Successful launch at Oak Ridge: six labs collaborate on the largest unclassified science construction project in the United States. SNS: By Bill Cabage Neutrons for ‘molecular movies’

Image source: ORNL, Sandbox Studio 14 The scene is reminiscent of Mission Control for a 1960s Why neutrons? Why the SNS? space shot—minus the skinny ties, pocket-protectors, When a fast particle, such as a high-energy proton, bom- and cigarette smoke. But the intensity in the control bards a heavy atomic nucleus, some neutrons are “spalled,” room at 1:30 in the afternoon on Friday, April 28, 2006 or knocked out from the nucleus, in a nuclear reaction approximates that of a lunar landing. called spallation. Other neutrons are “boiled off” as the The eyes of operators, project managers, and a few bombarded nucleus heats up. It’s something like throwing special guests track the numbers and graphs on control a baseball at a bucket of balls, resulting in a few being monitors. DOE’s Spallation Neutron Source at Oak immediately ejected and many more bouncing around and Ridge National Laboratory is about to attempt to send falling out. For every proton striking the target nucleus, a beam of trillions of protons to a target. If the effort 20 to 30 neutrons are expelled. Wave guides channel succeeds, it will mark the completion of a seven-year, beams of spalled neutrons to instruments that probe mate- $1.4 billion effort, the United States’ largest unclassified rial structures and properties. science construction project. Neutrons are abundant in the universe, making up Among the furrowed brows in the control room, Jack more than half of all visible matter. But for research on Carpenter’s smile stands out. Wearing a navy blazer, physical and biological materials, neutrons of the right with an expensive-looking camera around his neck, brightness are in short supply. Just as we prefer a bright Carpenter is a special guest. He gets credit for proposing, light to a dim one to read the fine print in a book, back in the 1970s, the idea of a spallation neutron source researchers need a bright source of neutrons that will that would combine the advantages of a pulsed neutron give detailed snapshots of material structure and make source with time-of-flight neutron scattering instruments— “movies” of molecules in motion. The SNS will provide the key concepts at the core of the SNS. He’s often these bright neutrons. Like a flashing strobe light provid- come to Oak Ridge from Argonne National Laboratory to ing high-speed illumination of an object, the SNS will help with the project. Now launch day is here. produce pulses of neutrons every 17 milliseconds, with symmetry | volume 03 issue 05 june/july 06

15 more than 10 times more neutrons than are pro- The applications of neutron scattering seem duced at the most powerful pulsed neutron almost limitless. Neutron scattering probes the sources currently available. Like water spraying behavior of internal magnetic fields in advanced from a rock splashed by a garden hose, neutrons high-temperature superconducting materials from a beam “scatter” from a target material such as yttrium-barium-copper-oxide, allowing in a way that reveals its structure and properties. scientists to view these fields directly. Neutron Why pulsed neutrons? To analyze the results scattering also reveals the structure and molecular- of neutron scattering, scientists need to know level dynamics of semiconductors used in the the initial energy of the neutrons spraying off the race to develop new materials for the electronics material they are studying. When all the neu- industry. Small-angle neutron scattering—aiming trons leave the starting gate at the same time— the garden hose just so—can reveal clusters as in a pulse—the time it takes each neutron to small as 50 atoms that form, for example, in the reach the target material—its time of flight—is steel of reactor pressure vessels after years of known, revealing its velocity and hence its exposure to radiation from the reactor core. The energy. With this key piece of information on neutrons can show whether heat treatment neutron energy, scientists can interpret what removes defects from the irradiated steel, making neutron scattering is telling them about a mate- it less brittle and less susceptible to failure. rial under study. In contrast, neutrons from a reactor leave the source continuously, rather Something new? than in a single pulse, making time-of-flight With a beam power of 1.4 megawatts, the SNS will energy determination impossible. become the world’s leading facility for neutron Although there are fewer accelerator-based scattering research—eight times more powerful neutron sources than reactor-based sources, than next-brawniest ISIS. A power upgrade and accelerator-based pulsed spallation sources second target station—with a different assortment represent the state of the art. Japan’s J-PARC of specialized instruments to add to its 24 beam- will have a spallation source coming on line in lines—are already on the DOE Office of Science 2008–2009 with one-megawatt power. The drawing board. UK’s Rutherford Laboratory is upgrading the The development of the neutron as an ana- ISIS facility to give it a power boost and a lytical tool came largely as a spin-off from World second target station. War II weapons research. The initial discoveries

Deuteron

Secondary Neutron

Primary Proton Nucleus γ Ray

π Particle Spallation Product

To produce neutrons, scientists smash protons into a material α Particle made of heavy atomic nuclei, which contain many protons and neutrons. Each collision shakes loose some neutrons and other particles, a process called spallation. The secondary particles hit surrounding nuclei and create even more neutrons.

