Use of high energy (1-12 GeV) protons for radiography of dynamic systems
Los Alamos National Laboratory and Lawrence Livermore National Laboratory
Result of NP research:
Basic Nuclear Physics research
Application currently being supported by:
Impact/benefit to spin-off field:
Provides capability to measure the dynamic performance of light materials encased in heavy metal objects. This is of crucial importance(rated in category A by Defense Programs) for measuring propagation of shock fronts and materials deformation in high explosive assemblies. Has had direct impacts on the security of the US nuclear stockpile.
In most implosion tests, scientists study a full-scale weapon mock-up. The pit of the mock-up is made of a surrogate metal that has mechanical properties similar to those of the fissile material but that is not fissile and so cannot produce nuclear reactions. During an implosion, the shock waves' high pressures and temperatures cause metals and other materials to flow like liquids. Because liquid behavior can be described by hydrodynamic equations, implosion tests are called hydrotests.
To see what happens during a hydrotest requires "x-ray vision", that is, the ability to see through the mock-up's thick, dense metal parts. In the first hydrotests, performed during the Manhattan Project, scientists took snapshots of imploding mock-ups with intense flashes of high-energy x-rays, much as medical x-rays are taken of bones or teeth. X-rays have been used for hydrotests ever since. LANL’s present state-of-the-art hydrotest facility is the Dual-Axis Radiographic Hydrodynamic Test facility, or DARHT. Recently completed at Los Alamos, DARHT is expected to be the centerpiece of the nation's hydrotest program for at least a decade.
In 1995, however, Los Alamos physicist Chris Morris thought of a way to use protons instead of x-rays for hydrotest radiography. "The extraordinary quality of the first proton radiographs made at Los Alamos surprised even the inventors us," says Morris. Later experiments confirmed the technique's potential.
One of proton radiography's many advantages over x-ray radiography is its ability to take twenty or more sequential radiographs per hydrotest, as opposed to DARHT's four. This multiframe capability will allow weapon scientists, a UK-US collaboration, to make detailed motion pictures of implosions for the first time.
Proton radiography is a new diagnostic technique that produces detailed movies of high-speed shock and implosion tests. This sixteen-frame proton-radiography movie at the left shows how a tin disk responds to the shock wave produced by detonating a high explosive beneath the disk. First, the detonation's spherical shock wave bulges the disk (about 2 inches in diameter and 0.25 inch thick). Then, when the compressive shock wave reflects from the disk's upper surface, the shock becomes tensile, dislodging and levitating the spall layer (the "flying saucer"). The shock wave's high pressure and temperature also melt the tin (light gray region connecting the flying saucer and bulge).
In addition, proton radiography will be able to provide simultaneous views from many directions during an implosion. Using the same computerized-tomography methods that produce three-dimensional medical images of the brain or other organs (commonly called CT scans), researchers can convert proton radiography's multiple two-dimensional images into a single three-dimensional view for each frame of an implosion movie. Such views will help scientists validate three-dimensional weapon codes as well as determine if the implosion geometry is correct.
Protons can also image subtle effects impossible to see with x-rays. For example, Lab Researchers have made proton radiographs of water coolant levels inside an internal combustion engine. The ability to distinguish between parts of an object with different densities will help researchers track the behavior of specific weapon components during implosion tests.
demonstration of proton radiography's ability to measure small density differences inside a test object. Two levels of coolant water in an engine block are clearly visible in these radiographs at the right. The ability to discriminate between small changes in density will help scientists track individual weapon parts during implosion.
A 150-cubic-centimeter model airplane engine (top left) and radiographs of it produced by 800-million-electronvolt protons (middle left) and ~100-thousand-electronvolt x-rays (bottom left). Because the x-ray attenuation length is much shorter than the engine's thickness and because the engine strongly scatters the x-rays producing "scatter background" the x-ray radiograph is less detailed. The details of the thicker parts of the engine are clearer in the proton radiograph because the proton attenuation length is longer and because the lens used to produce the radiograph greatly reduces proton scattering.
By giving weapons scientists 'x-ray vision'; that is more versatile and precise than now possible, proton radiography promises to meet stockpile stewardship needs well into the twenty-first century.