Proton radiography at Los Alamos
Sharper "X-ray vision" for hydrotests
Researchers are now using protons to probe the details of imploding weapon mock-ups. Proton radiography is enhancing the Lab's ability to predict the performance of stockpiled nuclear weapons.
LOS ALAMOS, N.M., May 2003 -- The first step in detonating a modern nuclear weapon is to rapidly compress, or implode, a sphere of fissile material such as plutonium. Imploding the sphere, also called a pit, forces the fissile atoms closer together so they can sustain a chain reaction, thereby releasing a huge burst of nuclear energy. The pit is imploded by shock waves produced by a shell of high explosives surrounding the pit.
Because the implosion geometry is crucial, implosion tests are needed to ensure that the weapons in the nation's nuclear stockpile will perform as expected--one of the goals of stockpile stewardship. These tests also help validate computer simulations of nuclear weapon performance, confirmation that is vital in the absence of nuclear testing.
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.
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.
Proton radiography is now being proposed as the key diagnostic technique for an Advanced Hydrotest Facility, or AHF. In the view of some Los Alamos officials, experiments conducted at this facility would "represent a focusing of all the predictive capability developed through stewardship and would validate our designers' abilities to predict, with greatest confidence, the nuclear performance of weapons. In short, AHF would represent the 'last stop' taken by stewardship before technical issues would lead us to demanding a nuclear test."
Protons versus X-Rays
The promise of proton radiography is best illustrated by comparison with the Lab's present state-of-the-art hydrotest facility, the Dual-Axis Radiographic Hydrodynamic Test facility, or DARHT. Recently completed at Los Alamos, DARHT should be fully operational in 2004 and is expected to be the centerpiece of the nation's hydrotest program for at least a decade.
During a hydrotest, DARHT will take four sequential x-ray radiographs along one axis and one radiograph along a perpendicular axis, providing the first-ever simultaneous views of an implosion from two directions. The radiographs' exposure time--60 nanoseconds, or 60 billionths of a second--will freeze the action of an imploding mock-up to within a millimeter. (The exposure time required is determined by the radiograph's spatial resolution and by the implosion speed, which can exceed 4.4 millimeters per microsecond, or 10,000 miles per hour.)
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 to make detailed motion pictures of implosions for the first time.
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.
To image such effects, researchers must sensitively measure how a test object reduces (or attenuates) the intensity of the imaging radiation, which, generally speaking, means measuring small changes in density within the object. To do so, researchers must match the proton's attenuation length in the test object to half the object's thickness.
The attenuation length is the distance in which the intensity of a proton beam decreases to about a third of its initial value as the beam passes through the test object. If the attenuation length is much shorter than the object's thickness, the protons will not penetrate the object, and the resulting radiograph will show a silhouette of the object but nothing inside it. If the attenuation length is much greater than this thickness, all the protons will sail through the object, leaving no trace of it in the radiograph. The optimum attenuation length is between these two extremes.
For most elements in the periodic table, a proton's attenuation length increases with its energy. Fortunately, modern accelerators can produce protons with the high energies required to match their attenuation lengths to the thickness of a hydrotest mock-up. The attenuation lengths for x-rays, however, peak at about 4 million electronvolts, the reason that DARHT produces x-rays with this energy. But even at its peak, the x-ray attenuation length is only a tenth or less that of high-energy protons, the main reason that proton radiographs of thick, dense hydrotest objects reveal much smaller changes in density than is possible with x-rays.
The longer attenuation lengths of high-energy protons also allow a much larger fraction of the protons to pass through the test object than is possible with x-rays, while still producing radiographs that detect small changes in density. Thus, the total amount of radiation, or dose, required to produce a proton radiograph is much less than that required for an x-ray radiograph.
For DARHT, the x-ray dose per radiograph will reach 1,000 roentgens--the equivalent of 60,000 chest x-rays. For a proton radiograph, the dose is currently about 2 roentgens per radiograph. In fact, the first proton radiographs were taken in 1973 to explore their potential for low-dose medical radiography, but the radiographs were unacceptably blurred by proton scattering.
In addition, the intensity of an accelerator-produced proton beam is so high that billions of protons can be focused on a test object in 50 nanoseconds. The resulting radiographs have excellent detail and tonal range and are not "snowy," as they would be if too few protons were used. In fact, Morris says that proton radiography owes much of its success to the ability of modern accelerators to provide high beam intensities as well as high proton energies.
Another reason proton radiography can measure small changes in density is that proton scattering can be controlled. Whereas a test object scatters x-rays uncontrollably into a wide range of angles--producing a diffuse background that reduces the radiograph's accuracy and sensitivity--scattered protons can be focused to form a sharp image with virtually no background. In fact, rather than being a hindrance, proton scattering can be used to identify the chemical elements in a radiograph, which will also help scientists track specific weapon parts during implosion tests.
In addition to these advantages, the proton-radiography technique proposed for hydrotests has a spatial resolution of several hundred micrometers, compared with DARHT's millimeter resolution. And recent proton-radiography experiments have demonstrated a spatial resolution of 15 micrometers, with the potential for 1 to 2 micrometers, making possible a new type of microscopic "x-ray vision."
