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Why Files
The more infamous nuclear reaction.

X-rays see the little and the not-so-little.

Composite materials.

Underground lab flooded.

SNS, the collaborative efforts of six national laboratories, will provide the most intense pulsed neutron beams in the world. Map: SNS.

Neutron diagram: SNS.

All photos this page (unless otherwise noted) by Sarah Goforth, on
assignment for The Why Files.

The Why Files got a peek at the place in October, during the annual meeting of the Council for the Advancement of Science Writing. And (forgive us for a little slip in journalistic detachment) trust us. It is a sight to behold.

But before we dazzle you with the details, a little background. The point of SNS is to let scientists see very small things very close up. We're talking molecules and atoms as well as crystals and cells. In the 1940s and 1950s, researchers discovered that when they shot neutrons at hunks of material or solutions of molecules, the neutrons scattered in a way that revealed the properties of the sample.

Neutrons are electrically neutral, and they have spins and are sensitive to magnetic fields. This impartial personality allows the tiny particles to scatter from the nucleus (the most interior part) of atoms without changing the atoms' qualities. Charged particles like protons and electrons, contrarily, can only interact with the shell. It's like seeing the cover without reading the book.

Unlike the other basic particles, neutrons have no electric charge. Neutrons also have what is known as a magnetic moments: When in a magnetic field, they twirl. By virtue of this property, scientists can bounce neutrons off materials to understand their magnetic properties.

We may not be as aware of it as gene chips and telescopes, but chances are that neutron scattering research already makes your life easier. Among the technologies that have benefited are jets, credit cards, CDs, shatter-proof windshields, and satellite weather information.

In fact, there is a striking range of applications:

Biology - Neutrons can "see" the light elements -- namely, hydrogen and oxygen -- that pepper nearly all biological materials. What structural biologists know about the heavy-element backbones of molecules thanks to synchrotron radiation, explains Geoff Greene, a physicist at Oak Ridge National Laboratory, neutrons can reveal about the light elements. Synchrotrons use X-rays to see patterns in the electrons that circle the outer portion of atoms.

"Neutrons, on the other hand, barely see the electrons, but they really see nuclei. Both are at a resolution of a few angstroms, so you've got the perfect match. X-rays to tell you where the electrons are and neutrons tell you where the nuclei are. Now you have a chance to see the whole shebang," says Greene.

Engineering - SNS scientists will be able to measure the energy and magnetic properties of a given pulse as the neutrons enter, and after they bounce off, a sample. Watching how the neutrons change can reveal valuable data about how materials react to stress. Such information will help predict, among other things, how cracks will form in critical welds on aircraft or spacecraft.

Soft materials - "Physicists like to divide matter into hard material -- like metals and ceramics -- and soft material like plastics," explains soft material man Rob Briber of the University of Maryland. "SNS has great opportunities to allow the study of soft material with much greater resolution and speed."

Superconductor Science - Neutron scattering is the traditional tool for superconductors research, explains Thom Mason, director of SNS. And with more intensity and higher resolution comes better data, and better superconductors.

Chemistry - A big question in chemistry is how molecules pass through biological membranes, the fatty films that hold our cells together. Since enzymes and hormones must pass through these membranes to do their jobs, understanding how the process works will be key in drug development.

The cosmos - As all armchair philosophers know, the best way to strengthen a theory can be to attack it face on. And that's just the approach taken by ORNL researcher Geoff Greene when it comes to deciphering the secrets of the cosmos. Instead of using neutrons to probe tiny samples, he and other physicists will use SNS to decipher the properties of the tiny particles themselves -- and in the process, poke holes in the Standard Theory, the closest physics has come to explaining everything.

Thomas Mason, director of SNS, waves an arm over the construction site near Knoxville, Tenn.

Neutrons aren't easy to harness, but there are two ways to do it. First, nuclear reactors like the one at Oak Ridge produce a steady stream of neutrons via nuclear fission reactions. The second way, producing neutrons in pulses instead of a steady stream, is called spallation. Each technique has advantages, Mason says, and they are complementary. But reactors tend to be more expensive and environmentally risky -- and for these reasons, politically charged. They remain plenty useful, but spallation sources are now the rage.

But partly because they are so expensive to build and maintain, no new neutron sources have been built in the U.S. in more than 30 years. This means that, for years, neutron scientists have gone abroad to conduct research at more modern sources. Europe now has the two best neutron sources in the world,the Institut Laue Langevin in Grenoble, France, a steady-state reactor, and the ISIS spallation source at the Rutherford Laboratory near Oxford in the UK.

