Editor’s note: This article is based on a June 21, 2001, interview with Sheldon Datz, who died August 15, 2001.
Sheldon Datz, ORNL's recent winner of the Department of Energy's Enrico Fermi Award and a senior corporate fellow, was a strong believer in curiosity-driven science. Throughout his remarkable career, he planted seeds in different places and recruited collaborators to help the seeds grow and bear fruit, opening up several new fields in atomic physics.
In 1951, after earning an advanced degree in physical chemistry from Columbia University, Datz came to ORNL. He was hired by Ellison Taylor, then director of ORNL's Chemistry Division and his former supervisor at Columbia's Manhattan Project labs.
Datz and Taylor were interested in finding out what happens at the molecular level during a chemical reaction. The proposed approach was to make a beam of one type of gaseous molecule and shoot it at a beam of another type of molecule. It was believed that measurements made where the two beams cross would shed light on the details of gaseous chemical reactions. In 1955, Datz and Taylor first demonstrated the use of crossed molecular beams in studying the mechanisms of chemical reactions. They crossed beams of potassium and hydrogen bromide, and the resulting reaction yielded potassium bromide and hydrogen.
Asked why he chose to work in Oak Ridge, Datz responded that because only extremely small amounts of product could be expected from crossed-beam experiments, the Oak Ridge Graphite Reactor could be used to perform neutron activation analysis on the deposited potassium bromide. After a few less-than-successful efforts to use this difficult technique, Datz invented a new method, "differential surface ionization," in which two thin, heated wiresone of tungsten and one of platinumcould be rotated in the reaction plane and electrically record the elastic and reactive scattered product. The method greatly simplified the experiment.
This pioneering work opened up the new field of molecular beam chemistry. Much of our detailed understanding of chemical reaction dynamics has emerged from this new field of research. The ORNL work also laid the foundation for the research recognized by the 1986 Nobel Prize in Chemistry. Dudley R. Herschbach (Harvard University), Yuan T. Lee (University of California at Berkeley), and John C. Polanyi (University of Toronto) received the 1986 Nobel Prize "for their contributions concerning the dynamics of chemical elementary processes."
After receiving a Fulbright Senior Research Fellowship, Datz went to the Netherlands in 1962 and worked as a guest scientist at the FOM Institute for Atomic and Molecular Physics in Amsterdam. There he and Cornelis Snoek discovered that, at low energies, an argon ion could be bounced off, or scattered from, a copper atom on a solid surface. In 1964 they reported the discovery of collisions between an ion and a single metal atom, a process known as ion-surface scattering. Other groups soon used this method to analyze the elemental composition and structure of surfaces.
While in Amsterdam, Datz proposed shooting the ion beam at a single crystal target at a small angle, with respect to the rows of atoms, to reduce interference from ions backscattered from atoms below the surface. "The experiment worked and, surprisingly," said Datz, "the entrance angle could be made much wider than theoretically predicted. The cause? Channeling." Upon his return, he immediately joined a small meeting at Chalk River Laboratory in Canada where others reported effects that could be explained by channeling.
"This remarkable effect had actually been 'discovered' in computer 'experiments' at ORNL," said Datz. Because of concern about neutron-induced radiation damage in nuclear reactors, in 1962 Mark Robinson and Dean Oen, two researchers in ORNL's Solid State Division (SSD), attempted to model the effects of an energetic copper projectile ion on a copper crystal lattice. "They wanted to know how far a copper ion goes before it stops," Datz said. "They let their Monte Carlo computer program run for a long time, but they sometimes couldn't find where the particle went. They changed the code and their simulation showed that the copper atom often came out the other side of the lattice." Their 1963 modeling led to their prediction that ions can travel through a crystal in the space, or channel, between rows of atoms and planes in the latticehence the term, ion channeling.
In 1964, then SSD director Doug Billington asked Datz to start a program on particle-solid interactions with emphasis on channeling. "We needed energetic heavy ions," Datz said. "Charlie Moak in the Physics Division produced them on the EN Tandem Van de Graaff accelerator. We also needed very thin crystals. Tom Noggle, an SSD expert in radiation damage in thin crystals, produced perfect gold crystals 300 atoms thick. We chose iodine-127 and bromine-79 ions at energies of 50 to 100 million electron volts because they could be viewed as synthetic fission fragments similar to what is produced in reactors."
In 1965 Datz, Moak, and Noggle demonstrated experimentally the phenomenon of energetic ion channeling in solids. They found that the ions moving along the channels of gold single crystals lost only about half the energy they would yield if they traveled through the same volume in a random direction. The explanation: Penetrating ions lose energy mainly through collisions with electrons, but the collective attractive action of nuclei in the planes gently pushes the positively charged ions away from the high concentration of electrons in the atomic cores.
