Intermediate energy Coulomb excitation is well suited to study low-lying excited bound states in exotic nuclei. For the case of light neutron-rich nuclei in the rr(sd)-shell this technique permits the experimental investigation of excited states close to the neutron dripline with exotic beam intensities of as little as 15 particles/second. Initial experiments have revealed the disappearance of the N = 20 shell closure and the weakening of the N = 28 shell closure for neutron-rich isotopes.
1. APPROACHING THE NEUTRON DRIPLINE EXPERIMENTALLY
Neutron-rich nuclei in the rc(sd)-shell (8 < Z _< 20) are unique in the sense that they have enough constituents to exhibit collective properties and at the same time it is experimentally possible to study them close to the neutron dripline. Until the advent of radioactive ion beams, the spectroscopic study of nuclei in this region of the table of isotopes has been limited to beta-decay studies and nuclei relatively close to the valley of beta-stability. There are several reasons. First, there are no suitable stable beam/target combinations to synthesize neutron-rich isotopes in the 7r (sd)-shell with fusion-evaporation reactions. Second, while one-and two-particle transfer reactions have been used in the past to study nuclei up to two mass units removed from the valley of beta-stability in this region, the yield falls off dramatically for multi-particle transfer reactions. With the availability of radioactive ion beams it has become possible to extend our experimental knowledge of simple collective properties to nuclei closer to the neutron dripline. The case of aZMg exemplifies the vast improvement in exotic ion beam intensity achieved in the last 20 years. In 1977, a2Mg was found to be particle stable in the natU(p,32Mg) reaction at LAMPF [2]. About 20 particles were collected in five days. The first spectroscopic study of 32Mg in 1983 at CERN [3] resulted in the identification of a low lying excited state in 32Mg [4]. The reaction n~tlr(p, a2Na) at 10 GeV/nucleon was used to produce a few thousand 3~Na particles and the decay a2Na --+ 32Mg+Β’3 was observed. Eighteen years after the discovery that a2Mg was particle stable Motobayashi and collaborators at RIKEN produced a beam of 32Mg with an intensity of 300 particles/second by projectile fragmentation (using the reaction 9Be(4Β°Ar,32Mg) at 94 MeV/nucleon). This beam intensity was high enough to Coulomb excite the projectile in a lead target and to observe the de-excitation photon in an array of NaI detectors. The energy of the photopeak confirmed the location of the first excited state in 32Mg seen in the/3-decay study twelve