Researchers have been able to use neutrons in this way for decades, but like most kinds of technology, neutron science evolves. The older tools, like the neutron source at Los Alamos National Laboratory don't exactly retire, but the newer generation offers science that just wasn't available before. The advantage of SNS, says Greene, will be the intensity of its neutron beams. "If you're a pollster trying to learn about public opinion," he explains, "it's much more useful to ask 10,000 people than to ask three. There's less uncertainty. SNS will produce a lot of neutrons, so we'll see a lot more." In fact, the neutron beam at SNS will be ten times more powerful than the current strongest beam, at ISIS in the UK.

Most notably, SNS will produce more neutrons more quickly (a scientist would say "higher flux") than previously available sources. "You're making a flash of neutrons happens 60 times a second," Mason explains. Still, "Neutron sources are pretty weak. The number of neutrons you can get at the most intense reactor now is a smaller flux than the flux of photons [particles of light] off a candle 15 feet away."

Greene explains it this way: "The high flux of the SNS will provide better statistics. There are a variety of schemes we can use to exploit this nature to make sure we're asking, and answering, the right questions."

The higher flux will let researchers do slow experiments faster, and probe smaller samples. The latter quality will be of particular interest to structural biologists who study proteins, Mason says, because some proteins are too tiny to elucidate with current methods.

This copper tube -- called the drift tube linac -- is the front end of the SNS linear accelerator. Before any neutrons can be produced, hydrogen ions stream through this 1000-foot-long tube, reaching a whopping 2.5 million volts.

It's just what it sounds like
The word spallation, Mason explains, comes from the German describing what happens when a ball peen hammer strikes rock, sending pieces flying. Similarly, when a heavy metal "target" is bombarded with very fast-moving, very energetic protons, it spews out neutrons. "Our ball peen hammer is a billion-volt proton accelerator," Mason says.

At SNS, hydrogen ions (basically, a proton encircled by two whirling electrons) is plunged through a linear accelerator, gaining energy and speed -- up to 90 percent the speed of light -- on the way. Because protons are charged, they are easy to accelerate. At the end, a graphite foil strips the ions of their electrons (with "brute force," says Mason), leaving behind bare protons that then speed into the sweet spot -- a giant vat of mercury.

Every proton knocks out 20 to 30 neutrons into the immediate region around the target, where the neutrons are cooled to the most useful energy range. The result? Neutron pulses, or waves, with wavelengths between half an angstrom and 20 angstroms -- comparable to the spacing between atoms in materials, Mason explains.

To sort out charged particles, the spray will pass through a large magnetic field, where charged particles will be deflected but the neutrons will hurtle through. From there, they will be channeled into a series of pulsed beams, each of which throbs forward to an instrument tailored for a particular kind of experiment. There are 16 such approved instruments being prepared for installation at SNS, but a decade from now there may be as many as 50.

The second half of the linear accelerator, where hydrogen ions dash toward a mercury target.

Spallation reactions produce neutrons in all kinds of energetic states. That's key. Scientists can tell how energetic any given neutron is, based on how fast it whizzes away from the site of spallation. Because low-energy neutrons are useful for some experiments and high-energy neutrons are suited for others, spallation enables many kinds of experiments to be conducted at once.

Mason hopes SNS will reinvigorate the American neutron scattering community and attract scientists from new fields, and from abroad. A similar spallation source is in the works in Japan, and a proposed spallation source in Europe is in "a holding pattern," he says. Does Mason expect such sources to be competitors or collaborators? "Both. But both are healthy for progress."

Also healthy, he says, is the collaboration going on within SNS. The project is a joint effort of six national Department of Energy laboratories, with each lab donating time and expertise to construct a piece of the whole. Science is increasingly an interdisciplinary, and international, effort, Mason points out. "SNS is an exciting model for what's to come."

-- Sarah Goforth

Bibliography
"Green light for long-awaited facility," A. Lawler, Science, 1/23/98.

"The case for neutron sources," B. Keimer et al., Science, 10/18/02.

"The spallation neutron source is taking shape," T.E. Mason et al., Appl. Phys. A, Issue A 74, 2002.

"Spallation Neutron Source," M. White, Advances in Cryogenic Engineering: Proceedings of the Cryogenic Engineering Conference, Vol. 47, 2002.