In 1978 Datz and his collaborators reported the discovery of resonant coherent excitation of channeled ions. In this phenomenon, an ion with only one electron is raised to an excited state by traveling at the right velocity (and frequency) past the local electric fields of a "picket fence" of atoms, along a channel in a crystal. Work in this area is currently ongoing in Japan.
In 1979, Datz and collaborators in California (Lawrence Livermore National Laboratory and Stanford University) published two papers on their discovery of channeling radiation. They observed experimentally that ultra-relativistic electrons or positrons entering a channel oscillate in response to the string of atoms' local electric fields, resulting in the release of strongly forward-directed, high-energy X-ray radiation.
The study of atomic collisions in solids was initiated largely in response to problems related to radiation damage. The physics learned from these studies has numerous applications, including development of ion implantation, a technique widely used in the fabrication of computer chips.
In the late 1970s John Clarke, then director of ORNL's Fusion Energy Division (FED), suggested to Datz that his group should look into some interesting atomic collision problems affecting fusion plasmas. Datz chose dielectronic recombination, which was at that time a little known but nonetheless important area, especially for highly charged ions. In a plasma, excited electrons may break free of nuclei (ionization) and free electrons may recombine with ions that have lost electrons (recombination). Datz and ORNL's Pete Dittner pioneered the study of electron-ion recombination, using merged beams of electrons and ions. They were the first to make recombination measurements on multiply charged ions.
An FED group led by Clarence Barnett merged with Datz's Atomic and Molecular Physics Section in ORNL's Physics Division to perform basic fusion energy research. In the mid-1980s, this group (which is today led by Fred Meyer) built the Electron Cyclotron Resonance Multi-charged Ion Source (ECR), which is used to obtain fundamental data on plasma–wall interactions of interest to fusion energy researchers. The ECR produces positively charged ions by using highly energetic electrons, heated by microwave radiation and confined by magnetic fields, to strip electrons from atoms of a specific element. Recently, DOE asked researchers in the Atomic Physics Section to focus on research using ions from an upgraded ECR.
DOE's investment in high-energy nuclear and particle physics is considerable. In the Large Hadron Collider being built at CERN near Geneva, Switzerland, lead ions will collide in a storage ring at speeds very close to the speed of light. It is expected that atomic physics processes will occur with much greater likelihood than the desired nuclear events, thus interfering with these experiments. A prime concern here is the loss of stored beam.
"In the collision region, a bare lead ion captures an electron that it has created as part of an electron-positron pair," Datz explained. The results of theoretical predictions differed widely in the early 1990s. Accordingly, Datz initiated a program to study atomic collision physics at ultra-relativistic energies, using 33-TeV lead ions from the Super Proton Synchrotron at CERN. This multinational effort included Randy Vane, Herb Krause, and Dittner from ORNL and collaborators from Sweden, Denmark, Germany, Switzerland, and South Africa. "The results, aside from quantifying expected phenomena," said Datz, "disclosed some new, totally unanticipated findings, which, in addition to their intrinsic interest, can be of use in designing very expensive particle physics experiments."
During his tenure as guest professor in Stockholm in 1990, Datz devised a method for using heavy-ion storage rings to study low-energy collisions between molecular ions and electrons. One result of these collisions is "dissociative recombination," in which molecular ions recombine and break up into neutral fragments. This process is of great importance to low-temperature plasma studies, astrochemistry, and aeronomy. Considerable work in this area is now being conducted at a number of storage rings around the world.
Although Datz did not win the Nobel Prize for pioneering the crossed molecular beam technique, his successes in opening up new fields of study in atomic physics brought him two other prestigious awards. In 1998 Datz received the American Physical Society's Davisson-Germer Prize in Atomic or Surface Physics for his research into atomic interactions with ions, electrons, and photons. On November 9, 2000, President Bill Clinton named Datz a co-winner of DOE's Enrico Fermi Award, one of the nation's highest prizes in science. On December 18, 2000, Datz received this award for his pioneering research in atomic and chemical physics.
Reflecting on DOE's decision to focus ORNL's atomic physics research on the ECR in light of his career, Datz said, "DOE thinks that we do too many things, that we should work on one or two projects. That is hard for us because we have developed many interests and capabilities. I contributed to a lot of different areas, all of which related in some way to DOE missions. To be sure, the main efforts at ORNL should be directed at larger projects, but I think it's vital to the Laboratory's health to also encourage more speculative, curiosity-oriented research."
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