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Scientific Opportunities with a Rare-Isotope Facility in the United States D Glossary Beta-nuclear magnetic resonance: In general, nuclear magnetic resonance (NMR) enables the study of local magnetic and electronic environments in condensed matter through the measurement of the spin precession and relaxation of a probe nucleus. In beta-NMR, a beam of appropriate radioactive, beta-decaying nuclei is created; then the nuclei are highly polarized, for example, by tuned laser hyperfine interaction with the radioactive atoms, and are finally implanted at the correct depth or sites in the material under study. The temporal response of the nuclear spin to the local environment is followed through the detection of beta-decay electrons preferentially emitted antiparallel to the nuclear spin, thereby tracking the probe’s spin response to its environment. This method has much in common with muon-spin resonance in which polarized muons are used as the local probe. In both cases, detection efficiencies are as much as 10 orders of magnitude greater than with conventional NMR. Density functional theory (DFT): A quantum mechanical method used in physics and chemistry to investigate the detailed structure of many-body systems. Basically, DFT describes an interacting system of fermions via its density and not via its many-body wave function. Electronvolt (eV): The energy acquired by an electron accelerated through a potential difference of 1 volt. Using the standard system of measurement prefixes, the following also holds: keV = 1,000 eV; MeV = 1 million eV; GeV = 1 billion eV.
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Scientific Opportunities with a Rare-Isotope Facility in the United States Exotic nucleus: A nucleus whose proton number (Z) and neutron number (N) are different from those nuclei in the valley of stability. The term is often used synonymously with “a nucleus far from stability” or “a rare isotope.” Such nuclei are unstable and hence decay to more stable configurations. Fast breeder reactor and fast neutron reactor: The fast breeder reactor is a type of fast neutron reactor designed to produce more fissile material than it consumes. More generally, in fast neutron reactors, fast neutrons maintain the chain reaction. This kind of reactor requires no moderator; instead, it uses enriched fuel and has an efficient neutron “economy.” In the fast neutron reactor, excess neutrons can be used either to produce extra fuel, as in the fast breeder reactor, or to transmute long-half-life waste to less troublesome isotopes, or to do both. Fission: A process in which the heavy nucleus rapidly divides into two lighter species of roughly equal mass, releasing energy. Fragmentation: A nuclear reaction process in which the primary high-energy heavy ions irradiate targets of light materials such as lithium or carbon. The breakup of the heavy ion produces short-lived nuclear fragments that have approximately the primary beam velocity. Fragmentation is the opposite of the spallation reaction. Gas catcher ion source: An apparatus used to provide high-quality beams of rare isotopes of any element except helium. In a gas catcher ion source, high-energy rare isotopes are decelerated by solid absorbers and finally slowed to rest in pure helium gas. Rare isotopes stopped in this way remain charged and can be extracted quickly from the helium gas by a combination of electric fields and gas flow. Inertial fusion: The achievement of controlled fusion through the tailored implosion of small deuterium-tritium capsules driven by lasers, ion beams, or pulsed power. There are several schemes for carrying out inertial fusion, including direct drive, indirect drive, and “fast ignition,” depending on how the lasers (for instance) are used to deposit their energy and drive the capsule. In-flight: A production method in which the fragmented exotic nuclei directly exit the production target at velocities similar to those of the primary beam and are isotopically separated and then directly used for experiments.
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Scientific Opportunities with a Rare-Isotope Facility in the United States ISOL: Isotope Separator On-Line. A method for producing exotic nuclei in which the nuclei are produced (often by the collision of an energetic light ion with a high Z target) in a thick hot target. These rare species diffuse out of the target, are ionized, and extracted to form a beam for reacceleration. Limitations arise owing to the time required (relative to the lifetimes of some exotic nuclei) of the diffusion process, the near impossibility of extracting refractory elements (those elements that are not sufficiently volatile at the elevated temperatures to effuse out of the ISOL target and diffuse into the ion source), and the peculiarity of the chemistry and surface physics of each element produced. For those nuclei that can be extracted by this method, it often provides the most intense beams. Isomer: A metastable nuclear excited state. Isomers can play significant roles in nuclear-reaction kinetics in astrophysics and stockpile stewardship applications. Isomers can also have technological significance—for example, the single photon emission computed tomography (SPECT) gamma-emitting isomer 99mTc. Linac: Short form of “linear accelerator,” which is a device used to accelerate ions or electrons. This type of accelerator is “straight” and comprises a series of resonators or cavities that provide the acceleration via high-frequency electric fields. One of its principle advantages is the ease with which the accelerated beam can be extracted from the accelerator. Monoclonal antibody: Derived from a single kind of immune cell that in turn is a clone of a single cell. In principle able to bind specifically to any antigen (such as those produced by cancers), monoclonal antibodies can both detect and target cancer cells by radioimmunotherapy. Mössbauer effect: The recoil-free, resonant emission and absorption of narrow-line-width gamma rays by atoms bound in cooled solids. Perturbed angular correlation (PAC): The angular correlations in the gamma-gamma decay of radioactive probe atoms due to perturbations induced by the neighboring atoms. Positron emission tomography (PET): A medical imaging method by which a metabolically active compound is tagged with a radionuclide decaying via positron emission. The positrons in turn annihilate with electrons mainly producing nearly back-to-back gammas that are detected in coincidence and used for the three-dimensional tomographic reconstruction of the local metabolic activity. 11C, a typical PET nuclide, with a lifetime of 20.3 minutes, can be produced via 14N(p,α).
