Giant resonances in unstable oxygen isotopes

Electromagnetic and nuclear breakup of the neutron-rich oxygen isotopes ranging from A=17 to A=22 is studied experimentally in reactions at energies around 600 MeV/u. The beams were produced in fragmentation reactions and separated by the GSI fragment separator FRS. By measuring the four-momenta of all decay products after inelastic scattering and neutron decay of the projectile, the excitation energy is determined. From the differential cross sections d#sigma#/dE"* for electromagnetic excitation, the E1-strength distributions can be deduced. For "1"8","2"0","2"2O, low-lying dipole strength is observed, exhausting about 5% of the Thomas Reiche Kuhn sum rule for energies up to 5 MeV above the continuum threshold. (orig.)Available from TIB Hannover: RO 801(98-54) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Neutron removal in peripheral relativistic heavy ion collisions

We investigate the relativistic Coulomb fragmentation of "1"9"7Au by heavy ions, leading to one-, two- and three-neutron removal. To resolve the ambiguity connected with the choice of a specific minimum impact parameter in a semiclassical calculation, a microscopic approach is developed based on nucleon-nucleon collisions ('soft-spheres' model). This approach is compared with experimental data for "1"9"7Au at 1 GeV/nucleon and with a calculation using the 'sharp-cutoff' approximation. We find that the harmonic-oscillator model predicting a Poisson distribution of the excitation probabilities of multiphonon states gives a good agreement with one-neutron removal cross sections but is unable to reach an equally good agreement with three-neutron removal cross sections. (orig.)Available from TIB Hannover: RO 801(94-54) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Photo-neutron cross sections for unstable neutron-rich oxygen isotopes

The dipole response of stable and unstable neutron-rich oxygen nuclei of masses A = 17 to A = 22 has been investigated experimentally utilizing electromagnetic excitation in heavy-ion collisions at beam energies around 600 MeV/nucleon. A kinematically complete measurement of the neutron decay channel in inelastic scattering of the secondary beam projectiles from a Pb target was performed. Differential electromagnetic excitation cross sections d#sigma#/dE were derived up to 30 MeV excitation energy. In contrast to stable nuclei, the deduced dipole strength distribution appears to be strongly fragmented and systematically exhibits a considerable fraction of low-lying strength, exhausting up to 12% of the energy-weighted dipole sum rule at excitation energies below 15 MeV. (orig.)Available from TIB Hannover: RO 801(2001-11) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Electromagnetically-induced nuclear-charge pickup observed in ultra-relativistic Pb collisions

A strong increase of inclusive nuclear-charge pickup cross sections, forming _8_3Bi from 158 A GeV _8_2Pb ions, is observed in comparison to similar measurements at 10.6 A GeV. From the dependence of these cross sections on target atomic number, this increase is attributed to the electromagnetic process of pion production by equivalent photons. The observed cross sections can be reproduced quantitatively using the recently developed RELDIS code. (orig.)Available from TIB Hannover: RO 801(2002-02) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Measurement of the dipole response of neutron-rich nuclei in the A #approx# 20 region

Coulomb break up of the neutron-rich "1"5","1"7C and "1"7"-"2"2O isotopes has been studied experimentally using secondary beams at energies of 500-600 MeV/u. A comparison between differential cross sections, d#sigma#/dE"*, with that obtained from a binary model shows that the main ground state configurations of "1"5C is "1"4C(0"+) x #nu#_S__1__/__2 as expected. For "1"7C, our preliminary data analysis reveals that the predominant (#propor to#80%) configuration of the ground state is "1"6C(2"+) x #nu#_s_,_d. For "1"7"-"2"2O, the low-lying E1 strength amounts upto about 10% of the energy weighted sum rule strength depending on neutron number. The energy weighted E1 strength (integrated up to 15 MeV excitation energy) increases up to "2"0O then decreases for "2"1","2"2O. These data are compared to a shell model calculation. (orig.)Available from TIB Hannover: RO 801(2000-18) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Fragmentation of unstable neutron-rich oxygen beams

