Los Alamos National Laboratory
James E. Rickman, (505) 665-9203
LOS ALAMOS SCIENTISTS PHOTOGRAPH SHOCK WAVE WITH PROTONS
LOS ALAMOS, N.M., Aug. 18, 1997 --- Researchers at Los Alamos National Laboratory have used protons from a linear accelerator to "photograph" the detonation wave from a small-scale explosion --- demonstrating for the first time a potentially important technology to support science-based stockpile stewardship.
The scientists detonated a small amount of high explosive inside a chamber specially designed to contain the explosion and let the proton beam at the Los Alamos Neutron Science Center enter and exit to produce a clear, high-resolution image of the explosion's shock wave.
"This is the first time in history that anyone has demonstrated the use of proton radiography for dynamic experiments," said Chris Morris, chief scientist for the project and a member of the Subatomic Physics Group in Los Alamos' Physics Division.
With the September 1996 Comprehensive Test Ban Treaty heralding an end to nuclear weapons tests, the United States is refining ways to ensure that the nation's nuclear weapons stockpile remains safe and reliable in the absence of weapons tests; this is the essence of the science-based stockpile stewardship program at Los Alamos and other national laboratories.
One way to study the behavior inside a weapon's non-nuclear, high-explosive components is to detonate them (so-called dynamic experiments) and look at the resultant shock waves using radiography. The radiography process is most often done with X-rays or neutrons, but in the recent Los Alamos experiment, researchers used protons --- which have several potential advantages over X-rays --- to take a picture of what happened shortly after detonation.
"In the era of the Comprehensive Test Ban Treaty we have a real need for new radiographic tools, especially those that can be used for dynamic experiments, if we are to understand how weapons components age over time," Morris said. "Ultimately, if the research at Los Alamos is successful, protons could even be used in future facilities for hydrodynamic testing of nuclear weapons mockups. Right now that idea is just at the conceptual stage.
"Although the idea of proton radiography has been around for a while, we have had to invent new techniques to make radiographs of dynamic systems," he said. "In early discussions we realized there were a number of technical challenges that would need to be overcome for it to work."
For these experiments, Morris said, researchers from many different divisions across the Laboratory assembled to figure out how to use Los Alamos' half-mile-long accelerator with its high-intensity, 800-million-electron-volt proton beam as a proton camera.
In principle, the idea of using protons to produce an image is somewhat similar to using X-rays --- high-energy photons --- to expose film. But instead of photons, protons are "shone" on the material to be studied. As protons interact with the material from which an image is to be obtained, some of the protons are absorbed or scattered. Protons that exit the material strike a recording medium, such as a special type of film or electronic camera, and create an image much like conventional photographic film or digital camera.
However, because protons are electrically charged, they undergo a large number of small-angle scatterings as they pass through the material. If not corrected, these scatterings would cause unacceptable blurring of the image as they pass from the object to be photographed to the recording medium.
To help overcome the proton scattering problem, John Zumbro of Los Alamos' Subatomic Physics Group and Tom Mottershead of LANSCE developed a magnetic lens to refocus scattered protons. The system uses a series of four quadrupoles --- devices that produce magnetic fields --- to focus the protons onto the image plane, a process somewhat analogous to a camera lens focusing light onto film. This new focusing system was the key to overcoming many of the problems that researchers believed would limit the usefulness of protons as radiographic probes.
John McClelland, leader of the proton radiography project and of Los Alamos' Subatomic Physics Group, said the complex interaction of the proton beam with the materials to be radiographed were calculated using the Blue Mountain supercomputer at Los Alamos.
The Blue Mountain machine, part of the Department of Energy's Accelerated Strategic Computing Initiative and another key element of science-based stockpile stewardship, is scheduled to be fully installed at Los Alamos in 1998, and will be the world's most powerful supercomputer --- capable of performing more than three trillion calculations a second.
"The ASCI Blue Mountain machine's role was vital to the project since two billion protons in a single burst from the accelerator had to be tracked through the radiography system in order to model the system's performance," McClelland said. "This traditionally would have taken weeks of intense computing to model. With a small fraction of the eventual capability of Blue Mountain, it can now be done in a couple of days. In this experiment, Blue Mountain already has demonstrated its usefulness in predictive modeling calculations."
With development of the magnetic lens, with calculations from Blue Mountain, and with data from proton radiography experiments on static objects performed in July 1996 at Brookhaven National Laboratory, the Los Alamos research team was ready to try proton radiography on a small explosion at LANSCE.
Researchers led by Roger London of Los Alamos' Dynamic Experimentation Division prepared a four-foot-diameter containment vessel in which the explosion would occur. The chamber was specially designed to have relatively thin windows through which the proton beam could pass in order to minimize blurring effects while still safely containing the explosion.
Researchers led by John Sarracino of Los Alamos' Applied Theoretical and Computational Physics Division designed the high-explosive systems used in the dynamic experiment to match experimental capabilities available at LANSCE with the interests of the science-based stockpile stewardship program at Los Alamos.
On April 19, the Los Alamos team and a team from Lawrence Livermore National Laboratory gathered at the LANSCE facility for the first dynamic experiment. A portion of the proton beam was diverted to an experiment area where the containment vessel loaded with 92 grams --- a little more than three ounces --- of high explosive stood ready. Three-and-one-quarter-millionths of a second after detonation, the proton beam snapped a picture of the explosion. The faint image on the plate shows the spherical burn edge --- discernible because of density changes created by the explosive shock wave.
"The dynamic experiment was a success on two levels," said McClelland. "First it succeeded in demonstrating on the very first attempt a new technology that may revolutionize how we probe the interior of rapidly changing objects, and second it showed that we can integrate people from across the Laboratory and even from other laboratories, to achieve an objective."
>From idea to implementation, the entire project took less than two years. Researchers didn't actually begin putting the experimental apparatus in place until December 1996, just five months before the first snapshot was made, Morris said.
After the April experiment, researchers performed a series of small-scale explosive experiments. In the course of these experiments, a research team led by Nicholas King of Los Alamos' Neutron Science and Technology Group developed a new electronic camera system capable of taking up to six snapshots over the duration of the explosion to produce a "motion picture" of the explosion process.
Livermore researchers provided two of the cameras used in the new system and are also developing another system that can detect the amount of energy a proton loses as it passes through a material, McClelland said. This will provide a new way of processing the data to better interpret radiographic images.
The researchers plan to continue using LANSCE for additional experiments to develop proton radiography, understand its potential and obtain scientific data on the behavior inside dynamic materials. Data obtained from this year's LANSCE experiments have shown researchers that proton radiography is a unique diagnostic tool that can be used to better understand the performance of high explosives.
Los Alamos National Laboratory and Lawrence Livermore National Laboratory are operated by the University of California for the U.S. Department of Energy.
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