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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.