Source: ORNL

16 were made at Oak Ridge, originally a pilot plant Six-lab partnership for the reactors that would produce plutonium The SNS rose from the ashes of a reactor project. for the Manhattan Project. Scientists working at Scientists had proposed the Advanced Neutron Oak Ridge’s Graphite Reactor realized that light Source, a research reactor to succeed ORNL’s atoms as well as heavy ones could be probed High Flux Isotope Reactor, with state-of-the-art by exposure to neutrons. Two researchers, Ernest beamlines and instrumentation tailored particularly Wollan and Clifford Shull, approached ORNL’s to the mission of neutron research. When con- then-scientific director, the esteemed Eugene cerns over its $3-billion-plus cost and highly Wigner, with their work. “Maybe there is something enriched uranium fuel doomed the project in the new here,” Wigner remarked. mid-1990s, DOE’s Office of Science decided Now neutrons are a favored tool for investi- to pursue instead a more affordable spallation gating the properties of materials. Because a source located at Oak Ridge. neutron is electrically neutral—hence its name—it Project organizers developed an innovative can penetrate deep into a material. Unlike the plan to involve five DOE national laboratories in case of X-rays, the probability of neutron scatter- the design and construction of the project. ing does not depend on the number of protons The original partnership of Lawrence Berkeley in a material’s nucleus, so it can probe light ele- National Laboratory, Los Alamos National ments as well as heavy ones. Neutrons easily Laboratory, Brookhaven National Laboratory, reveal hydrogen atoms, a capability especially Argonne National Laboratory, and Oak Ridge useful in studying biological materials. National Laboratory represented one of the Neutrons are magnetic; the size and direction largest of its kind in US scientific history. Each of a neutron’s magnetic field is called its magnetic laboratory would design and build a key SNS moment. Beams of neutrons can be polarized component. Oak Ridge, the home base, would according to their magnetic moments and used put them all together and make it work. to investigate the magnetic properties of materi- als. The range of energies represented in thermal and chilled neutrons (those with short and long Top: Operators remotely maintain the SNS target wavelengths, respectively) makes them ideal for area. Bottom: The linear accelerator of the SNS is analyzing soft materials such as proteins and 330 meters long. polymers. Photos: ORNL The SNS will provide neutrons that are bright enough to create detailed characterizations of material structures, from crystals to DNA mole- cules; and to make “movies” of molecules in motion. Neutrons complement X-rays in studying proteins for critical information in pharmacology, agriculture and biotechnology. Determining the structure of enzymes in the human body, for example, will speed the development of more effective drugs. symmetry | volume 03 issue 05 june/july 06

