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ORNL's Search for Rare Isotopes

Our earth is home to 81 stable elements, including slightly fewer than 300 stable isotopes. Several other elements (e.g., thorium and uranium) and more than 130 unstable isotopes also exist on the earth, but all of them eventually decay. That is, the atomic nuclei of these isotopes eventually capture or emit electrons, positrons, or alpha particles, or they undergo spontaneous fission, making the isotopes radioactive. Many radioactive isotopes are quite useful in the diagnosis or treatment of diseases, biological and environmental studies, archeology, national security, and energy generation.

Each unstable isotope is characterized by its half-life—the time it takes for half of the sample to decay. Isotope half-lives range from less than a thousandth of a second to billions of years. Short-lived isotopes cannot be found naturally on the earth because they have long since decayed in that our planet was formed about 4 billion years ago. Yet, thousands of short-lived isotopes are continually created in the cosmos. Their existence may be fleeting, but they play a crucial role in the ongoing formation of the elements in the universe. In fact, synthesis of heavy elements involves unstable nuclei at every stage.

HRIBF has the largest tandem electrostatic accelerator in the world, which is located in ORNL's landmark tower. (Photo by Jim Richmond.)

The properties of most rare isotopes are unknown and can be only inferred, with considerable uncertainty, from theoretical calculations. Nevertheless, at several laboratories around the world, including the Department of Energy's Holifield Radioactive Ion Beam Facility (HRIBF) at ORNL, it is possible to produce beams of some rapidly decaying, rare-isotope nuclei that can be used to study properties of nuclei that are difficult to access.

NATURE OF THE NUCLEUS

The nucleus is the core of the atom, containing more than 99.9% of the atom's mass. Nuclei are composed of protons and neutrons, together called nucleons. The attractive force between nucleons is responsible for holding them together to form a nucleus. However, this force is to some extent counterbalanced by the electrostatic repulsion between the protons, which are positively charged. The binding energy that results from the combination of these forces determines the stability of nuclei—that is, which combinations of protons and neutrons are stable, which are not, and how unstable nuclei decay. One of the greatest challenges in nuclear physics is to understand the boundaries of nuclear existence, which are commonly depicted on a nuclear chart.

The cornerstone of nuclear structure for over half a century has been the shell model, in which each nucleon (neutron or proton) is assumed to move in average potential generated by its interactions with all of the other nucleons. This potential leads to the prediction that the quantum levels in a nucleus form shells within which nucleons reside. This picture of nucleon motion explains a host of phenomena, such as the existence of particularly stable “magic” nuclei corresponding to completely filled shells (corresponding to particle numbers 2, 8, 20, 28, 50, 82, and 126). The left-hand-side diagram shows the shell structure characteristic of nuclei close to the valley of stability. The right-hand-side diagram shows schematically the shell structure predicted in drip-line nuclei, which corresponds to a more uniform distribution of energy levels and the quenching of magic gaps. Radioactive ion beam facilities such as HRIBF offer unparalleled access to exotic nuclei where such predictions can be tested. (Illustration enhanced by LeJean Hardin)

One of the most recognized nuclear physics theorists in the United States is Witek Nazarewicz, a University of Tennessee physics professor and the deputy director for science at HRIBF. According to Nazarewicz, neutron-rich nuclei pose many fascinating questions. They have a "neutron skin" consisting of numerous neutrons with a greater radius than that of bound neutrons deep within the nucleus. The question of how many neutrons are too many for a given isotope is one of great interest to nuclear physicists. They talk about the "neutron drip line" beyond which the nucleus cannot exist as a bound system. If one neutron too many is added to a nucleus, a neutron will "drip out." It's similar to what happens when water is added with a medicine dropper to a full cup of water; one drop will cause the water to spill over, or drip out of, the cup.

