Unambiguous identification

Of three copper isomers has been achieved in a radioactive-beam experiment at CERN. Isomers are excited states of a specific nucleus (they differ slightly in their masses)—not to be confused with isotopes, which are nuclei having the same number of protons but different numbers of neutrons. Isomers relax through the emission of gamma rays or by alpha or beta decay. Almost half of all known nuclei have isomers, whose half-lives range from nanoseconds to beyond the age of the universe. At the ISOLDE (On-Line Isotope Mass Separator) facility, the researchers created the exotic nuclide copper-70 by bombarding a thick target with energetic protons. Different isomers were selected using mass spectrometry and then captured and contained in a Penning trap, where the ions zipped around in looping trajectories. An isomer’s mass could then be deduced from its orbital period (the cyclotron frequency). The new observations have cleared up some mysteries about specific assignments of spin and mass in the isomers of ⁷⁰ Cu. Furthermore, because ⁷⁰ Cu has 41 neutrons, the results are an important step toward understanding the complex structure of nuclei with a closed subshell of 40 neutrons. (J. Van Roosbroeck et al., Phys. Rev. Lett. 92, 112501, 2004 http://dx.doi.org/10.1103/PhysRevLett.92.112501 .)