are probing the nature of matter at DOE's scientific user facilities.
The Department of Energy's scientific user facilities are
national treasures. They provide a gathering place for
world-renowned scientists and engineers, and they house unique resources and state-of-the-art equipment for research. Without these large facilities, researchers throughout the world would be denied access to precious information about a material's smallest features.
ORNL Research at User Facilities
ORNL researchers use protons, photons, neutrons, electrons, and ions to probe the nature of matter and recreate conditions that may at one time have existed in the universe. Among the sources of these probes are cyclotrons, electrostatic accelerators, reactors, synchrotrons, and now electron microscopes that can be controlled remotely over the Internet. In addition to facilities at ORNL, DOE facilities used by ORNL scientists include those at Argonne and Brookhaven national laboratories. DOE has also funded international experiments involving ORNL scientists at the European Laboratory for Particle Physics (CERN) near Geneva, Switzerland, and Oak Ridge researchers are involved in DOE's five-laboratory effort to design and build the Spallation Neutron Source user facility at ORNL (see Target Test Facility: Replica of the SNS Mercury Target).
The Impossible Beam
The founders of radioactive ion beam physics said it couldn't be done, but ORNL proved it could. Conventional wisdom held that a beam of radioactive fluorine ions (fluorine-17) that would be sufficiently intense for research could not be formed because fluorine is too reactive. But, in 1998, beams of short-lived fluorine-17 were generated, accelerated, and used for research at ORNL's Holifield Radioactive Ion Beam Facility (HRIBF), a DOE user facility dedicated to the production of beams of short-lived radioactive nuclei that do not exist naturally on the earth. The experiment paves the way for nuclear astrophysics studies of how elements heavier than oxygen were formed in exploding celestial bodies by proton collisions with radioactive nuclei like fluorine-17, found only in the debris of stellar explosions.
The experiment—which served as the Ph.D. thesis work for Dan Bardayan, a doctoral student from Yale University—was led by Michael S. Smith, group leader for astrophysics research in the Physics Division. Smith established a program at ORNL to use radioactive ion beams from the HRIBF to measure reaction rates critical to our understanding of the production of the heavy elements in stellar explosions such as novae.
"Michael Smith and the large group of outside and in-house collaborators he organized have developed the unique experimental apparatus needed to carry out the experiments," says Fred Bertrand, director of the Physics Division. "The development of the very difficult-to-produce fluorine-17 beam was made a top priority of the scientists and engineers working at the Holifield facility. Their success has allowed the work of Smith and collaborators to proceed."
The half-life of fluorine-17 is only 64 seconds, so the HRIBF operations staff had to devise a clever way to produce, extract, and prepare a beam of negative ions for acceleration before running out of material. This was complicated because fluorine is one of the most reactive of all the elements.
The beam of radioactive fluorine-17 was formed by a series of events, beginning with a beam of deuterons that was accelerated using the Oak Ridge Isochronous Cyclotron at HRIBF. A deuteron consists of one proton and one neutron.
After experiments with different targets, ORNL scientists learned to produce a beam of fluorine-17 ions of acceptable intensity by slamming the deuterons into a target of hafnium oxide, a material that functions at higher temperatures than any material tried previously. The deuterons transmute the oxygen-16 atoms in the target to fluorine-17 atoms. Aluminum vapor from an oven is then pumped through fibers of the thumb-size target. At a high temperature the vapor reacts with the fluorine-17 formed in the target and makes aluminum fluoride. This gas diffuses quickly out of the target and into a positive ion source, and the extracted beam is then passed through a chamber containing vapors of cesium, whose atoms' loosely bound electrons are easily snatched by fluorine ions. The positively charged aluminum fluoride is turned into negatively charged fluorine-17 ions that, after being selected by a magnetic field, can be accelerated in the HRIBF's 25-megavolt tandem electrostatic accelerator for use in the experiment.