Fragmentation of secondary beams of neutron-rich, unstable "1"9","2"0","2"1O isotopes at beam energies near 600 MeV/nucleon was studied by measuring the production cross sections for carbon, nitrogen, and oxygen fragments. Data for stable "1"7","1"8O beams were obtained as well. The measurements serve to illuminate the isospin dependence of the fragmentation process. The experimental results are compared those from an empirical parametrization and to those from abrasion-ablation models. (orig.)Available from TIB Hannover: RO 801(2002-15) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Direct experimental evidence for strong admixture of different parity states in "1"1Li

Peripheral fragmentation of a 287 MeV/u beam of the halo nucleus "1"1Li incident on a carbon target has been studied in a fragment-neutron coincidence experiment. The momentum distribution of the "1"0Li fragments gives access to the momentum distribution of the removed neutron in "1"1Li. From the shape of the distribution, the (1s_1_/_2)"2 contribution to the mixture of (1s_1_/_2)"2 and (0p_1_/_2)"2 components in the ground-state wave function was determined to be 45#+-#10%. The angular correlation between the knocked out neutron and the one from the decay of "1"0Li shows a skew distribution signalling interference of these two different parity states. (orig.)22 refs.Available from TIB Hannover: RO 801(99-22) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Continuum excitations in "6He

The three-body break-up "6He#->#"4He+n+n is studied experimentally, using a secondary "6He ion beam of 240 MeV/u incident on carbon and lead targets. Integrated cross sections for one- and two-neutron knock-out and differential cross sections d#sigma#/dE"* and d#sigma#/d#theta# for inelastic nuclear or electromagnetic excitations into the "6He continuum are presented. The E1-strength distribution is deduced from electromagnetic cross sections and is found to exhaust (10#+-#2)% of the energy-weighted TRK sum rule or (40#+-#8)% of the cluster sum rule for excitation energies below 5 MeV. Both the energy-weighted and non-energy-weighted dipole cluster sum rules are almost exhausted integrating the strength up to 10 MeV, a fact, from which a root-mean-square distance between the #alpha# core and the two valence neutrons of r_#alpha#_-_2_n=(3.36#+-#0.29) fm is derived. The known I"#pi#=2"+(1.80 MeV) resonance in "6He is observed in nuclear inelastic scattering; model dependent values of the quadrupole deformation parameter #delta#_2=(1.7 #+-# 0.3) fm or B(E2,0"+#->#2"+)=(3.2#+-#0.6) e"2fm"4 are derived. No clear signature could be obtained for predicted higher-lying 2"+ resonances, but low-lying continuum strength of multipolarity other than dipole, likely of monopole and quadrupole multipolarity, is indicated by the data. Two-body correlations in the decaying "4He+n+n system are investigated. The astrophysical relevance of the data with regard to the two-neutron capture process "4He(2n,#gamma#)"6He is briefly discussed. (orig.)SIGLEAvailable from TIB Hannover: RO 801(99-01) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Impact-parameter dependence of giant resonance excitations in relativistic heavy-ion collisions

Angular distributions of Xe fragments produced in peripheral collisions of a "1"3"6Xe beam (700 MeV/nucleon) with "2"0"8Pb and "n"a"tSn targets were measured. Equivalent sharp-cutoff minimum impact parameters were derived on the basis of a semi-classical description for the electromagnetic excitation of one-and two-phonon giant resonances. The results are compared with current standard parametrizations of minimum impact parameters and with the soft-spheres model using realistic mass density distributions for projectile and targets. (orig.)15 refs.Available from TIB Hannover: RO 801(99-32) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Multiphonon giant resonances in nuclei

We review the present knowledge of multiphonon giant resonances in nuclei. Theoretical concepts approaching the intrinsic structure and excitation mechanisms of multi-phonon states are discussed. The available experimental results are summarized, including a brief description of applied techniques. This review emphasizes electromagnetic excitations of double dipole resonances. Open questions and possible routes toward a solution are addressed. (orig.)Available from TIB Hannover: RO 801(98-36) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman