USER FACILITIES 100,000 HOURS OF OAK RIDGE ELECTRON LINEAR ACCELERATOR OPERATION (This article also appears in the Oak Ridge National Laboratory Review (Vol. 26, No. 1), a quarterly research and development magazine. If you'd like more information about the research discussed in the article or about the Review, or if you have any helpful comments, drop us a line. Thanks for reading the Review. When the Oak Ridge Electron Linear Accelerator (ORELA) went on line in 1969, the U.S. nuclear power industry was enjoying its heyday, the Soviet Union was supporting communist insurgencies in the Far East and Central America, and Richard Nixon occupied the White House. Nearly a quarter-century later, both the world and the field of nuclear research have seen a lot of changes, but ORELA is still going strong. In fact, on October 13, 1992, ORELA logged its 100,000th hour of on-line operation--an indication of the enduring vitality and value of the research done at the facility. The facility's accelerator, target room, and several of the test stations are located under a group of grass-covered mounds not far from the Laboratory's main entrance. The more distant test chambers, located up to 200 meters (650 feet) from the target room, can be seen poking up out of adjacent parking lots. At the heart of ORELA is a 180-MeV electron linear accelerator that uses electromagnetic waves to fire bursts of electrons pulsed from a hot-cathode gun down a 23-meter (75-foot) acceleration tube at nearly the speed of light. At the end of the tube this joyride comes to an abrupt end as the electrons slam into a water-cooled tantalum target, where collisions with the target and the surrounding water produce neutrons with a wide range of energies. This spray of neutrons is then channeled to one or more of the test chambers. Seven stations, located from 10 to 200 meters (33 to 650 feet) from the target, are used because neutrons of varying energies and intensities are needed for different experiments--and these qualities vary with the distance the neutron travels from the target. ORELA'S ROOTS Jack Harvey, director of ORELA and a research physicist since 1950, recalls that in 1969 the facility represented a quantum leap forward in the technology used to obtain neutron cross sections for various materials. Cross sections are measurements of the likelihood that neutrons of a given energy will interact with the nucleus of a material, rather than passing through it. Because most nuclear reactions produce a lot of neutrons, nuclear researchers at the time of ORELA's construction were interested in learning more about neutron cross sections to help them develop more effective shielding materials for use in production reactors or nuclear research facilities. "Thirty or forty years ago," says Harvey, "investigating neutron cross sections became an area of great interest both for people building reactors and those studying basic nuclear physics." Most early research facilities measured neutron cross sections using a "fast chopper" in conjunction with a reactor. "The chopper," says Harvey, "was a rotating cylinder with slits in the sides, which provided microsecond-long bursts of neutrons. This was adequate for measuring cross sections in low-energy regions, up to a few thousand electron volts, but the resolution was not good enough to work at higher energies." In 1965 plans were announced to build plutonium fast breeder reactors, including Oak Ridge's Clinch River Breeder Reactor project, which was ultimately canceled in 1983. Breeder reactors rely on neutrons given off during the process of nuclear fission by reactor fuel, such as plutonium-239, to transform a blanket of non-fissionable material, like uranium-238, into plutonium-239. This approach to reactor design harnesses the fission process for the production of both fuel and power. Because plutonium breeds fissionable material best at high energies, researchers needed high-resolution information on nuclear interactions that occurred at energies between a few thousand electron volts and a few hundred thousand electron volts. "As a result," says Harvey, "funding was approved for a facility capable of measuring the cross sections for reactions in these regions. That's why ORELA was originally built." ORELA was designed primarily to measure neutron cross sections of materials thousands of times more precisely than could be done using fast choppers. At the time of its construction, the facility produced neutron bursts that were 10 times as intense and 5 times narrower than those of comparable facilities, filling a gap between experimental results provided by low-energy fast choppers and high-energy cross sections provided by Van de Graaff accelerators then operating at ORNL and other laboratories. STRETCHING THE NEUTRON Measuring neutron cross sections of various materials isn't the only activity going on at ORELA, however. Because the facility's pulsed neutron source is so intense compared to its background "noise," it has recently been used to probe the structure of the neutron itself. The neutron is believed to be held together by the so-called "strong" force, one of the four physical forces in the universe, a group that also includes gravity and electromagnetism. Previous experiments have suggested that neutrons are made up of three subatomic particles, known as quarks. One quark has two-thirds of a positive charge, and the other two have one-third of a negative charge each, making the total charge of the neutron zero. To measure the force holding these quarks together, researchers had to find a way to stretch the neutron, slightly separating the positive quark from the negative quarks. To accomplish this feat, a group of Austrian researchers, along with Harvey and Nat Hill, a retired researcher from the Instrumentation and Controls Division, fired beams of low-energy neutrons at a target of lead-208, an isotope with an extremely strong electric field around its nucleus. The interaction between this electrical field and the neutron's charged quarks causes the quarks to separate slightly. Using a new detector and other special electronics provided by the Austrian research team, it was determined that, as a result of this separation, more of the neutrons were scattered by the lead-208 sample, rather than passing through. By comparing the results of this experiment to those obtained when no target was present, researchers