Beams of radioactive (unstable) nuclei. xvi xIn several nuclear physics laboratories, a capability exists to produce such beams and, before these nuclei spontaneously decay, use them to gain insight into the reactions on and structure of nuclei never before accessible. Radioactive beams are particularly useful to study stellar explosions such as novae, supernovae, and x-ray bursts. These explosions are some of the most catastrophic events in the universe, generating enormous amounts of energy while synthesizing the elements that make up lifeforms and the world. These spectacular explosions involve, and in some cases are driven by, reactions where the atomic nuclei of hydrogen (protons) and helium (alpha particles) fuse with (are captured by) radioactive isotopes of heavier elements to form new elements. The capability to produce beams of radioactive nuclei allows direct measurements of these reactions, providing crucial information needed to theoretically model cataclysmic stellar events and to understand the origin of many chemical elements.
One approach to radioactive beam production is the isotope separator on-line (ISOL) technique. One accelerator bombards a target with a beam of stable nuclei, and a small number of the radioactive atoms of interest are produced through nuclear reactions. These atoms are transported, by various techniques, including thermal diffusion, to an ion source where they are ionized (removing or adding electrons to give atoms an electrical charge) and extracted. The radioactive ions are then mass-separated from other ions and accelerated to energies needed for nuclear physics experiments by a second accelerator. The ISOL technique can produce very high beam qualities, purities, and intensities; the disadvantages are that only a few radioactive beam species can be generated from each combination of production target and primary beam, and that beams with short lifetimes (less than 1 s) are difficult to produce. See Ion sources
A complementary radioactive beam production technique is projectile fragmentation. When a high-energy beam of stable heavy ions passes through a thin target, the beam particles (projectiles) can break up into fragments—some of which are the radioactive isotope of interest. The desired fragments are then mass-separated from other ions and steered toward a target to undergo the reaction of interest. The projectile fragmentation technique can produce beams of very short lifetimes (10-6 s or less), and the same setup can be used to produce many different beam species; the disadvantages are that high beam quality, purity, and intensity are difficult to obtain. See Nuclear fusion, Nuclear reaction, Particle accelerator