Finally, researchers can arbitrarily select the time at which a proton radiograph is exposed, a great advantage for hydrotests. For example, researchers can use longer time intervals between successive radiographs to freeze the mock-up's slower motion during the early phases of an implosion. However, in the later phases, when implosion speeds are higher, they can use shorter time intervals to freeze the motion. In this way, they can obtain the most important information about the implosion using a limited number of radiographs.
This timing flexibility is possible because the Los Alamos accelerator used for proton radiography produces protons in a continuous succession of "microbunches," each lasting about 100 picoseconds and spaced 5 nanoseconds apart. (Other accelerators may have slightly different parameters but operate in basically the same way.) Typically, a series of eight to twenty microbunches is used to expose each radiograph.
In contrast, the timing of DARHT's radiographs is determined by electrical circuits that produce x-ray pulses whose temporal separations and durations are fixed. Thus, DARHT's radiographs will be taken at set intervals, and researchers will be able to specify only the time of the first radiograph.
The Right Source
Many of the advantages of protons over x-rays arise from differences in the sources of radiation. At an x-ray hydrotest facility, an intense "point" source projects x-rays through the weapon mock-up onto a scintillator plate. The radiograph is produced without using lenses or mirrors to focus the x-rays. In fact, there is no known way to focus the 4-million-electronvolt x-rays required for hydrotests. As a result, the radiograph's sharpness, or spatial resolution, is determined by the diameter of the source.
But the source's diameter cannot be reduced arbitrarily because of the way the x-rays are produced: an intense, pulsed electron beam impinges on a tungsten plate, and the electrons emit x-rays as they are deflected by the tungsten's positively charged nuclei. However, since the electrons have like charges, they repel one other and try to expand the beam radially, increasing the beam's cross section. As a result, the minimum possible beam diameter is 1 to 2 millimeters.
The radiation source for the proton radiographs produced at Los Alamos is a beam of 800-million-electronvolt protons generated by the half-mile-long linear accelerator at the Los Alamos Neutron Science Center (LANSCE). Although the beam is used primarily to produce neutrons, it can also be diverted to produce radiographs. The protons in the beam move at about 84 percent of the speed of light, and about 4 billion protons pass through the radiograph's field of view in 50 nanoseconds.
Los Alamos scientists have also made radiographs with 12- and 24-billion-electronvolt protons produced by a synchrotron accelerator at Brookhaven National Laboratory in Upton, New York. These energies are closer to the Advanced Hydrotest Facility's 50-billion-electronvolt design energy, which is in the range required for hydrotest radiography.
The Right Prescription
Proton radiography's potential would never have been realized if the blurring evident in early proton radiographs had persisted. In 1995, however, Morris realized that a magnetic lens could focus the protons and eliminate the blurring. Initial experiments with an existing lens design proved him right. Later, John Zumbro, another Los Alamos physicist, improved this lens design. The resulting Zumbro lens has been used for proton radiography ever since.
According to Morris, the main virtue of the Zumbro lens is its low chromatic aberration. Chromatic aberration causes protons with different energies to be focused differently, blurring the radiograph. The protons in an accelerator beam do not have quite the same energy. In addition, proton scattering by the test object increases the spread in proton energies, which is why chromatic aberration could have been a showstopper.
For the Zumbro lens to minimize chromatic aberration, however, the protons must have an "angle-position correlation," which means that the angle a proton makes with the optical axis of the lens is proportional to the proton's radial distance from the axis. A series of quadrupole electromagnets, common at accelerator facilities such as LANSCE, gives the protons the necessary correlation.
Because the Zumbro lens preserves the correlation, several Zumbro lenses can be used in series without one of the lenses degrading the images produced by later lenses, an essential feature for elemental identification. The correlation also simplifies sorting protons by their scattering angles, which is also required for elemental identification.
With the basic setup shown below, many proton-radiography studies have been performed at Los Alamos and Brookhaven in the last few years. About 150 of these studies have been done at LANSCE in collaboration with scientists from Sandia National Laboratories in New Mexico, Lawrence Livermore National Laboratory in California, and the Atomic Weapons Establishment in the United Kingdom.
The studies have focused on the propagation of detonation waves in high explosives, the response of metals and other materials to shock waves generated by high explosives, the performance of explosive neutron generators, and the implosions of half-scale weapon mock-ups with aluminum pits. (Because of the energy of LANSCE protons, their attenuation length is better matched to the thickness of a half-scale mock-up with an aluminum pit than to that of a full-scale mock-up with a surrogate pit.)
These experiments have confirmed proton radiography's ability to meet the demanding goals of stockpile stewardship for decades to come. In fact, proton-radiography data have already influenced stockpile decisions, prompting conceptual designs for the Advanced Hydrotest Facility. The current schedule calls for the facility's preliminary design to begin in 2006, with facility construction starting in 2008 and full facility operation set for 2016. The new facility would be built near LANSCE, possibly using a second accelerator to boost LANSCE protons to the energies required for full-scale hydrotest mock-ups. The facility's main proton beam would be split into twelve subbeams, which would illuminate a mock-up from twelve angles, providing the data needed for tomographic reconstruction of high-resolution three-dimensional implosion movies.
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.
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.
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By Brian Fishbine