Once the "impossible" beam of fluorine ions was produced, it was accelerated by the tandem accelerator and directed at a target of polypropylene, which consists of carbon and hydrogen atoms. The scientists measured the number and energies of fluorine-17 ions that scattered from the hydrogen in the target at different angles. When they studied the neon-18 nuclei produced when fluorine-17 fused with hydrogen, they found a new quantum state, which is at the correct energy—and possibly has the correct properties—to significantly enhance the rate at which fluorine-17 fuses with hydrogen in a hot stellar environment. "Such an enhancement," Smith says, "may have significant implications for the particular isotopes that are synthesized in a stellar explosion and ejected into space, as well as for the rate at which energy is prodigiously generated in the explosion."
An additional experiment is needed to determine if the properties of this new nuclear state in neon-18 will enhance the fusion rate of the fluorine-17 and hydrogen. In this experiment, a fluorine-17 beam from the HRIBF will be directed onto a polypropylene target at the proper energy to form the neon-18 nucleus in this special quantum state, and a sophisticated research device—the Daresbury Recoil Separator—will be used to count the number of such nuclei formed. In this way, the nuclear reaction that helps power the stellar explosion will be measured in a laboratory here on the earth.
|This Hubble Space Telescope image (left) of Nova Cygni 1992 was taken about three-and-a-half years after this exploding new star (right) was discovered in February 1992. The atomic spectra of the ring of debris surrounding such stars indicate that novae are sources for the synthesis of heavy elements. The nuclear state discovered in ORNL experiments using beams of radioactive fluorine-17 is crucial for determining the rate at which nuclear reactions that power such stellar explosions occur.|
HRIBF's electrostatic accelerator enclosed in its landmark tower will provide the energy needed to overcome the natural repulsion between the positively charged fluorine-17 and hydrogen nuclei, allowing the "stellar reaction" to occur. In a star the fusion of these nuclei is made possible by the extreme heat produced in the stellar explosion. The number of the outgoing neon-18 nuclei detected in the experiment will indicate the rate at which this reaction occurs in space. Such nuclear reactions occurring in stellar explosions synthesize the heavier elements that are then dispersed into space, including those that make life on the earth possible.
Neutron Scattering Upgrades for HFIR
|The HFIR neutron scattering upgrades include a new cold source, flared beam tubes, cold and thermal neutron guide systems, and new and upgraded instrumentation. The cold guide hall (including a proposed extension) is shown above.|
When the planned neutron scattering upgrades are completed, ORNL's High Flux Isotope Reactor (HFIR) will exploit the world's highest thermal neutron intensities, and it will provide cold neutron intensities comparable to the world's best (and significantly higher than those currently available in the United States). ORNL will then have 14 of the most competitive steady-state neutron scattering instruments anywhere. Currently, two buildings are being constructed at HFIR to house the cold-source refrigeration equipment and the cold neutron guides and instruments. The ORNL team leading the neutron scattering upgrades includes Jim Roberto, Herb Mook, Colin West, Doug Selby, and Mike Farrar. The upgrades will be implemented in FY 2001 after completion of the scheduled HFIR outage to replace the beryllium reflector.
An important step toward explaining high-temperature superconductivity, one of the major scientific challenges of our time, has been taken as a result of the neutron scattering research performed by ORNL scientists at HFIR and the ISIS spallation source in the United Kingdom. The work demonstrated similar magnetic behavior in two principal families of high-temperature superconductors, suggesting that a single mechanism is responsible for high-temperature superconductivity.
|Images of the magnetic excitations near the (1/2, 1/2) reciprocal lattice position in the high-temperature superconducting material yttrium-barium-copper oxide (YBa2 Cu3O6.6), where Tc = 62.7K, for an energy transfer of 24 MeV in the superconducting state.|
The new result obtained by Herb Mook and Pengcheng Dai, both of the Solid State Division, corrects previous misconceptions about disparate behavior of magnetic fluctuations in the different families and greatly simplifies the theoretical quest to explain high-temperature superconductivity. The research was published in 1998 in Nature and Physical Review Letters.
