It's an imager! It's a diagnostic! It's...Super cpRad

CHARGED-PARTICLE RADIOGRAPHY—TAKING THE "X" OUT OF X-RAY

Suppose for a minute that Superman had come to Earth endowed with proton vision, whereby he could emit beams of high-energy protons from his eyes and thus see through any substance, including dense materials like lead or plutonium that are opaque to his celebrated x-ray vision. He would also be able to follow with astonishing clarity and detail the nearly instantaneous changes an object goes through when subjected to extreme forces, such as when hit by explosion-driven shock waves. Accordingly, Superman's proton vision would have been a remarkable boon to the nuclear weapons community, which, since the days of the Manhattan Project, has wanted to peer inside a detonated nuclear weapon and watch its plutonium core implode and go critical.

prosthetic hand

Above: A prosthetic hand (manufactured with an internal bone structure) illustrates the capability of charged-particle radiography. The false-colored image was obtained using high energy protons.

Then suppose again that the Man of Steel could see the streams of cosmic-ray muons that continually race down from the upper atmosphere. Highly penetrating, the ghostly muons can whiz through a mountain of rock and dirt, but they scatter in characteristic ways from plutonium or uranium, even when those bomb-making materials are hidden behind layers of lead or otherwise concealed. If Superman were stationed at a border crossing, his "muon vision" would allow him to spot those telltale ripples in the muon stream and quickly intercept the illicit materials. It goes without saying that with proton or muon vision augmenting his powers, Superman would have been a lynchpin for nuclear stewardship and a champion in the war on terror.

Now admittedly, most people would consider the enhanced vision of a fictional superhero largely irrelevant to their lives. It's just that proton and muon vision are both as real as the Earth is round, and are already being used by scientists to confront the problems of a complex world. And that kind of vision is anything but irrelevant.

Charged-Particle Radiography

Protons and muons are charged particles that for years have been employed to make x-ray-like radiographs of an object's interior (a radiograph being a photograph made with non-visible light), as well as utilized to measure thicknesses, identify materials, and provide information about dynamic events. Indeed, muons were used as early as 1955 to measure the depth of a mineshaft within a mountain. Over the years, Los Alamos scientists helped drive the development of proton radiography (pRad) and muon tomography (µTom), and have recently demonstrated the capabilities of high-energy electrons in electron radiography (eRad). Collectively, pRad, µTom, and now eRad go by the catchphrase "charged-particle radiography," or cpRad.

While similar to each other in a block-diagram sense, each radiography has its pros, cons, and appropriate applications. Proton radiography can make movies of ultra-fast events, and is often used to probe components of nuclear weapons, obtaining information that can help keep our nuclear deterrent safe and reliable. Muon radiography is able to probe giant objects such as tractor trailers or shipping containers, and is close to being deployed at points of entry to guard against the smuggling of nuclear contraband. Researchers are even eyeing eRad as a tool that can help them develop the next generation of materials. So while not as awesome as a superhero, cpRad is still pretty super.

Super pRad

Proton radiography goes back at least 40 years, with an early pRad system described in a 1968 issue of Science. The key to making that early system work was understanding the relationship between a material's density and the distance a proton travels in the material before stopping.

A fast-moving proton passing through a substance loses energy to the electrons and nuclei of the substance's atoms—a little at first, but more and more as the proton slows down. If the material is thick enough, then similar to the way a spinning top teeters and abruptly falls, ending its spin, the energetic proton travels a certain range within the material, after which it quickly loses the bulk of its energy and abruptly stops moving (is absorbed). If a pulse of many billions of protons enters a material, all the protons will stop in relatively close proximity to one another, scattered about that range.

The early pRad system exploited this effect. The proton energy was chosen such that its range was about equal to the object's thickness. Then some of the protons were absorbed while the rest emerged from the object. The actual number that exited from a particular location depended sensitively on the amount of material encountered by each proton; that is, the transmission was proportional to the thickness of the object at that location.

The exiting protons would hit a piece of photographic film, exposing a small spot—the fewer protons that emerged, the less the exposure. In the black and white photo made from the developed film, blacker regions corresponded to the object's thicker, denser parts. Variations in thickness as small as 0.05 percent were discernable.

The pRad system housed at the Los Alamos Neutron Science Center (LANSCE) is as evolved from that early system as a camcorder is from a pinhole camera. The major advance is that instead of using the proton's range to map out the object's thickness, the angular spread of the emerging protons is used to map out the areal density; the latter is the amount of material encountered by the proton as it traverses the object. It's equivalent to the material's density times the length of the path taken by the proton (in units of grams per square centimeter).

