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Researchers use "magic nuclei" to unlock the secrets of heavy elements.
Inner space and outer space: Representing the bookends of atomic discovery, they are the two big attractions for the hundreds of visiting scientists who each year conduct research at ORNL's Holifield Radioactive Ion Beam Facility. Holifield's ability to create and analyze isotopes that exist for only seconds gives researchers a unique glimpse into the inner workings of atomic nuclei, as well as how they interact with each other and with high-energy particles. Understanding these processes provides astrophysicists with insights they will need to continue to unravel the mystery of how the same processes could have created of all of the heavy elements in the universe, both in the hearts of stars and through hyperviolent stellar events such as supernovae. Holifield's users are primarily scientists from national and international universities. "This year we have about 200 users either on-site or working on studies waiting to be run," says Carl Gross, who manages the facility's experimental systems. "Additionally, we have more than 500 'potential users' who stay up-to-date with Holifield's research and capabilities. Last year's workshop on Holifield's wideranging capabilities attracted more than 150 participants from 44 institutions and 10 countries. Scientific director Witold Nazarewicz believes interest is increasing because of Holifield's unique capability to produce and study beams of the shortlived isotopes created by fission reactions. Holifield holds the distinction as the only American facility that generates radioactive ion beams using the isotope separator online technique. The technique accelerates protons that strike a uranium target, which then fissions into a spray of different isotopes. Researchers focus and accelerate this assortment of short-lived elements into a beam for analysis. The isotopes produced by this process include those found in stars, as well as those created as fuel is consumed in nuclear reactors. "Many of these isotopes have only been detected and never studied in any detail," Gross says. "We are able to analyze them with a range of sophisticated measurement tools to see how they interact with one another." "Magic" nuclei One scientist benefiting from Holifield's singular capabilities is Kate Jones, an assistant professor at the University of Tennessee. Her studies focus on what she describes as "a junction point between three areas of physics research: nuclear structure, nuclear reactions and nuclear astrophysics." The object of many of Jones's subatomic inquiries is the behavior of the somewhat enigmatic isotope tin-132. Although its fleeting, 40-second half-life makes working with tin-132 difficult, the isotope is significantly more bound than its atomic neighbors and has a number of characteristics that endear the isotope to nuclear physicists. Researchers are exploring how neutrons and protons (known collectively as nucleons) in the nucleus influence its shape and behavior. Nuclei carry a number of layers, or "shells," of either protons or neutrons. Each shell in turn can carry a limited number of nucleons. Once a shell is full, or "closed," adding or removing a nucleon becomes more difficult, resulting in nuclei with full shells that tend to be more stable than those with incomplete or "open" shells. Tin-132 is of special interest to nuclear structure physicists because it has closed shells of both protons and neutrons. Because tin-132 has many more neutrons than protons, the isotope also appeals to researchers seeking to understand why such nuclei tend to be unstable. With its closed shells, neutron-heavy tin-132 provides a platform upon which to study the effect of adding a single nucleon to the nucleus outside the doubly closed shells. "At Holifield," Jones says, "unstable nuclei like tin-132 nuclei can be accelerated to an energy that enables us to add a neutron to the nucleus, creating tin-133. The neutron is added to tin-132 by causing it to collide and react with a deuteron (a deuterium nucleus, consisting of a neutron and a proton). By measuring the energy and angle of the proton emerging from the reaction, invoking conservation of energy and momentum, we can determine exactly how the neutron is incorporated into the new tin-133 nucleus." Jones adds that Holifield is the only place in the world with the capability to study these isotopes in this manner.
Hearts of stars One of the most important quests in nuclear science is the effort to understand how heavy elements are created. From hydrogen to iron, elements can be created by nuclear fusion in the hearts of stars. However, creating heavier elements requires energy consumption—rather than energy production, as is the case of lighter nuclei—suggesting there is another path for the creation of the elements from iron to uranium. Scientists theorize this creation process for heavier elements occurs in two basic contexts: very quickly (on the order of seconds) in supernovae and other violently energetic cosmic events, and very slowly (typically thousands of years) in the hearts of light- to intermediate-mass stars late in their lives. Isotopes like tin-132 that have nuclei with closed shells of both protons and neutrons and an overabundance of neutrons are of interest to physicists not only because of what they reveal about the structure of nuclei, but also because they are thought to influence key astrophysical phenomena, such as the formation of heavier elements. The structure and behavior of tin-132 provide scientists insight into the nuclear reactions that occur within astrophysical phenomena such as supernovae. Some theorize that during these explosions neutrons are captured by nuclei, leading to the formation of the heavier elements. Tin-132 represents an extremely important benchmark, both for understanding nuclear structure and for calculating the properties of the plethora of nuclei involved in producing heavy nuclei in explosive cosmic events such as supernovae. Most such nuclei are so exotic and so short lived that they are not available for experimental study. Their properties and reactions, therefore, must be modeled. Jones explains that "rather than fusing nuclei to create heavier elements, these elements can be created by fusing neutrons into the nuclei of atoms." This process does not create a heavier element, producing instead a heavier isotope of the same element. When enough neutrons have been added to the nucleus to make the element unstable, the nucleus decays. This process converts one of the added neutrons into a proton, thus creating a different element. Computer models that incorporate the rate at which neutrons are captured by isotopes neighboring tin-131 in supernovae have indicated these isotopes may play a critical role in determining the quantity of heavier elements produced by supernovae. Jones and her colleagues are experimenting with tin-132 and nearby isotopes to try to identify the details of this process. If the team is successful, their findings can be used in models to determine how other factors, like a star's size or magnetic fields, contribute to the synthesis of new elements. The only game in town While there are many approaches to studying the mechanics of the atomic nucleus, Jones notes that Holifield has the distinction of being America's only facility that produces beams from the fission of uranium. "That makes very neutron-rich isotopes," she says. "If users require the range of neutron-rich nuclei we work with, they must come to Oak Ridge."
Nazarewicz also emphasizes Holifield's one-of-a-kind capabilities. "Although there are other radioactive ion beam facilities in the U.S. performing similar work, they specialize in other isotopes because they use a different process to produce their beams. Nazarewicz points out that Holifield users specialize in creating and analyzing isotopes to help answer key questions about nuclear structure, as well as the origin of the elements from iron to uranium. "These are actually very hot areas of science," he says, "particularly with regard to nuclear theory and astrophysics. For users who are interested in these areas, we are the only game in town."
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