Plastics of the Future
Developing completely new polymers to meet U.S. needs for plastic products has become an expensive business. Over the past 50 years industrial companies have spent billions of dollars inventing today's polymers, so they are naturally reluctant to invest billions more inventing new ones that must compete with existing materials produced in large volumes at competitive prices using current technologies. Thus, researchers have begun blending known polymers to obtain "polymer alloys" that possess new, desired properties. Currently, a time-consuming trial-and-error process is used to find compatible polymers and predict the properties of the resulting polymer alloys. However, a more direct approach may be possible, thanks to an integrated research effort that combines new theories of polymer mixing with experimental findings about polymers obtained using scattering methods.
|Tony Habenschuss (left), George Wignall, postdoctoral researcher Man-Ho Kim, and Brian Annis examine a cell near a polymer alloy target at the Small-Angle Neutron Scattering facility at HFIR.|
Neutron scattering experiments at HFIR reveal the structure of various polymer alloys and the ability of different polymers to mix with each other to form alloys with desired properties. These experiments are being performed by Brian Annis and Tony Habenschuss, both of ORNL's Chemical and Analytical Sciences Division (CASD), and George Wignall of ORNL's Solid State Division, in collaboration with researchers at Sandia National Laboratories, the University of Illinois-Urbana, and the University of Minnesota.
"The refined understanding that we gained should lead to a cost-effective, efficient engineering approach to polymer blending that is guided by sound scientific principles," Wignall says. "The long-term result for consumers may be higher-quality but lower-cost plastic products."
X-ray Microbeams Commissioned
at Advanced Photon Source
|Schematic drawing of an X-ray microbeam experiment. Curved mirrors focus the synchrotron X rays down to a diameter of less than one micron on the sample. The microbeam penetrates each layer of the sample, and an area detector measures the directions of the scattered X rays. Here, the sample consists of a roll-textured nickel substrate covered with two epitaxial films: a buffer layer and a superconductor (YBCO). The detector image provides a grain-by-grain description of the atomic structure, orientation, and strain of each layer. |
Materials ranging from massive steel girders to the microscopic aluminum wires in computer chips are made of grains—tiny crystals with diameters measured in millionths of a meter (microns). If scientists could "see" these individual grains, they could determine their orientation, as well as the effects of stress and chemical activity on them. They might also be able to determine how best to jam more circuits together in microelectronic components, making them smaller and faster, so computers can perform complex functions—such as speech recognition—more quickly. They could also find out to what extent grains of a superconducting material mimic the alignment of the substrate on which the material is grown; discovery of such orientation replication involving deposited thin films is essential to the design of effective high-temperature superconductors.
Scientists are now able to study the fine details of grain behavior in materials, thanks to new X-ray beam lines at the Advanced Photon Source (APS), an intense synchrotron X-ray source at DOE's Argonne National Laboratory. ORNL is a leader in efforts to develop microbeams at the APS. Microbeams are X rays that are focused down to beam diameters smaller than one micron, allowing researchers to see a material's microstructural features within individual grains. The instrumented beam lines now enable researchers to perform micro-diffraction, X-ray scattering using beams of submicron dimensions. These microbeams will provide access for the first time to the mesoscale, the length scale that determines the macroscopic properties of many materials.
"This is an essentially untapped research area of enormous scientific and technological interest," says Gene Ice of the Metals and Ceramics Division. "Understanding mesoscale dynamics will revolutionize our understanding of several key materials problems for the next decade—stress-driven grain growth, aging, and materials failure. For example, important materials properties—such as the brittle fracture that led to the sinking of the Titanic and the magnetic response allowing VCR tapes to be watched—are controlled by mesoscale features."
Ice and Solid State's Ben Larson are leading ORNL's efforts to develop these new microbeam capabilities. Other ORNL scientists, including John Budai, Jon Tischler, Eliot Specht, Jin-Seok Chung, Nobumichi Tamura, and Mirang Yoon, have performed experiments using microbeam analysis with a resolution of <1 µm. They are studying strain in integrated circuit wires, a major source of electrical problems in developing smaller, denser microelectronic components for the next generation of computers. They are also studying the epitaxy of oxide films on nickel foils and the defects introduced by ion-implantation processing in silicon to help them understand and improve properties of materials.