Map of bound nuclear systems as a function of the proton number Z (vertical axis) and the neutron number N (horizontal axis). This nuclear landscape forms the territory of radioactive nuclear beam physics. The black squares show stable, or nonradioactive, nuclei, which form the "valley of stability." The yellow color indicates nuclei that have been produced in laboratories and that live a short time. Adding either protons or neutrons (called nucleons) to a nucleus can move it away from the valley of stability, allowing it to reach the "drip line," where nuclear binding ends because the forces between neutrons and protons are no longer strong enough to hold these particles together. The nuclei beyond the drip lines are unbound to nucleon emission. Many thousands of exotic radioactive nuclei with very small or very large neutron-proton (N/Z) ratios are yet to be explored. In the (Z,N) landscape, they form the "terra incognita" indicated in green. The lines of astrophysics r- and rp- processes, which are responsible for the production of heavy elements in stars, are indicated. The red lines show the magic numbers known around the valley of stability. However, because the structure of nuclei is expected to change significantly as drip lines are approached, it is not known how the nuclear shell structure evolves at the extreme N/Z ratios. The doubly magic radioactive nuclei—neutron-poor tin-100 (50 protons and 50 neutrons) and neutron-rich tin-132 (50 protons and 82 neutrons)—are indicated by the red dots. Research at HRIBF addresses these unknowns. (Illustration enhanced by LeJean Hardin)

"Our challenge is to find out where the drip line is for each neutron-rich nuclide," says Nazarewicz. "Knowing the drip line will help us better understand how well neutrons are bound to protons in a nuclear cluster—that is, the limits of nuclear existence for a given element. Helium-5 doesn't exist. If you add a neutron to helium-4, it flies away, but you can make helium-8. We know the limits of nuclear existence up to oxygen. It is possible to make oxygen-24, but not oxygen-25 or oxygen-26. Many more experiments are needed to find the limits of nuclear existence for heavier elements. It won't be easy."

Nazarewicz cites the difficulty of finding the limit of nuclear existence for polonium, which has several neutron-rich isotopes (e.g., polonium-210 has 84 protons and 126 neutrons and has a lifetime of 138 days). "Polonium-218 was studied by Ernest Rutherford back in 1904," he says. "It took 94 years for scientists to find a way to add one neutron to polonium-218 to get polonium-219 and then 2 neutrons to make polonium-220. The work was done in 1998 in Darmstadt, Germany."

Are the basic properties of very neutron-rich nuclei much different from those of nuclei closer to the valley of stability that have been studied? According to Nazarewicz and his collaborators, who have performed some of the most important calculations in this field, the answer is, "Yes." Nuclei close to the drip line provide us with a new form of nuclear matter—something that resembles a dilute neutron gas with many new properties. One of the most tantalizing predictions is that the shell-like structure of atomic nuclei, introduced more than 50 years ago by Maria Goeppert-Mayer and Hans Jensen, may alter dramatically as we approach the neutron drip line. That is, the magic numbers* that by and large determine nuclear properties as we know them may be washed out or rearranged. (See ORNL Theorists and the Nuclear Shell Model.) However, these predictions are based on experimental information obtained from nuclei that are not too far from stability. They need to be validated by studying unstable nuclei that are located much farther out. Understanding of these far-from-stability nuclei is also the key to understanding nuclear synthesis in the cosmos and origin of the elements.

These experiments, however, are not easy; many of the most interesting nuclei cannot be produced from reactions involving beams of stable nuclei. Therefore, several nuclear physics laboratories around the world have developed, or are developing, new capabilities to produce beams of unstable nuclei that would extend their reach. HRIBF is the pioneering U.S. facility for producing and accelerating radioactive ion beams (RIBs) for nuclear physics and nuclear astrophysics studies.

