Berkeley Lab Part of Collaboration to Build Spallation Neutron Source
October 17, 1997
By Paul Preuss, paul_preuss@lbl.gov
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 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."
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.
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 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