The initial design, microbeam optics, and associated techniques for materials analysis are being developed by ORNL and Howard University at the APS on the MHATT-CAT beam line constructed by the University of Michigan, Howard University (a historically black university), and Lucent Technologies Collaborative Access Team. To exploit microbeam capabilities fully, ORNL is developing a dedicated microbeam facility directed toward 0.1 µm resolution on the recently commissioned UNICAT synchrotron beam line. (UNICAT stands for University National Laboratory Industry Collaborative Access Team.) UNICAT is a $10-million beam line collaboration involving ORNL, the University of Illinois, the National Institute of Standards and Technology (NIST), and UOP Research Inc. UNICAT provides access to the nation's most intense X-ray beams for a wide range of studies of the structure and properties of materials. ORNL has received funding through the new-initiative competition of the Division of Materials Science in DOE's Office of Basic Energy Sciences to develop a mesoscale materials program at the APS using microbeams. It's a big project, and the reward will be insights into structures of materials that are very small.
Materials Microcharacterization Collaboratory:
Can scientific research at DOE national laboratories be made friendlier and more accessible? Can science students in college classrooms watch the progress of a national laboratory experiment? Can a scientist working at home run an experiment at a DOE facility? The answer is yes, thanks to the Internet.
Several years ago, students at Lehigh University remotely operated an electron microscope at ORNL. By tapping on a computer keyboard, they moved the subject specimen and changed the microscope's focus and image magnification. They studied the various images on the monitor screen. Using teleconferencing tools, they had live video and audio contact with their ORNL collaborator.
Remote operation of the electron microscopes has been successfully demonstrated in a number of other venues (e.g., for Vice President Gore's visit to ORNL on January 21, 1998). Ford Motor Company scientists used ORNL's electron microscopes in research conducted remotely from Michigan.
Today ORNL researchers are supporting DOE's Materials Microcharacterization Collaboratory (MMC). Their work is focused on making materials characterization tools and our expertise accessible over the Internet to scientific users and students across the country. This project involves all DOE electron microscopy user facilities, NIST, several microscope and ancillary equipment manufacturers, the neutron residual stress user facilities at the HFIR, and the ORNL beam lines at DOE's National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). The goal of the multipartner collaboration is to promote the development of common technologies for the remote operation of research equipment.
Remote operation of scattering instruments for experiments at HFIR and the NSLS is under way. Lessons learned from these operations should affect plans for enabling researchers to remotely operate neutron scattering instruments at DOE's proposed Spallation Neutron Source, which may be ready for experiments in 2006.
Participants in the research at ORNL are Michael Wright of the Instrumentation and Controls Division; Edgar Voelkl, Ed Kenik, Cam Hubbard, and Larry Allard, all of the M&C Division; and Jim Rome of the Computing, Information and Networking Division. The MMC is jointly funded by DOE's Mathematical, Information and Computational Sciences Division and the Division of Materials Sciences, both in DOE's Office of Science, and the Office of Heavy Vehicle Technologies in DOE's Office of Energy Efficiency and Renewable Energy.
For DOE, one of the appeals of remote access to and remote operation of research facilities is the reduced need for travel and energy for transportation. Instead of traveling to the scientific user facility, the facility can be brought to the home or office through the Internet. It is hoped that more and more researchers will be doing science from a distance.
Ultrarelativistic Collision Results
Like two remotely controlled cars careening around a circular racetrack in opposite directions, two beams of gold ions will smash into each other at a velocity of over 99% the speed of light. The site of this subatomic "demolition derby" is the Relativistic Heavy Ion Collider (RHIC), now being completed and tested at BNL.
Bode Well for DOE Collider
The trick is to magnetically confine those positively charged ions traveling at ultrarelativistic velocities to the appropriate orbit for a long enough "storage time" to make use of the machine for experiments economical. If some ions in one beam collide with each other and change their nuclear mass or if some lose or gain an electron and change their charge, they may be lost from the orbit at too great a rate for supercollider operations to continue effectively.
