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A revolutionary target station and 24 instruments will reshape the study of materials.
The Spallation Neutron Source is the first scientific facility to use pure mercury as a target for a proton beam. ORNL designed and built this "first"—a mercury target in which neutron beams are produced by spallation for use by researchers wishing to analyze materials samples. "We were ready to take beam on our target in April 2006," reports Tony Gabriel, one of the principal developers of the mercury target. "Like the rest of the SNS project, the mercury target was designed and built on time and within budget." The SNS will soon become the most powerful, pulsed-neutron source in the world for materials and biological research. One of the principal responsibilities for ORNL staff was the development of the target, where accelerated protons flung from the SNS accumulator ring unleash beams of neutrons by the process of spallation. This specific task was in addition to ORNL's other responsibilities of managing the entire project, helping develop neutron beam instruments for experiments, and ensuring the SNS works properly by linking the subsystems designed and built by five other Department of Energy national laboratories. Over the next two years the beam power at SNS will be gradually increased to its full operating level of 1.4 megawatts. This slow ramping up of beam power will give the SNS staff experience in operating at new power levels as well as insight into processes that limit the lifetime of the target. The principal ORNL developers of the target are optimistic that over time the target lifetime will be increased to support reliable user operations and the eventual power upgrade. A Mercurial Solution One day in 1995 Gabriel was invited to a meeting on neutron-source targets in the office of former SNS director Bill Appleton. Present at that meeting was Günter Bauer, a German visitor who was spearheading the development of the European SNS (which was never built). The three men discussed the best-known target options—tantalum and tungsten, which are solids, and mercury, the only metallic element that is liquid at room temperature. Bauer, who had pondered all three elements, convinced Appleton and Gabriel that a mercury target should be developed for the SNS.
"With 120 neutrons and 80 protons, mercury has a high atomic number, making it a source of numerous neutrons," Gabriel says. "Mercury has a high density, which means the neutron brightness can be maintained. Because mercury is liquid at room temperature, mercury does not have to be heated to make it flow. Also, mercury is better than a solid at dissipating the heat deposited in the target by the proton beam. Unlike a solid target, liquid mercury is not damaged by radiation and does not have to be cooled if the proton beam should suddenly shut off by accident." The mercury target, which is only one cubic meter, will be bombarded 60 times a second with a proton pulse lasting less than a microsecond and consisting of 1014 protons. The bombardment of the mercury target with protons dumped from the accumulator ring will lead to the transmutation of only 0.1% of the mercury into radioactive isotopes, which emit gamma radiation. The 20 tons of mercury circulating at one meter per second in the loop of pipes connected to the target module will become radioactive. But this mercury will be recycled, which will prevent escape into the environment. When the target module reaches the end of its useful life, the mercury will be drained into a storage tank and the module will be retracted via cart into a shielded bay, where the module will be removed from the mercury system's permanent portion. After a new module has been installed through the use of a remotely controlled servomanipulator's robotic arms, the stored mercury will be pumped into the new vessel. Operators in the control room, separated from the target service bay by 40 inches of high-density concrete and lead glass windows, will remotely replace the target module and other important hardware, such as the motor that drives the pumps that circulate the mercury completely through the loop in one minute. The mercury loaded in the target in December 2005 was delivered from the Oak Ridge Y-12 National Security Complex, where the liquid metal had been stockpiled for more than 40 years. The Pits The goal of ORNL's mercury researchers has been to design a mercury target that lasts for weeks instead of days. Because replacing a mercury target requires a week, the best way to reduce downtime is to keep the target in operation as long as possible for SNS users. "Our research indicates that the target module will last only two weeks if the SNS were to operate at full design power of 1.4 megawatt in 2006, which fortunately for us is not the case," Gabriel says. "We had hoped that the target module would be good for 1250 hours, or about seven weeks, before it must be replaced."
