Berkeley Lab Part of Collaboration to Build Spallation Neutron Source

October 17, 1997

By Paul Preuss,

Berkeley Lab is playing a vital role in a five-laboratory collaboration aimed at bringing the United States back to leadership in neutron science after two decades of playing catch-up with Europe, a goal that Energy Secretary Federico Peña has put at the very top of his list of the department's energy-research priorities.

Members of the Ion Beam Technology Program in the Accelerator and Fusion Research Division (AFRD) will build the "front end" -- the ion source, radio-frequency quadrupole, and associated beam transports -- of the $1.3 billion accelerator-based Spallation Neutron Source, or SNS, to be located at Oak Ridge National Laboratory in Tennessee.

The planned $1.3 billion accelerator-based Spallation Neutron Source (SNS) at Oak Ridge National Laboratory will have a front end (right) provided by Berkeley Lab

Testifying before Congress in March of this year, Secretary Peña enumerated some of the fundamental discoveries and practical benefits offered by neutron science: "Chemical companies use neutrons to make better fibers, plastics, and highly efficient and selective catalysts; automobile manufacturers use the penetrating power of neutrons to understand how to cast and forge gears and brake discs...; airplane manufacturers use neutron radiography for nondestructive testing of defects in airplane wings, engines, and turbine blades; and drug companies use neutrons to design drugs with higher potency and fewer side effects."

The primary mission of the SNS is to characterize materials; once in operation, it will have an anticipated 1,000 users a year. AFRD's Jose Alonso, who is coordinating all accelerator and target elements for the SNS project, explains why neutrons are uniquely valuable to materials-sciences investigators.

"The closest analogy is to x-rays, but while photons interact with an atom's electrons, neutrons interact with its nucleus. If you were to look at a uranium-oxygen compound with x-rays, the uranium has so many electrons that the oxygen would disappear in the noise. Neutrons, on the other hand, pick out light elements -- hydrogen has a strong signal in neutron scattering--so the oxygen would stand out. Knowing where the oxygen sits is vital to understanding such compounds as high-temperature superconductors of the barium-copper-oxygen type, and indeed, neutrons unraveled that puzzle."

Alonso notes that while x-ray sources will always be much brighter ("neutron sources and bright light sources like the ALS are truly complementary," he said) neutrons have other advantages: "They have a magnetic moment and can measure a material's magnetic properties, something that is very difficult to do with photons. They penetrate more deeply than electrons, so they can look into bulk materials. The effective energies of research neutrons are very low, measured in milli-electron volts, close to the phonon excitation energies in crystals, which means you can look at the dynamics of crystalline materials, not just at their structures." In chemistry, biology, earth and environmental sciences, solid-state physics--virtually every science dealing with real-world problems--neutrons promise to open new research frontiers.

Accelerators were the first sources of copious quantities of the remarkable particle that James Chadwick discovered in 1932, a trend that culminated in the 37-inch cyclotron built by Ernest Lawrence and his colleagues in 1938. During World War II and into the 1970s, U.S. fission reactors dominated the field, notably those built at Oak Ridge and Brookhaven National Laboratory. Inexorably the lead passed to reactors built abroad. Recently, powerful accelerators like ISIS in England and LANSCE at Los Alamos have become important sources of intense neutron beams.

Rick Gough of the Ion Beam Technology Program explains that while reactors are capable of a high-quality, high-flux, continuous flow of neutrons, accelerators complement reactor sources by providing pulsed beams; the pulses are exceedingly bright and, because time-of-flight measurements are possible, the energy and wavelength of the individual neutrons can be determined and selected, making pulsed beams "user friendly" in important ways.

The SNS is poised to begin construction in fiscal year 1999 and is scheduled for completion at the end of 2005. It will be built by what Martha Krebs, Director of the Office of Energy Research, has called "a system of laboratories"--five national labs each taking responsibility for a different piece of the machine. The front-end team led by Gough is responsible for delivering a beam of 2.5 million electron-volt negative hydrogen ions to a half-kilometer-long linac to be built by Los Alamos National Laboratory. After the ions are accelerated to a billion electron-volts they will be transported to an accumulator ring--and stripped of electrons in the process -- to be built by Brookhaven. From there a one-megawatt beam of short-pulse protons (less than a microsecond's worth per pulse) will be routed to the target area to be built by Oak Ridge.

Jose Alonso compares the multi-part arrangement to a strobe light: "The linac is the battery, the accumulator ring is the capacitor, and the charge is released all at once in a really bright flash."

The target is a self-cooling flow of liquid mercury circulating past the proton-beam window at a rate that has been described as "a Volkswagen per second." When the protons hit the mercury the result is spallation, a word derived from chipping or cracking a stone but adapted by Glenn Seaborg in the late 1940s to describe what happens when an energetic particle blows a nucleus apart. One result: "You get lots of neutrons," Rick Gough remarks, deadpan.

The business end of the SNS is the research area, with instrument stations built by Oak Ridge and Argonne National Laboratories. The neutron bunches arrive 60 times each second, but to be useful, the neutrons have to be slowed to thermal velocities -- reduced to a billionth of their energy -- by moderators of water and liquid hydrogen placed around the target. Time-of-flight measurements are used to precisely calibrate the wavelengths of the neutrons arriving at the research instruments, which assures great versatility in meeting research demands. To the 10 instruments available when the source turns on in 2006, one or two others will be added each year, up to a total of 18 beam lines.

The SNS has been designed to upgrade power and capacity quickly and inexpensively so that users experience minimum disruption; even anticipated major upgrades will require no more than six months' downtime. Within that time-frame the power will be increased--first to two, then to four megawatts or more by adding more linac radio-frequency power and a second accumulator ring. A second target and experimental hall will also be added. Such improvements will help the SNS sustain a competitive edge, even if powerful European and Japanese spallation accelerators now on the drawing boards are built in the next century.

The Department of Energy recently gathered a group of 65 reviewers to review the SNS design--"so many we couldn't outnumber them," Gough commented. The group, chaired by the Energy Research Office's Director of Construction Management, Dan Lehman, endorsed the technical choices and agreed with the budget estimates. Thus the SNS is firmly on track.

Rick Gough says the Ion Beam Technology group's expertise with ion sources, radio-frequency quadrupoles, and the manipulation and transport of ion beams at low energies "positioned us well to respond" to the needs of the front end of the SNS. Moreover, Jose Alonso, past manager of the Bevalac and a long time champion of innovative applications of accelerators -- including advanced accelerator-based neutron sources -- is now commuting between Oak Ridge and Berkeley. Gough says, "We think of him as a kind of personal contribution from Berkeley Lab to the SNS."

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