Los Alamos Researcher Explains How Protons Create Movies Of Nuclear Weapon Models

Proton radiography uses a series of proton pulses from an accelerator to create a time sequence of images and literally make a movie of a dynamic event with unprecedented resolution of the interior constituents.

Los Alamos physicist Tom Mottershead explained the technique, especially the array of magnetic lenses that resolves the transmitted and scattered protons into sharp radiographic images, in an invited talk at the American Physical Society meeting today in Albuquerque. Mottershead works in the Accelerator Physics and Engineering Group at the Los Alamos Neutron Science Center, or LANSCE.

In dozens of experiments over the past seven years, researchers have refined the proton radiography technique to the point that they can see fine details in weapon components and other dense objects.

For decades, nuclear weapon scientists have built mockups of weapon primaries, substituting lead, tantalum or other surrogates for nuclear materials. Then they implode the mock primary at a firing site in front of flash X-ray machines and study the resulting images, much like a dentist studies x-rays to diagnose and recommend treatment. The x-rays and related data allow them to quantify the effects of aging on stockpile weapons, and refine the three-dimensional computer codes that provide the best prediction of weapon performance in the absence of nuclear testing.

Unlike X-ray machines, the unique, 800-million-electron-volt accelerator at LANSCE fires a series of proton pulses on demand, creating an intense proton beam. This makes it possible for a unique system of cameras put together by Los Alamos and industrial partner Bechtel Corp. to catch and separate the image made by each burst in the sequence, producing a motion picture. A typical 16-image time-lapse "movie" lasts just one fifty-thousandth of a second.

A high-resolution radiographic motion picture of a model primary helps scientists understand precisely how a weapon behaves upon implosion. Data from these experiments provide the strongest technical justification for determining nuclear weapon reliability, short of a nuclear test.

Los Alamos Director John Browne told Congress earlier this month that "Proton radiography has already provided data that has influenced stockpile decisions. This technology supports and strengthens LANSCE as the Laboratory's flagship user research facility."

The secret behind the breakthrough technology is a unique system of magnetic lenses that remove blurring and aberrations and provide uniform fields of view, Mottershead explained.

First, an expansion-lens system spreads the proton beam before it strikes the object. As the protons travel through the object, they undergo millions of small-angle scatterings, or strike nuclei and are absorbed, or encounter electrons and lose energy. They emerge from the other side of the object attenuated, spread out over a range of energies as well as spread out in direction.

The magnetic lenses placed on the other side of the object refocus the scattering angle distribution to produce a radiographic image.

"All the particles have lost energy. The spread in energies of the beam coming out depends on the object itself, but very little on the energy spread going in," Mottershead said. "By focusing the lens for a specific energy, we eliminate the angle spread at that particular energy and leave other energies slightly out of focus."

The lenses transform a fuzzy picture to into a sharply focused image by sorting all the protons, based on their angle as they emerge from the object, a property that is independent of their varied energies. Thus the lenses eliminate some blurring by focusing the protons.

The multiple lenses after the object also allow scientists to identify specific materials in the object.

"Working on proton radiography has been gratifying because our results consistently have been better than we originally estimated," Mottershead concluded. "The root of that is the wonderful symmetry of the lens system. The image produced by the lens on the detector plane is an inverted copy of the proton distribution exiting the object, but 30-meters away."

Proton radiography movies are a natural result of the multiple pulses produced by proton accelerators. Among other advantages of protons over x-rays are the fact that the higher penetrating power of protons is ideal for clear images of thick, dense objects, providing sub-millimeter resolution.

In addition to its weapon stewardship applications, other proton radiography experiments have shown promise for day-to-day applications. In one, scientists from the Laboratory and industrial partners took pictures of water running through an auto engine cylinder head. In another, they captured a movie showing precisely where industrial high explosives fail to burn efficiently when used inside insensitive high explosives.

Researchers at Los Alamos are exploring the optimal configuration for a proton radiography system. A high-energy version of the lens was tested in a series of experiments at the 24-billion-electron-volt accelerator at Brookhaven National Laboratory. Proton beams generated at higher energies near 50-billion electron volts, delivered with flexible time-sequencing in tens of pulses, could provide an unprecedented capability to directly see images of the very thick and very complicated geometries of surrogate weapon configurations

Los Alamos researchers are studying beam distribution systems that can provide multiple-view directions, which would allow tomographic reconstruction of interior densities, resulting in 3D movies.

Los Alamos National Laboratory is operated by the University of California for the National Nuclear Security Administration (NNSA) of the U.S. Department of Energy and works in partnership with NNSA's Sandia and Lawrence Livermore national laboratories to support NNSA in its mission.

Los Alamos enhances global security by ensuring safety and confidence in the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction and improving the environmental and nuclear materials legacy of the cold war. Los Alamos' capabilities assist the nation in addressing energy, environment, infrastructure and biological security problems.