The nuclear shell model remains an essential tool in describing the structure of nuclei heavier than carbon, with shells corresponding to the “magic” numbers of protons (Z) or neutrons (N) associated with particular stability. A good way to probe the shell model is through the study of the magnetic dipole moment of a nucleus. Indeed, the model should describe particularly well the magnetic dipole moment of an isotope with a single particle outside a closed shell, as in this case the moment should be solely determined by this last nucleon. Copper isotopes (Z = 29), with one proton outside the closed shell of nickel (Z = 28), provide an example of such a system, which has been systematically studied at CERN’s ISOLDE facility with the Resonance Ionization Laser Ion Source (RILIS). The COLlinear LAser SPectroscopy (COLLAPS) collaboration uses collinear laser spectroscopy on fast beams and the NICOLE facility employs nuclear magnetic resonance on oriented nuclei.
Unfortunately, the tendency for chemical compounds to form in the thick target of the ISOLDE facility, does not permit the efficient release of the short-lived isotope 57Cu (T½= 199 ms), This isotope is of particular interest as it can simply be described as the doubly-magic 56Ni plus one proton, but a recent measurement of its magnetic moment strongly disagreed with this picture (Minamisono et al. 2006).
The Leuven Isotope Separator On-Line (LISOL), a gas-cell-based laser ionization facility at the Cyclotron Research Centre in Louvain-La-Neuve in Belgium, is perfectly suited for 57Cu. Beams of protons at 30 MeV and of 3He at 25 MeV impinge on a thin target of natural nickel. The radioactive copper isotopes produced recoil directly out of the target and are thermalized and neutralized in the argon buffer gas. The flow of the buffer gas then transports the isotopes to a second chamber where two laser beams, tuned on atomic transitions specific to the element of interest, give rise to resonance ionization of the atoms.
Resonance ionization has provided very pure beams of radioactive isotopes for more than a decade. It also enables in-source resonance ionization laser spectroscopy, as at ISOLDE’s RILIS. The new feature recently developed at LISOL is the implementation of laser spectroscopy in a gas-cell ion source (Sonoda et al. 2009). Its first on-line application has been the measurement of the magnetic dipole moment of the interesting copper isotopes 57,59Cu.
A team at LISOL observed the hyperfine structure spectra of several isotopes of copper, namely 57,59,63,65Cu, and extracted the hyperfine parameters, which yield the magnetic dipole moments. They were able to perform the measurement of 57Cu with yields as low as 6 ions a second, showing the high sensitivity of the technique (Cocolios et al. 2009). The accuracy is demonstrated by the very good agreement with known hyperfine parameters for 63,65Cu and with the measured magnetic dipole moments for the stable isotope 65Cu and for the radioactive isotope 59Cu, studied previously at ISOLDE. This meant that the team at LISOL was able to disprove with confidence the previous measurement of the magnetic dipole moment of 57Cu. Moreover, the new value is in agreement with several nuclear shell model calculations based on the N = Z = 20 40Ca core and the N = Z = 28 56Ni core, thereby confirming understanding of nuclear structure in this region.
This new technique opens the door for the study of short-lived refractory elements, which are not accessible at ISOLDE, to be probed in new radioactive ion beam facilities, such as at the accelerator laboratory at the university of Jyväskylä (JYFL), GANIL in Caen, RIKEN in Tokyo and the National Superconducting Cyclotron Laboratory at Michigan State University.
Further reading
T E Cocolios et al. 2009 PRL 103 102501.
K Minamisono et al. 2006 PRL 96 102501.
T Sonoda et al. 2009 Nucl. Instr. and Meth. B 267 2918.