The relationship between exit angle and areal density exists because a proton tends to scatter, or change its direction of travel, when it runs into atoms—their electrons, or their nuclei. Higher areal densities lead to more scattering and a broader angular distribution for the transmitted protons.

Once they exit, the protons begin to follow the magnetic field lines produced by several pairs of tractor-sized electromagnets. The carefully designed, high-efficiency magnetic "optics"—briefly discussed on the facing page—transport a proton from a point on the back of the object to a pixel on a multi-pixel detector. But scientists can also adjust the magnets to enhance the image contrast and resolution. Furthermore, they can insert a magnetic "magnifier" that, like a microscope objective, provides magnification at the expense of a reduced field-of-view.

The magnetic optics also creates images in two locations at once, so that the modern pRad system can do something that no other radiograph can manage—it can take a series of images of an object that is changing substantially faster than the blink of an eye.

"We do many dynamic experiments, which is a euphemism for saying we typically blow our test object up," says Andy Saunders, deputy group leader for the Subatomic Physics Group at Los Alamos. "We'll detonate a high-explosive and watch how the detonation wave advances through the material. We're able to capture the entire process with high resolution, routinely taking 30 or more images of the event."

Remarkably, pRad technology was developed on a shoestring budget. Initially, a large fraction of the money came from Laboratory Directed Research and Development (LDRD) funds, but funding was dicey. "Let's just say that we never had any money problems, because we never had any money," jokes Chris Morris, one of the driving forces behind both pRad and µTom. "That was half the fun, figuring out what we didn't have the money to do, then figuring how to do it anyway."

It seems that sheer scientific tenacity was what kept pRad going—that and a strong sense of purpose, or perhaps a sense of irony. For while the technology grew out of basic research—developed by scientists whose bread and butter was measuring nuclear-reaction parameters—the motivation was weapons related. Given the proton energies available at LANSCE, pRad would be almost ideal for seeing how materials performed inside a detonated nuclear weapon.

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Super Application

Top scientists still lack a complete understanding of the complex phenomena that occurs inside a nuclear weapon after it is detonated, when exquisitely timed explosions send the shock-wave equivalent of a tsunami racing towards a thin plutonium shell. Slammed everywhere at once, the shell is instantly driven inward at supersonic speeds (it implodes). Its density skyrockets, the number of fission reactions increases exponentially, and at some point, the total energy released by fission exerts enough pressure to blow the compressed plutonium to smithereens (along with everything else in sight). It's all over in a tiny fraction of a second, and it all has to work perfectly.

Or maybe it just needs to work within a smidgeon of perfectly? How does one begin to answer that question, or begin to quantify "smidgeon"? One place to start is to understand all aspects of the implosion process, including how fluid instabilities and material defects impact the exponential growth of fissions.

The United States, however, ceased nuclear weapons tests nearly twenty years ago, making it virtually impossible to obtain new data under relevant conditions. So instead, the community has settled for testing individual components and materials under close-to-relevant conditions, then piecing together the results using various models. Then there are experiments to test the models, simulations to check the experiments, tests to verify the simulations, and occasionally a so-called hydrotest on a full-scale (non-nuclear) weapon surrogate to check everything.

With the energy available from the linear accelerator at LANSCE, a proton will pass through a piece of lead on the order of 10 centimeters thick, corresponding to an areal density that is appropriate for the objects that weapons scientists want to study. So in terms of scale, pRad is a good match for problems of interest to the nuclear weapon's community. But about half of its Los Alamos workload is unclassified.

If there's one disadvantage to a pRad imaging system it's that with the over-sized magnetic lenses and the need for a source of energetic protons, the entire setup is forbiddingly complicated, sort of the way the Hubble Space Telescope differs from a pair of binoculars. At present, proton radiography resides at LANSCE, tethered to its proton accelerator. It's the only place in the country where proton radiography is performed.

A copper projectile forms and takes flight in this series of pRad images. The newly designed projectile, intended to generate shock waves in a test object by impacting it at roughly 3 kilometers per second, forms after a small explosive charge is detonated beneath a thin copper dish. The dish deforms and the projectile moves out. The images verify that the projectile formed as expected, but they also highlight pRad's unique ability to take multiple frames of a rapid, dynamic event.