17 Work began at the end of 1999. During the Researchers will use the Powder Diffractometer, planning process, advances in superconducting the most flexible and versatile of its kind, for study- technology led to the addition of a supercon- ing small samples and performing parametric—or ducting section to the SNS’s linear accelerator, effect-over-time—experiments. The Backscattering which would operate at a temperature two Spectrometer will probe atomic-scale dynamics degrees above absolute zero and greatly improve at high resolution, providing data up to 100 times the machine’s efficiency. Thomas Jefferson faster than existing instruments. Probing the National Accelerator Facility, which helped pio- dynamics of atomic or molecular motion is impor- neer superconducting accelerator technology, tant for materials with large surface areas. would deliver the cold section, completing the Researchers will use the Ultra-High Pressure six-lab collaboration. Diffractometer to study atomic structures at An SNS first is its target station, a modular ultra-high pressures equal to those found deep unit that recirculates 20 tons of liquid mercury. in the earth or in brown-dwarf stars. A Disordered Designers realized that mercury’s neutron-rich Materials Diffractometer will be used for small- atoms would be ideal receivers for the SNS’s angle studies of atomic structures of glasses and powerful, pulsed proton beam, which spalls neu- liquids, while the Single-Crystal Diffractometer trons from the nuclei of mercury atoms. will be optimized for rapid data collection from very small crystals, crucial for characterizing the Instruments of discovery large number of materials for which it is impos- Scientists from around the globe will come to sible to grow large crystals. do experiments using the instruments stationed The SNS will have a class of spectrometers on the 24 SNS beamlines. Some 2000 users called “choppers,” so named because a mechan- annually will be selected by Oak Ridge through ical device interrupts the neutron beam and a peer-reviewed proposal system. allows only neutrons of a certain energy to reach The SNS will have two highly specialized the sample. The speed of the neutrons as they instruments known as reflectometers. The bounce back from the sample gives researchers Magnetic Reflectometer is designed for studies the data they need to analyze the characteristics of thin films; the Liquids Reflectometer will, not of the materials. surprisingly, study liquids. The SNS beam intensity These chopper spectrometers offer research- will allow the added dimension of “off-specular” ers enhanced sensitivity, broader ranges, and measurements—that is, measurements of molecular structures parallel as well as perpen- dicular to the surface. The Extended Q-SANS (Small Angle Neutron Scattering) Diffractometer can cover a range of spatial scales from several hundred angstroms to a fraction of an angstrom in one measure- The SNS target station circulates 20 tons of liquid ment. The Extended Q-SANS instrument, with mercury, a material that contains a large number its ability to analyze multiple scales at once, of neutrons per atom. When hit by protons, neutrons will be especially suitable for studying complex emerge from the liquid and travel to the surrounding experimental areas with their instruments. materials, particularly biological samples such as cell components. Photo: ORNL

18 improved intensity, in some cases by factors of Staffers gather in the lobby of the SNS Central approximately 100. They include the High- Laboratory. Project Director Thom Mason and Resolution Fermi Chopper Spectrometer, opti- ORNL Director Jeff Wadsworth stand on a stair- mized for single-crystal studies; the Wide-Angle well holding plastic flutes of champagne to lead Fermi Chopper Spectrometer, a lower-resolution the gathering in a toast to success. The Physics- instrument for wide-angle studies; and the 10-100 Diagnostics Station electronic logbook entry Microvolt Multi-Chopper Spectrometer, a flexible reads: “Here is what so many have labored toward general-purpose analytical tool capable of sending for so many years.” up to 100 times more neutrons to the sample. Clifford Shull, who, in the 1940s, along with Finally, the SNS will have a beamline dedicated Ernest Wollan saw the potential of neutrons for to the study of the neutron itself, addressing exploring the structure of materials, lived to questions about the big-bang theory and phys- share the 1994 Nobel Prize for his work in neu- ics beyond the Standard Model of particles and tron scattering. Shull also saw the beginnings their interactions. of the new neutron research facility on a ridge top in Oak Ridge overlooking the old reactor Wigner was right where he had worked just after the war. As the At 3:30 p.m., the control room at Oak Ridge breaks prescient Eugene Wigner foresaw, the SNS out in a roar. There is lots of back-slapping, hand- represents a long-awaited “something new” for shaking and cheering. The SNS, an hour and a the neutron science community. half after producing its first neutrons—and a first “We’ve come a long way,” says Jack Carpenter, round of cheers—has delivered its 1013 protons the control-room guest who proposed the SNS to the target—the beam power mark that signals idea more than 35 years ago. “I would have never official completion. envisioned how far.”

Instrument

The Spallation Neutron Source will deliver high- intensity neutron beams to a variety of instruments that feature special detectors to examine the Sample interaction of neutrons with biological samples or other materials. Detectors

Source: ORNL

Neutron Guides

10–80 meters

Moderator A moderator slows down the neutrons produced in the spallation process and illimi- nates a neutron guide. symmetry | volume 03 issue 05 june/july 06 Target Nucleus

Energetic proton When a high-energy proton bombards a heavy atomic nucleus, causing it to become excited, 20 to 30 neutrons are expelled.

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