effect, but never as the main outcome in order to retain muscle or reduce fat."
While the main goal of the work is to study the "normal" aging process in order to discover how exercise and nutrition can increase our quality of life, Kehayias' technique also provides feedback on the effectiveness of new drugs designed to fight obesity, or carchexia in HIV and other patients. Body mass analysis is also important for studying the effects of renal failure and improved treatment of dialysis patients, he added.
As scientists such as those at Tufts University and elsewhere search for new ways to help people and industry, the general populace will come to see penetrating radiation not as a dangerous area of science, but one that can benefit everyone when combined with the proper knowledge, responsibility, and expertise.
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