Recent atomic physics data obtained through an international collaboration at the Super Proton Synchrotron at the European Laboratory for Particle Physics (CERN) near Geneva, Switzerland, suggest that it is feasible for BNL to attain hoped-for storage times at RHIC. Key contributors to this collaboration were Sheldon Datz, Herb Krause, and Randy Vane, all of ORNL's Atomic and Molecular Physics Section in the Physics Division.
In the CERN experiments, solid and gaseous targets were bombarded with an ultrarelativistic beam of "bare" lead ions that were completely stripped of their electrons before being accelerated at 33 trillion electron volts. The solid targets used were gold, copper, tin, aluminum, carbon, and beryllium films of different thicknesses. In some experiments using a thin gold target, up to 0.1% of the bare lead ions in the projectile beam each picked up an electron, changing their positive charge from 82+ to 81+.
The explanation? When a lead projectile ion comes close to a target nucleus, the electromagnetic field of the two nuclei is so strong that a virtual photon arises, generating at least one electron-positron pair. Because the lead ion has a strong positive charge, the pair's negatively charged electron may be captured into the lead electron shell and its positively charged mate, the positron, repelled. The probability of electron capture was determined by measuring the fraction of lead ions emerging from the target that changed their charge.
The researchers also participated in experiments that revealed the results of nuclear collisions between bare ions traveling at ultrarelativistic energies in the same beam. Normally, nuclei of like charge repel each other, but at these high energies, they get close enough together to allow exchanges of protons and neutrons that can alter some ions' nuclear mass.
In other experiments, many of the accelerated bare ions captured single electrons as they traveled through air to the beam tube leading to the target. The researchers found that the probability that a one-electron lead ion projectile would lose its electron in collisions with target atoms is lower at ultrarelativistic energies than once believed. As a result of these measurements, a Danish theorist has proposed a new theory that explains observed experimental results and predicts the outcome of new experiments.
In their 1998 experiments with targets of noble gases (argon, krypton, xenon) at various pressures (which mimic different thicknesses for solid targets), the ORNL physicists and their colleagues were surprised to find that the probability that the projectile ion would lose an electron was lower for collisions in gases than solids. It had been thought that the probability was the same for both types of matter. The physicists determined this probability by measuring the fraction of projectile ions that pass through the target with their single electrons intact. They found the probability for capturing an electron in dilute gas targets is lower than expected.
The small probabilities observed suggest that beam orbit losses at RHIC should be acceptably low. The ORNL researchers' atomic collision findings should also be of value to collider operations at CERN, which is building the Large Hadron Collider for international experiments.
Electronics for Nuclear Physics Detectors
Physicists have long desired to understand the environment that may have existed moments after the Big Bang. International experiments involving ORNL and other researchers were performed between 1992 and 1997 at CERN to gain this understanding. Physicists sought to verify the theory that, within the first 10 microseconds of the Big Bang, quarks existed in the free state, called the quark-gluon plasma. After that brief time, as the universe cooled, it is thought that the free quarks formed the protons and neutrons that 300,000 years later became the atomic nuclei of our universe.
|ORNL researchers are developing electronic components for the PHENIX detector, shown here under construction. |
To do these experiments, special detectors were needed. A group led by Chuck Britton in ORNL's I&C Division supported the Physics Division group led by Frank Plasil and Glenn Young by developing detector electronics. The experiments produced questions as well as answers. Physicists still don't know exactly what happened during the first few moments of the early universe, and they are still wondering how the proton gets its spin. Now, ORNL physicists and their colleagues plan to bombard heavier ions together at almost the velocity of light to simulate more closely a quark-gluon plasma so they can find answers they couldn't get at CERN. The place to do that is RHIC. There the PHENIX detector is being built to collect particles spraying out from RHIC's high-energy collisions and measure their energies to answer the physicists' fundamental questions.
Data from all particle collisions at RHIC will enter some 350,000 detection channels in PHENIX. Some data will be more important to physicists than other data. ORNL researchers are developing the special electronics needed to sort through the data and select only the meaningful information the physicists want to see from every one of PHENIX's channels. Tune in next century for an update.