Unambiguous Identification of Copper Isomers

Unambiguous identification of copper isomers has been achieved in a radioactive-beam experiment at CERN, helping scientists better understand what goes on inside nuclei and learn more about the personality of the nuclides.

Isomers are the excited states of a specific nucleus (they differ slightly in their masses) and should not be confused with isotopes, which are nuclear species with the same number of protons but a different number of neutrons.

Excited atoms return to their resting (groundstate) levels by emitting photons. Excited nuclei, by contrast, relax by emitting alpha particles (nuclear fragments consisting of two protons and two neutrons), beta particles (the antiquated name for electrons and positrons from a time before the nature of this radiation was known), and gamma rays (the highest energy photon type).

The excited states of atoms can be populated by inputting laser energy (in electron-volt lumps or less). Populating nuclear isomers isn't quite that easy since the difference in energy states is measured in millions of eV. Almost half of all known nuclides have isomers. Their half-lives range from nanoseconds to beyond the age of the universe.

Here's how isomers are excited and studied. At the ISOLDE (the On-Line Isotope Mass Separator) facility an exotic radioactive species can be created by proton bombardment, preselected by conventional mass spectrometry and then captured and contained in a Penning trap, an enclosure using both electric and magnetic fields that makes the ions whiz in looping trajectories. The mass of the ion can be deduced from its orbital period (the cyclotron frequency) to better than one part in 10 million (for an earlier paper establishing the masses of several argon isotopes to unprecedented precision, see Blaum et al., Physical Review Letters, 31 Dec 2003).

In the present experiment, the isotope in question is copper-70. Having caught the specimens in their trap, the physicists were able to distinguish two excited states and the ground state (all of which relax via beta decay) by measuring with the Penning trap mass spectrometer ISOLTRAP the cyclotron frequency of each isomer, and thus their mass (excited states are a bit heavier than the groundstate).

The new observations have cleared up some mysteries as to specific assignments of spin and mass in the isomers of Cu-70. Furthermore, since Cu-70 has 41 neutrons, the results provide an important step in understanding the complex structure of nuclides with one neutron less (N=40) which correspond to a closed sub-shell. Moreover it demonstrates the power of the techniques used for future nuclear structure studies. (Van Roosbroeck et al., Physical Review Letters, upcoming article; contact Klaus Blaum, klaus.blaum@cern.ch,; text at www.aip.org/physnews/select ; also see lab website.)