SNS: A Camera for Molecular Structures

by Kurt Riesselmann

From shatter-proof windshields to therapeutic drugs, from compact discs to cosmetics, many products have benefited from research with one of the building blocks of atoms: neutrons. Increasingly powerful neutron sources have paved the way for scientists to design and to produce new materials and better products with a wide range of applications.

Norbert Holtkamp, head of the SNS Accelerator Division - Photo by Kurt Riesselmann

Similar to x-rays illuminating the inside of the human body, bunches of neutrons can unveil the interior of materials in a non-destructive way. Using pulsed neutron beams, scientists can even record the motion of atoms and molecules inside small samples of matter.

In contrast to x-rays, which cannot penetrate metal or other dense materials, neutrons traverse more or less all types of material, shedding light on their internal structures. For example, neutrons have been crucial in understanding how bones mineralize during development and how they degenerate during osteoporosis, a bone-demineralizing disease. The results have allowed scientists to devise and to test treatments for osteoporosis and other diseases.

"Increasingly, we are trying to master very complex materials: polymers, proteins, nanomaterials and superconductors, all of which are made of large molecules," said Thomas Mason, associate director for the Spallation Neutron Source currently under construction at Oak Ridge National Laboratory in Tennessee. "From a scientific point of view, what we need to understand is structure. Structure determines the properties of materials."

The SNS, built by the Office of Science of the U.S. Department of Energy, will be the world's premier accelerator-based neutron source, surpassing existing machines in Europe and Japan. Unlike reactor-based neutron sources, the 1.4-megawatt SNS facility, scheduled to be completed in June of 2006, will create pulses of neutrons rather than a continuous stream. Scientists will use this capability to take quick snapshots and to make "movies" of molecules in motion.

Neutron diffraction studies have revealed the structure of insulin. The studies are performed on crystals of insulin molecules containing zinc ions (balls in graphic). - Image courtesy of Brookhaven National Laboratory

The SNS pulses will contain almost 10 times more neutrons than the most powerful, pulsed neutron source in the world, the ISIS in the U.K. With the new facility, experimenters will be able to study small amounts of physical and biological materials, advancing areas of research that deal with tiny crystals and samples. The SNS also will accommodate testing of large-scale equipment such as jet engines to study deformation and failure from prolonged stress. A future upgrade, included in the 20-year DOE science facility plan announced on November 10, would raise the power of the SNS to 2-4 megawatts.

To create neutrons, the SNS will use a 1,000-foot-long Linear Accelerator that delivers a proton beam of one GeV. An Accumulator Ring at the end of the Linac, 750 feet in circumference, receives the protons at close to the speed of light, merging and compressing them into a high-intensity proton pulse that can deliver all particles within less than a millionth of a second. Sixty times per second, the ring ejects a pulse ultimately containing more than 10¹⁴ protons, all of which hit a target container filled with mercury.

"It's like a snowball hitting a wall," said Norbert Holtkamp, head of the SNS Accelerator Division. "Neutrons fly out in all directions."

The proton collisions knock neutrons out of the heavy mercury nuclei, a process called spallation. On average, each proton creates about 20 neutrons. In contrast to a nuclear reactor, a spallation neutron source creates no chain reactions. Operators can stop the neutron production at any time by simply switching off the linear accelerator that provides the proton beam.

MULTILABORATORY COLLABORATION
The construction of the SNS on an 80-acre site on top of Chestnut Ridge began in December 1999. The civil construction of all accelerator buildings is complete. The construction of the central office building and the building for the target and the instruments is in progress. According to Holtkamp, the $1.4-billion project is on budget, and more than 70 percent complete. Six DOE national laboratories contribute to the construction of the facility: Lawrence Berkeley, Los Alamos, Thomas Jefferson, Brookhaven, Argonne and Oak Ridge. ORNL is responsible for the civil construction, project management, design integration, and ultimately for operating the SNS. When completed, the SNS facility will employ about 400 permanent staff.