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Scientific Opportunities with a Rare-Isotope Facility in the United States Reaccelerated beam: A mode of operation for a rare-isotope facility based on bringing short-lived isotopes to rest using irradiation of targets with a primary beam, and then using a second or “post” accelerator to create beams of these stopped isotopes at the energies required for nuclear science or other applications. Reacceleration can follow either an ISOL or gas catcher ion source method. Reaction notation (n,γ), (n,xn), (n,p), and so on: In nuclear reactions that have two bodies interacting to produce two bodies in the final state, the reaction is denoted as (x,y) with x, y being the light bodies entering and leaving the reaction: n + 88Y → 89Y + γ, or 88Y(n, γ)89Y is an example of a (n,γ) reaction on the nucleus 88Y. Single photon emission computed tomography (SPECT): The attachment of a gamma emitter such as 99mTc to a biologically active compound aimed at specific tissues or biochemical pathways. The spatial and angular dependence of the gamma emission is then “inverted” to produce a metabolism-dependent three-dimensional image of the target. Slow neutron-capture process (s-process): A nucleosynthesis process that occurs at lower-neutron-density, lower-temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta decay. Spallation: A nuclear reaction process in which a high-energy light ion such as a proton or deuteron irradiates a thick target of heavier nuclei to produce rare isotopes. Spallation is different from fragmentation in that the heavy nucleus is at rest in the case of the former. Specific activity: The fraction of radioactive atoms in a sample that have a specifically desired radioactive property. Statistical reaction model: A now widely applied model proposed by Hauser and Feshbach in 1952. It is often used in cases in which neutron cross sections on excited nuclei are desired and it is sufficient to apply approximations based on the idea that the neutron plus nucleus forms an intermediate “compound” nucleus subject to simple statistical rules. Storage rings: The storage of energetic exotic nuclei for use in experiments. The energetic nuclei are guided in a circular orbit by magnetic fields. A storage ring has the advantage that thin targets can be used, since the beam of exotic nuclei can be
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Scientific Opportunities with a Rare-Isotope Facility in the United States cooled and recirculated to pass through the same target thousands of times. It has the disadvantage of being typically limited to exotic nuclei with half-lives on the order tenths of seconds or more. Superconducting driver accelerator: A high-power primary accelerator or linac employed for the production of rare isotopes. In a superconducting linac, the acceleration of the particles is provided by electric fields in a series of superconducting resonant cavities. In a superconducting cyclotron, the magnetic field keeping the particles in circular orbits is generated by a superconducting magnet, but the accelerating fields are created by room-temperature structures. Surrogate method: In cases in which it is difficult to measure a desired cross section directly because the target has too short a lifetime or otherwise cannot be obtained, it is sometimes possible to infer the cross section from a surrogate reaction that exploits different initial particles but shares a common intermediate product nucleus with the desired reaction. As a point example of the surrogate method, consider the partial cross section for n + 155Gd → 156Gd** → 156Gd* + γ. One can infer the cross section from the “inverse” neutron removal reaction 3He + 157Gd → 156Gd* + α + γ, under the assumption that the common intermediate excited nucleus, 156Gd** equilibrates (the Weisskopf-Ewing approximation). Recently, the surrogate method has been experimentally and theoretically revisited to successfully measure the energy-dependent fission cross section for 235mU. Furthermore, the equilibration and angular momentum constraint assumptions that underlie the surrogate method have been the subject of experimental tests. Two-step method: A production method for exotic nuclei in which the primary beam impacts a first target, which produces secondary projectiles that produce exotic nuclei in a secondary target. The most frequent case refers to a primary deuteron beam impinging on a target nucleus to produce an intense beam of neutrons, which bombards a heavy target such as uranium to produce exotic neutron-rich nuclei. This technique has the advantage of separating the area of intense beam heating (the first target) from the exotic-nucleus production target.
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