Atomic nuclei are generally deformed due to shell effects. Sometimes nuclei can exist in states of different shapes; this is called shape coexistence. The shape of a nucleus can be measured via the quadrupole moment and the transition strengths between states of different shapes. These quantities are sensitive probes to test nuclear structure models. We performed Coulomb excitation measurements to study shape coexistence in neutron-deficient krypton isotopes with ^74,76Kr beams from SPIRAL. In these measurements we applied the reorientation technique for the first time with a radioactive beam and could resolve the complex shape coexistence scenario of these nuclei. The next step would be to perform a similar measurement for the N=Z nucleus ⁷²Kr. However, the present beam intensity is not sufficient. Higher primary beam intensities are needed, which would require the installation of a new ion source, e.g. the GTS source. We have another on-going Coulomb excitation program to study the neutron-rich argon isotopes with the same technique. The main goal there is to investigate the evolution of the N=28 shell. Coulomb excitation is a straightforward technique that is applicable with beams of a few thousand ions per second. It is an old technique that is now being revived at radioactive beam facilities all over the world, and it will certainly be employed with beams from SPIRAL-2. Especially the light fission fragments around mass 100 exhibit very rich shape effects. But SPIRAL-2 offers also less conventional possibilities, for example Coulomb excitation of fusion-evaporation residues produced with the very intense stable beams of the LINAG. This can be an alternative to study elements that are not accessible with the ISOL technique.

        While Coulomb excitation is the tool to study nuclear shapes at low angular momentum, the most exotic shape effects occur at very high spins, where the rapid rotation can stabilize extreme deformations such as superdeformed and hyperdeformed shapes. These very large angular momenta can only be reached in fusion-evaporation reactions, which were so far limited to stable beams. SPIRAL-2 will deliver sufficiently intense beams of 10⁹ pps or more to be used in fusion reactions, even though there is still a lot to be learned in order to handle such intense beams and the background that they produce. Such experiments will be difficult, but there is very high potential for new and exciting physics, because with the radioactive beams we can reach more neutron-rich compound systems that can sustain higher angular momenta. With every additional neutron we bring into the system, we also push the spin limit by about two units, giving access to a new regime of ultra-high spins. These experiments will involve very high gamma-ray multiplicities, and of course we want to detect as many of the emitted gamma rays as possible. EXOGAM was not designed for high multiplicities; therefore we need a better and more efficient gamma detector. This detector is currently being built; it is called AGATA. Since we need a full 4pi sphere of AGATA detectors and the SPIRAL-2 beams at maximum intensity, which both will take time to be achieved, these experiments will probably not be realistic before 2014 or so.

More info:
    Andreas Görgen's talk
    Gamma spectroscopy session


Words collected by K. Turzó at the XV^e Colloque GANIL, Giens, France, from May 29^th to June 2^nd, 2006.