THE BEAM IS THE TARGET

In the 1980s, the late Paul Stelson, former director of the Physics Division, led experiments at the Holifield Heavy Ion Research Facility in which uranium targets were bombarded with protons and alpha particles. Through these early Coulomb excitation experiments, they inferred that the uranium-238 nucleus is shaped like a diamond. Nearly 20 years later, ORNL researchers began producing neutron-rich radioactive ion beams by bombarding a uranium-238 (²³⁸U) target with 40-million-electron-volt (MeV) protons from the Oak Ridge Isochronous Cyclotron (ORIC) and then extracting the fission-fragment nuclei out of the ion source to make beams. (See Beam Technologies Enable HRIBF Experiments.) This technique is referred to as isotope separation on-line (ISOL).

"We get more than 100 different RIBs from a three-dimensional matrix of carbon fibers with a thin layer of uranium carbide on the fiber surface," says Jim Beene, director of HRIBF. "These beams include atomic species ranging from gallium to lanthanum. The trick is to capture the fission fragments diffusing out of the target and to make them into an accelerated beam (which may be as weak as 1000 nuclei per second) before they decay."

HRIBF is the first ISOL facility in the United States; it specializes in low-energy nuclear physics and nuclear astrophysics research. However, unlike other ISOL facilities, it can accelerate rare radioactive ions to energies sufficiently high to produce nuclear reactions.

The Holifield Radioactive Ion Beam Facility (HRIBF) is an international user facility that produces beams of short-lived, unstable nuclei for research in nuclear structure physics and nuclear astrophysics. A production beam (e.g., hydrogen nuclei) from the Oak Ridge Isochronous Cyclotron (ORIC) strikes target material. Reaction products diffuse out of the target and into the positive ion source where they are ionized. The positively charged ions may be several different isotopes and atomic species, so they are separated in the mass/isotope separator. The ions are then converted to negatively charged ions in the charge exchange cell so they can be accelerated . The mass separator filters out the desired isotope for the radioactive ion beam, which is accelerated by the 25-MV tandem accelerator in the Holifield tower for use in experiments. (Illustration enhanced by LeJean Hardin)

"The Holifield facility is the only place in the United States where we can do preliminary experiments with accelerated ISOL beams," says Beene. "We have developed radioactive ion beams of fluorine-17 for nuclear astrophysics by making a novel target and ion source. We can now make neutron-rich nuclei by causing a uranium-238 target to fission after bombarding it with protons. Uranium-238 is itself neutron rich, having 92 protons and 146 neutrons, so its fission fragments are also rich in neutrons."

"Our special niche in this field," says Cyrus Baktash, head of the Radioactive Ion Beam Physics Section of ORNL's Physics Division, "is our ability to make neutron-rich beams to allow studies of nuclei far from stability. In addition, ORNL can accelerate beams of neutron-rich nuclei to high energies, whereas at CERN near Geneva, Switzerland, these nuclei can be produced only at low energies. At Holifield, we can accelerate these nuclei against a target to study nuclear excitations. We are now in a position to break the barrier to creating the next level of neutron-rich nuclides. This opens up a new area of experimental investigation."

"Besides being the only place where neutron-rich beams can be produced and accelerated for use in experimental physics studies, ORNL now has the best suite of detectors in the world for radioactive ion beams," Baktash continues. "ORNL staff members have developed sophisticated gamma-ray detectors and charged-particle detectors that can detect emission of particles and radiation following nuclear reactions, even though the background radioactivity is high and beam intensities are 10,000 to a million times weaker than stable beams. For example, a tellurium-136 beam is the weakest beam we have worked with. It has 20,000 particles per second, making it a million times weaker than the stable tellurium-128 beams we use for experiments."

David Radford and his colleagues in the Physics Division have used these experimental tools and the neutron-rich beams in several pioneering experiments to study nuclei close to the "doubly magic" tin-132. In the past, these nuclei could be studied only in a limited way via beta decays of their parents. The availability of accelerated neutron-rich beams at the HRIBF opens up many new and exciting possibilities. It is not easy to make an intense beam of tin-132, but a beam of tellurium-134 (¹³⁴Te) or tellurium-136 (¹³⁶Te) nuclei can be produced from the ²³⁸U ion source at the HRIBF. "Because tellurium-134 is only two protons away from tin-132," Baktash says, "it teaches us a great deal about the magic proton number 50."

