Search

Dissociation of a 240 MeV/u beam of He-6, incident on carbon and lead targets, has been studied in kinematically complete experiments to investigate low-lying excitation modes in the halo nucleus He-6. It is shown that alignment effects characterize the inelastic scattering and allow an unambiguous assignment of the spin of a narrow resonance observed in the excitation energy spectrum. The differential cross sections for the He-6 inelastic scattering on carbon and lead targets were deduced from the measured moments of the two neutrons and the a-particle. An analysis of these distributions shows that quadrupole and, possibly, monopole excitations characterize the hadronic interaction, while the dipole mode is dominating in Coulomb dissociation. Neither theoretically predicted new resonance states in He-6 nor nuclear excitation of a dipole mode were found. Direct evidence has been obtained for strong suppression of Coulornb post-acceleration in direct Coulomb breakup in a lead target.

Photoabsorption cross sections of trapped V 13 + clusters were measured for energies between 1.4 eV and 6.1 eV by use of a two-photon fragmentation technique. Using two laser pulses separated by a 100-ns delay period the cross sections of hot clusters (up to 1800 K) could be compared to the values at room temperature. Whereas sodium clusters are known to exhibit pronounced temperature dependence below 300 K, no continuation of these effects to higher temperatures was found for V 13 + .

13 pags, 5 figures.-- PACS nrs.: 24.60.−t; 25.70.Ef; 27.20.+n., Momentum distributions and invariant mass spectra from the breakup of He-6 ions with an energy of 240 MeV/u interacting with a carbon target have been studied. The data were used to extract information about the reaction mechanism which is influenced by the structure of He-6. It is found that the dominant reaction mechanism is a two-step process: knock out of one neutron followed by the decay of the He-5 resonance. The shape of the (α+n) two-body invariant mass spectrum is interpreted as mainly reflecting the 5He ground state which is a J(π) = 3/2(-) resonance. However, no evidence for correlations between cu particles and neutrons is observed in the momentum widths of the distributions. It is demonstrated that a combined analysis of the two-body invariant mass spectrum and an appropriate correlation function may be used to determine the properties of the intermediate resonance., This work was supported by the German Federal Minister for Education and Research (BMBF) under Contracts 06 DA 820, 06 OF 474 and 06 MZ 476 and by GSI via Hochschulzusammenarbeitsvereinbarungen under Contracts DARIK, OF ELK, MZ KRK and partly supported by the Polish Committee of Scientific Research under Contract PB2/P03B/113/09, EC under Contract ERBCHGE-CT92-0003 and CICYT under Contract AEN92-0788-C02-02 (MJGB) and by Deutsche Forschungsgemeinschaft (DFG) under Contract 436 RUS 130/127/1. One of us (B.J.)acknowledges the support through an Alexander von Humboldt Research Award.

4 pages, 1 table, 4 figures.-- Printed version published Mar 11, 2002.-- PACS nrs: 24.30.Gd; 25.60.-t; 27.20.+n, Low-lying resonance states in He-7(He-6 + n), formed after fragmentation reactions of a 227 MeV/nucleon He-8 beam on a carbon target, have been studied. Coincidences between He-6 nuclei and neutrons, corresponding to the one-neutron knockout channel in He-8, were selected. The relative energy spectrum in the He-6 + n system shows a structure, which is interpreted as the He-7 (Iπ = 3/2(-)) ground state, unbound with 0.43(2) MeV relative to the He-6 + n system and a width of Γ = 0.15(8) MeV overlapping with an excited (Iπ = 1/2(-)) state observed at 1.0(1) MeV with a width of Γ = 0.75(8) MeV., This work was supported by the BMBF under Contracts No. 06 DA 915I, No. 06 OF 838, and No. 06 MZ 864 and by GSI under Contracts No. DA RICK, No. OF ELZK, and No. MZ KRAK and partly supported by the Polish Committee of Scientific Research under Contract No. KBN/2P03B/144/10, by the EC under Contract No. ERBCHGE-CT92-0003, by CICYT under Contract No. AEN92-0788-C02-02 (M. J.G. B.), by RFBR under Contract No. 01-02-16685, and by WTZ under Contract No. RUS 674-98. One of us (B. J.) acknowledges support by Alexander von Humboldt Research.

19 pages, 3 tables, 6 figures.-- PACS nrs.: 27.20.+n; 24.70.+s; 25.06.-t.-- Available online Dec 4, 2000., Dissociation of 227 MeV/u 8He in a carbon target has been studied in kinematically complete experiments. The data include the relative energy spectrum, angular distributions in the neutron knock-out channel (6He+n) as well as diffractive dissociation and inelastic scattering into the (6He+2n) channel. The data are compared with corresponding results from the well-known halo nucleus 6He. In both cases it is found that neutron knock-out is the dominating reaction channel. The relative energy spectrum (6He+n) shows a structure, which is interpreted as being due to the Iπ=3/2− resonance in the 7He ground state with about equal contribution from its Iπ=1/2− spin–orbit partner., The 7He resonance shows a spin alignment similar to that observed in 5He, but with a smaller anisotropy indicating that the structure of the 8He ground state is more complicated than that of 6He., The data in the (6He+2n) channel were used to identify resonances in the excitation energy spectrum of 8He. If the spectrum is interpreted as two overlapping resonances, the spin-parity assignment for these is found to be 2+ and 1−, respectively., This work was supported by the BMBF under Contracts 06 DA 820, 06 OF 474 and 06 MZ 864 and by GSI via Hochschulzusammenarbeitsvereinbarungen under Contracts DA RICK, OF ELZ, MZ KRK and partly supported by the Polish Committee of Scientific Research under KBN grant No. 2 PO3B 14418, EC under contract ERBCHGE-CT92-0003, CICYT under contract AEN92-0788-C02-02 (MJGB) and WTZ under contract RUS 674-98. One of us (B.J.) acknowledges the support through an Alexander von Humboldt Research Award. The authors acknowledge fruitful discussions with Horst Lenske, Sergei Fayans and Leonid Grigorenko.