About 180 employees work on the construction of the SNS accelerator and storage ring, including the former Fermilab employees (from left to right) Norbert Holtkamp, David Brown, Saeed Assadi, Alan Jones, Thomas Neustadt, Hengjie Ma, Manuel Santana and Mark Champion. Not present: Wim Blockland, John Crandall, Craig Deibele, Kerry Potter, Don Richied and Ted Williams. - Photo by Kurt Riesselmann

The installation of the accelerator components is in full progress, and SNS scientists are already testing the front-end systems built by LBNL, achieving a test beam of 2.5 MeV. Right now, particles leaving the front-end systems enter the first radiofrequency drift tube linac tank produced by LANL for the first phase of acceleration that brings the beam to 7.5 MeV. Five additional RF tanks that have already arrived at the SNS site will accelerate the beam to 87 MeV.

The remainder of the Linac features one of the big technological advances incorporated in the SNS design. Final acceleration will be achieved by superconducting RF cavities from Jefferson Lab. The cavities, cooled to 2 kelvins, have a larger aperture (beam opening) than conventional cavities, allowing higher beam intensities while reducing beam losses and residual radiation. The first three of 23 cryogenic modules, each containing three superconducting cavities, are now in the SNS Linac tunnel. Each cavity will receive power from a klystron produced in industry. Scientists expect to have the first beam passing through the entire SNS Linac in March 2005. When complete, the Linac will be the second largest RF installation in the world.

The Accumulator Ring, which collects the protons, relies on equipment produced by BNL. The ring already has received 12 of the 32 modules needed to steer the proton beam around the ring and to shape its properties. Exiting the ring, the protons will fly through a small opening into the center of the target station constructed by ORNL. The target vessel is surrounded by 15 feet of steel and concrete.

Twenty-four openings allow neutrons to escape the target area and to travel to a corresponding number of experimental areas surrounding the target station. ORNL and ANL are building five of the 24 instruments required for neutron measure-ments. The other instruments, at a cost of approxi-mately $10 million apiece, will come from scientific collaborations involving universities and other scientific institutions. So far, a total of 16 instruments have been approved, none of which involve classified research. Two of the instruments will come from collaborations in Canada and Germany.

The first cryogenic tanks with superconducting RF cavities, built by Jefferson National Laboratory, are now in their final location within the SNS Linac. - Photo by Kurt Riesselmann

Experts from Fermilab
Although Fermilab is not an official member of the SNS project, it provides significant expertise in form of employees and technical advice. At least 14 people working in the SNS Accelerator Division are former Fermilab employees, including Holtkamp. Since the SNS project includes the construction of the first large accelerator in the history of Oak Ridge National Laboratory, the influx of accelerator experts is more than welcome. Alan Jones, who joined Fermilab in 1972, now is breaking new ground as part of the SNS team.

"The most exciting thing is to switch things on for the first time and see them running," he said. "Right now, we've got the SNS front-end systems running. We've produced beam."

Of course, work at the scientific frontier on a not-yet-complete research campus has its challenges.

"At Fermilab, there was a procedure for everything," said Mark Champion, recalling manuals and documentation associated with the scientific equipment at Fermilab. Champion, who has worked at several accelerator labs, also remembers another aspect that made work easy at Fermilab: "Fermilab has a great stockroom."

A 1-GeV proton beam will hit a mercury target at the center of this vessel, creating neutrons flying in all directions. Twenty-four "windows" allow neutrons to travel to experimental set-ups surrounding the vessel. - Photo courtesy SNS

Other former Fermilab employees responded with their own memories of a good work environment: the Fermilab Users Center, the library, the recreational facilities, the interaction with people in the cafeteria, the strong sense of community. With the main SNS office building still under construction and almost none of the expected 2,000 users on site, the SNS employees will have to wait a little longer for better amenities and a more vibrant science community. This doesn't diminish the overall success of the SNS project so far.

"The multi-laboratory SNS partnership will likely be a model for future large science projects," said Mason. "It will be a model for ITER, the Next Linear Collider, and other large science facilities listed in the 20-year DOE plan."

Oak Ridge National Laboratory
www.ornl.gov