determined the change in the "size" of the neutron resulting from its interaction with the electrical field of the lead-208 nucleus, and from that they determined the magnitude of the force holding the neutron's charged quarks together. "Twenty earlier attempts at measuring this force at other research centers have resulted in values with margins of error that were greater than the values obtained from the measurements," said Harvey. "The value obtained in the work with the Austrians was four times larger than the margin of error." PHONONS AND SUPERCONDUCTIVITY ORELA also provides researchers with an opportunity to study the role of phonons in superconductivity. Superconductivity occurs when electrons in a metal are attracted to each other, but because electrons are all negatively charged, they usually repel each other. Under certain conditions, however, this repulsion can be overcome. For example, a fast-moving electron traveling through an array of positively charged, slow-moving metal ions attracts the ions, causing them to move toward it as it passes and leaving a concentration of positive charges in its wake. These positive charges may then attract other electrons, causing an indirect attraction between electrons. The units of vibrational energy that cause the metal ions to move toward the electron in this example are called phonons. Studies of the vibrational spectra, or phonon spectra, of materials that conduct electricity normally in one temperature range and are superconductors in another have shown systematic differences between the two states, but until recently, the hypothesized change in the phonon spectrum at the transition point between the two states had not been observed for the new, high-temperature superconductors. To get a good look at phonon behavior at the superconducting transition point, Herb Mook and other researchers at ORELA used a technique called neutron resonance absorption spectroscopy (NRAS) to measure the vibrational spectra of the high-temperature superconductor modes of bismuth, strontium, calcium, and copper (BSCCO). The NRAS technique has several advantages, including being able to study the vibrations from each element in a sample separately, obviously a plus when dealing with molecules as complex as BSCCO. Another advantage of NRAS is that it enables researchers to study the structure of non-uniform material by varying its orientation in the neutron beam. Using this technique, scientists at ORELA were able to observe a change in the phonon spectrum of the BSCCO's copper component at the transition point. This finding shows that there is a close relationship between phonons and high-temperature superconductivity and suggests that any explanation of the high transition temperatures of high-temperature superconductors should take the role of phonons into account. NEW TOOLS The latest addition to ORELA is an intense "slow" positron- generating facility developed by Lester Hulett of ORNL's Analytical Chemistry Division. Positrons are electron-sized particles that have positive charges equal to the electron's negative charge. The new facility takes advantage of gamma photons that are scattered beyond ORELA's primary tantalum target by placing a secondary tungsten target in their path. When the photons strike the tungsten, they are converted to slow, or low-energy, positrons and channeled by a magnetic field into an adjacent experiment room. If this gamma radiation weren't a cost-free by-product of ORELA's neutron-producing activities, it would cost about $300,000 annually to generate it. Reseachers are taking full advantage of this windfall by using the resulting positrons to conduct a range of slow-positron spectroscopy studies, such as those planned for a cooperative research and development agreement between ORNL and AT&T Bell Laboratories. This joint effort will employ slow-positron spectroscopy to develop more effective semiconductor devices. Inefficiencies in semiconductor performance are often the result of defects, such as missing atoms, which trap positrons. The number and distribution of these defects determine how the device's performance will be affected. Once snared by a defect, a positron's lifetime is extended, changing the energies of the gamma rays produced when it finally interacts with an electron and is annihilated. Measuring these gamma emissions enables researchers to determine if defects are present in a semiconductor and whether or not they occur in areas critical to the device's function. The positron facility is also being used to try to get a fundamental understanding of how organic molecules like benzene are ionized--how electrons are removed--by comparing spectra produced by positrons with those produced by electrons and photons. This line of study may eventually lead to better ways of doing analytical mass spectrometry. Understanding the ionization process in medium-sized organic molecules, like benzene, may also result in a better understanding of how larger organic molecules, such as DNA, are ionized. Preliminary studies using bacteria have found that extremely low doses of positrons may have mutagenic effects. WHAT'S NEXT? Over the years, thousands of publications, reports, and presentations have documented research results from ORELA, but researchers are not content to rest on their laurels. Plans for the future include continuing the research initiatives mentioned previously, more collaborations with the Austrians--this time hoping to measure the charge distribution of the neutron at lower energies, and further forays into the realm of astrophysics to determine the properties of heavy elements formed in and around stars as a result of lighter elements capturing neutrons. ORELA is also involved in detector development for DOE's multibillion-dollar Superconducting Super Collider project, as well as other radiation damage and characterization studies. "Normally, accelerators have less than a 20-year life span, but we're 23 and we're still going strong," says Harvey, who shows no signs of slowing down either. "We really have an excellent facility here, and it's running well. We've done a lot with it, and I think there's still a lot of good work to be done." Jim Pearce (keywords: nuclear research, linear accelerators, neutrons) ------------------------------------------------------------------------ Please send us your comments. Date Posted: 1/26/94 (ktb)