Photoionization at relativistic energies

At MeV energies and beyond the inner-shell vacancy production cross section associated with the photoelectric and Compton effect decrease with increasing photon energy. However, when the photon energy exceeds twice the rest energy of the electron, ionization of a bound electron may be catalyzed by the creation of an electron-positron pair. Distinctly different from all other known mechanisms for inner-shell vacancy production by photons, we show that the cross section for this ''vacuum-assisted photoionization'' increases with increasing photon energy and then saturates. As a main result, we predict that vacuum-assisted photoionization will dominate the other known photoionization mechanisms in the highly relativistic energy regime. (orig.)Available from TIB Hannover: RO 801(2000-45) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Invariant mass spectrum and #alpha# - n correlation function studied in the fragmentation of "6He on a carbon target

Momentum distributions and invariant mass spectra from the breakup of "6He ions with an energy of 250 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 "6He. It is found that the dominant reaction mechanism is a two-step process: knock out of one neutron followed by the decay of the "5He resonance. The shape of the (#alpha#+n) two-body invariant mass spectrum is interpreted as mainly reflecting the "5He ground state which is a J "#pi# =3/2"- resonance. However, no evidence for correlations between #alpha# 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. (orig.)Available from TIB Hannover: RO 801(97-61) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Neutron momentum distribution from ''core break-up'' reactions of halo nuclei

Neutron angular distributions from violent break-up reactions of "1"1Li and "1"1Be have been measured at 28 MeV/u and 280 MeV/u and at 41 MeV/u and 460 MeV/u, respectively. The derived neutron momentum distributions show a narrow component in transverse momentum that is within uncertainties independent of beam energy and target charge. This component is suggested to be simply related to the momentum distribution of the loosely bound halo neutron(s) in the projectiles. (orig.)SIGLEAvailable from TIB Hannover: RO 801(95-02) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

"6He and neutron momentum distributions from "8He in nuclear break-up reactions at 240 MeV/u

Neutron and "6He momentum distributions from "8He break-up reactions in a C-target have been measured at 240 MeV/u. The two-neutron removal cross section was found to be #sigma#_-_2_n=0.27#+-#0.03 b. The nature of the momentum distributions is interpreted in some simple reaction scenarios. (orig.)32 refs.SIGLEAvailable from TIB Hannover: RO 801(96-04) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman