Systematic investigations on the decay properties of the Isoscalar Giant Dipole Resonance

Complete text of publication follows. An experiment to study the direct proton-decay properties of the Isoscalar Giant Dipole Resonance (ISGDR) was carried out at the superconducting cyclotron facility of KVI using of the (α,α'p) reaction on 90Zr and 208Pb targets at Eα 200 MeV. A proton-decay experiment was first performed by our group for the ISGDR in 208Pb, and later in Osaka in 58Ni, which yielded partial branching ratios of the ISGDR for the first time. The primary motivation was to extend these studies and to pursue a systematic investigation of the direct-decay branching ratios to the single hole-states. These can be compared to the results of recent continuum-RPA calculations in order to learn about the microscopic structure and the subsequent damping mechanism of the ISGDR. Previously, systematics were established only for the direct-neutron decay of the ISGDR in 90Zr,116Sn and 208Pb, but the moderate resolution (∼ 1500 keV) of the neutron energies in the TOF measurements did not allow to resolve the individual final states in the daughter nuclei, and therefore the determination of partial branching ratios was not possible. The experiment was performed with the Big-Bite Spectrometer (BBS) in conjunction with the EuroSupernova focal-plane detector- system. The BBS was set at Θlab = 4 deg and 5 deg or 208Pb and 90Zr targets, respectively, covering a solid angle of 6 msr in both cases. The decay-protons were detected by a Si-ball consisting of 16 Si(Li)-detectors with a thickness of 5 mm each. These were placed in the backward hemisphere of the scattering chamber at a distance of 10 cm from the target, as in our previous experiment. In Fig. 1 the final-state spectrum populated by proton decay from a part of the ISGDR region (20-24 MeV) in 90Zr is shown, which well represents the effective selection of single hole-states (2p1/2, 2p3/2, 1f5/2) in 89Y. Projecting events populating these hole-states resulted in a better definition of the giant resonance strengths compared to multipole decomposition analyses of singles data. This is due to suppression of the underlying nuclear continuum, which decays by statistical evaporation of particles to complex configurations. The multipolarity to be studied (L=1 in our case) was selected in the coincidence spectra by the application of the difference-of-spectra technique, which exploits differences in the shapes of the angular distributions for various multipolarities. Preliminary values of the partial branching ratios were determined for the ISGDR strength, and were compared to theoretical predictions. The analysis and the interpretation of the results is still in progress. (author)