Super µTom

A particle source is not a problem for muon tomography, because muons—a middleweight version of the flyweight electron (the tau particle being the super heavyweight version)—are everywhere. They emerge from the aftermath of a collision between an atom and a cosmic ray, typically a very energetic proton, that against all odds slams into Earth's upper atmosphere after crossing more (possibly much more) than a trillion miles of space. About 10,000 muons hit a square meter of the Earth's surface every minute.

In the mid-1960s, Luis Alvarez—Berkeley physics professor, Nobel Laureate, and one of the proposers of the theory that it was a giant asteroid that did the dinosaurs in—built the first high-angular-resolution system that could detect the direction a muon was going as it entered his detector. He and his collaborators used this system to search for hidden chambers within the second-largest pyramid in Egypt, the Pyramid of Chephren.

Like electrons, muons are unaffected by the strong force that holds a nucleus together, although the charged muon does feel the electric field produced by the protons in the nucleus. The result is that if a muon passes close to a nucleus, it's likely to be deflected from its line of travel by a small amount (small angle scattering). The denser the material, the more deflections and the greater the scatter. On the flip side, if a muon travels some distance through a hidden room instead of solid rock, it deviates less from its initial trajectory.

"It worked," recalls Morris. "They were clearly able to see the outlines of the rock structure. They could even discern the six inches of limestone that remains on the pyramid's cap. Too bad they didn't see any hidden chambers."

Fast forward approximately forty years to a world that depends on the peta-scale movement of goods around the globe, and lives with the prospect of a terrorist group detonating some type of nuclear or dirty radiation bomb in a population. Heavy nuclei contain many protons and will redirect muons through larger deflection angles more often than will light nuclei. And hardly any material has more protons per nucleus than plutonium.

With an ever present, natural particle source, µTom can be set up anywhere in the world. On the other hand, the low natural muon flux (compared to an accelerator source as in pRad) limits the amount of information one can gather about the object in a reasonable amount of time. Thus, µTom makes no sexy radiographs showing clearly defined chunks of fissile material, but it provides enough information to identify material that should never enter this country.

Pyramid of Chephren photo

(A) Because high-energy muons are created continually in the atmosphere, muon tomography can be set up anywhere. Back in the mid-1960s, it was used to look for hidden rooms within the Pyramid of Chephren in Egypt. (B) It can also be used to look for special nuclear materials hidden in everything from large shipping containers to small vehicles.

Super Electrons?

Yet another type of radiography is being developed jointly by scientists at Los Alamos and the Idaho Accelerator Center. Electron radiography, or eRad, is ideal for imaging the interiors of thin objects (less than a few millimeters thick).

Scientists at Los Alamos are looking into employing eRad simultaneously with x-ray radiography in their proposed MaRIE facility. The goal for MaRIE is to obtain an unprecedented understanding of material behaviors within environments that range from the ordinary to the extreme, thus paving the way to develop the next-generation materials that will likely be needed to sustain society's technological growth.

For a slab-like target, high-energy x-rays would pass through the wide dimension of the slab and provide information about micron-scale material properties, such as what happens to crystal domains or grain boundaries as the target is blown up, crushed by shock waves, or stressed in some other extreme manner. High-energy electrons would pass through the slab's thin dimension, providing information on surface properties or material interfaces.

"It's also possible to simultaneously probe the sample with pRad," says Frank Merrill, one of the developers of eRad. "That would allow us to observe how a severely-stressed material behaves over four orders of magnitude, from microns to centimeters, or over what is likely to be all relevant length scales."

Even if eRad isn't used at MaRIE, that will hardly dampen the enthusiasm of the ordinary, largely unacknowledged non-superheroes who, through mostly personal dedication, developed an extraordinary technology.

"If you think about it," says Saunders, "we developed pRad to address a need of the weapons community, and now it's being used to study unclassified, non-weapons-related objects. The knowledge gained from refining pRad was then used to turn muon tomography—developed for scientific reasons—into a viable means to screen cargo for special nuclear materials. Whichever side of the fence you're on, charged-particle radiography is a terrific example for how the pursuit of fundamental science can benefit society in unforeseen ways." Super rad, man!

—Jay Schecker

(Left) The Los Alamos National Laboratory acronym was the first radiograph made with electron radiography. Each letter is about 0.08 inches wide. (Right) The 1-inch wide "eRad" sign has letters less than 0.001 inch thick. The handwritten letters are visible because they were written with the Pilot Gold Metallic Marker, which contains 15–25 percent copper.

 

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