David Radford adjusts a gamma-ray detector at HRIBF. (Photo by Curtis Boles)

"David Radford and his colleagues have used reactions that range from 'gentle' Coulomb excitation to more energetic ones—like nucleon transfer and fusion. When energy is deposited in a nucleus by either bombarding it with a beam or scattering it off a target, the nucleus becomes excited. This excitation can take different forms. For example, the nucleus can vibrate, rotate, or simply rearrange the orbital motion of a few of its nucleons. This excitation, however, is short-lived. The nucleus gives up its excess energy by emitting particles or gamma rays."

"The Coulomb force, which causes the incoming beam to scatter off a target nucleus, such as carbon-12, also excites the projectile to a higher energy level," Beene says. "Upon de-excitation, the nucleus emits gamma rays. The angles and energies of these gamma rays are recorded by our sensitive gamma-ray detectors. We simultaneously detect carbon nuclei knocked out of the target to verify that the gamma rays we are detecting are the consequence of excitation in nuclei that have collided with the carbon target. This information allows us to determine how long it took the excited-beam nucleus to decay back to its ground state which, in turn, tells us about the nature of the excited state."

Coulomb excitation of neutron-rich nuclei is shown in this schematic in which a beam of tellurium-132 (132Te) nuclei collides with a target containing carbon-12 (12C) nuclei. Information about the shape of and charge distributions within a nucleus may be obtained from its electromagnetic properties. One of the best techniques for gleaning this information is to force an accelerated nucleus to collide with an appropriate target at energies just below the Coulomb barrier. Here, a 132Te nucleus in a beam is excited to higher energy levels through collision with a 12C nucleus. The excited 132Te nucleus then releases this additional energy by emission of gamma rays. Simultaneous detection of charged particles (the scattered target 12C nuclei) and gamma rays allows experimentalists to determine electromagnetic properties. In pioneering experiments at the HRIBF, this technique was used to investigate several radioactive isotopes of tin and tellurium for the first time. (Illustration enhanced by LeJean Hardin)

"In addition to being a unique facility now," says Nazarewicz, "the HRIBF will be a bridge to the DOE's proposed Rare Isotope Accelerator (RIA), which will be built somewhere in the United States in the next 10 years. Given the compelling physics that can be addressed with radioactive ion beams, the nuclear physics community in the United States has given construction of this facility its highest priority."

Nazarewicz and his collaborators in nuclear physics theory group have played a leading role in predicting the properties of nuclides far from stability," Baktash says. "They have also developed a compelling physics case for RIA." The proposed RIA will make it possible to produce and study more than 1000 new rare isotopes in the laboratory. The primary accelerator at RIA would be capable of delivering intense beams of many elements from hydrogen to uranium, with beam power in excess of 100 kilowatts and beam energies per nucleon up to 400 MeV.

RIA will combine the two major techniques of rare isotope production: fragmentation or fission of the primary beam with in-flight separation of the reaction products, and target spallation or fission ISOL, followed by acceleration of the isotope of interest. The integration of both production techniques into a single, advanced rare-isotope research facility will allow use of the full arsenal of experimental techniques being developed worldwide. In the meantime, HRIBF will serve as an important facility where new physical phenomena will be explored using radioactive ion beams, the necessary teams of researchers will be trained, and new research tools and experimental techniques may be developed.

*In the nuclear shell model, the constituent nucleons (protons and neutrons) move in nuclear-energy levels, or shells, that are filled, or closed, when the number of protons or neutrons equals 2, 8, 20, 28, 50, 82, or 126. These are the "magic numbers" because they indicate especially stable nuclei (e.g., helium-4 and tin-132).

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Related Web sites

Holifield Radioactive Ion Beam Facility
Oak Ridge Isochronous Cyclotron
ORNL Physics Division
CERN
DOE's Proposed Rare Isotope Accelerator (as envisioned by Argonne National Laboratory)