To help address the issues affecting target lifetime, the ORNL researchers designed, built, and operated two test facilities, collected data from reactors and accelerators at ORNL and two other national laboratories, and obtained insights from a dozen international collaborators. Experiments at a Los Alamos National Laboratory accelerator confirmed predictions that, when short pulses of protons bombard a mercury target, the mercury is heated so rapidly that a pressure wave is generated. Similar tests at Brookhaven National Laboratory confirmed these findings. In 2000 Japanese researchers demonstrated that pressure waves produced by only 100 or fewer pulses on an off-line impulse test apparatus can pit the wall of a stainless-steel container. ORNL researchers quickly prepared for and conducted a series of in-beam tests at Los Alamos, which confirmed that pitting damage occurs after only 100 proton pulses. "We became aware that the pressure wave could cause cavitation-induced erosion after only a few pulses," Gabriel says. "The pressure wave produces bubbles that collapse near the vessel wall, pitting the inside surface of the target module." A visiting scientist from Germany suggested that ORNL try "kolsterizing" the inside surface of the mercury target module to make the stainless-steel wall more resistant to pitting. ORNL researchers believe kolsterization, a low-temperature carburization method used to treat stainless steel for commercial products, might extend the life of the SNS target module. Researchers have found that another way to lengthen target life is to reduce cavitation-induced erosion by mitigating the high-energy pressure wave. "The Japanese SNS will operate at 20 hertz and our SNS will operate at 60 hertz," Gabriel says. "During laboratory testing with their mechanical impulse device, the Japanese found that increasing the frequency level from 20 to 60 hertz substantially reduced the potential for cavitation-induced erosion." Gabriel believes that operating the Oak Ridge SNS at 60 Hz may generate more bubbles in the mercury, making it "spongy" so as to soften the impact of the pressure wave on the module wall. Researchers are now testing their ability to inject bubbles into mercury at ORNL's Target test facility. The facility's original purpose was to help researchers design and test remote-handling techniques and gain experience at operating a large mercury loop. Another ORNL facility, the High Flux Isotope Reactor, was used to test the ability of the target module's stainless steel to resist damage from neutrons released from the mercury by spallation. HFIR's neutrons hardened the steel. "We've had ups and downs in this project over the past 10 years, but we're in an upswing now," Gabriel says. He is optimistic that ORNL researchers can transfer their gift of time to the target. Translating Power into Discovery When the highly energetic pulses of neutrons leave the target, they are traveling too fast to be useful for research. The neutrons are slowed down roughly 1000 times, however, through collisions with molecules of liquid in four moderators designed and built by ORNL staff. One moderator is filled with water. The other three contain supercritical hydrogen. The SNS Target Building can accommodate 24 research instruments at the end of neutron beam lines of different lengths that link to the mercury target and moderators like spokes of a wheel. The size and nature of the instruments are related to the specific material and properties that researchers wish to examine. To guide neutrons from the target-moderator assemblage to the instruments, designers had constructed beam lines of glass shielded with concrete. The concrete shielding, built to protect workers from neutron radiation, encloses and supports neutron beam optics formed from transparent glass mirrors manufactured by commercial vendors in Europe. These mirrors have complex shapes and a multilayer coating optimized for reflecting neutrons. For the backscattering spectrometer—one of the three SNS research instruments to go on line this year—the neutrons are reflected along a slightly curved path some 84 meters (90 yards) to reach an experimental sample suspended in the middle of a large tank. The tank wall is partially covered with panels of single-crystal, silicon, hexagonal plates that will reflect, or back-scatter, to 112 detectors only those neutrons scattered from the sample that have one particular energy, or wavelength. Each SNS instrument includes a neutron beam line, sample holder, sample environment, and detector system optimized for a particular type of measurement. All the detector systems will use the internationally acclaimed, world-class "event mode" data acquisition system. Some 80 people are developing SNS neutron scattering instruments, says Kent Crawford, leader of the Instrument Systems Group in the SNS Experimental Facilities Division. He previously worked for Argonne National Laboratory, which has operated the Intense Pulsed Neutron Source as a scientific user facility since 1981. As an SNS partner lab, Argonne was responsible for the development of the instruments, which Crawford describes as "an interesting mix of high precision and massive construction." Argonne instrument scientists initially carried out the engineering work and project management for the instruments and trained or advised ORNL scientists who assisted.
Instrument Development Teams or any other group proposing an instrument must present their proposals to an external Experimental Facilities Advisory Committee, which makes recommendations to SNS management. Thus far SNS management has followed the committee's recommendations. Poor planning had nothing to do with the fact that not all the instruments fit inside the Target Building. Rather than construct a much larger Target Building, SNS management decided to save money by constructing attached "satellite buildings" to enclose 6 of the 17 approved instruments, which stick out of the building because of instrument size and geometry or the need for a very long beam line. In 2006 three instruments will be "commissioned," or tested with a neutron beam. These instruments, funded by the Department of Energy, are the backscattering spectrometer, the liquids reflectometer, and the magnetism reflectometer. Health and safety considerations are resolved before any instrument is commissioned. Each of the instruments will be used for neutron-scattering research. While DOE provides funding for most SNS instruments, other instrument funding agencies include a national laboratory in Germany and a foundation in Canada. The mercury target and the research instruments housing advanced electronics and experimental targets represent the highest plateau of scientific design and construction. As the remainder of the SNS provides the enormous power needed to produce neutrons, the instruments will translate this process into what should be an endless journey of scientific discovery.
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