Fragment kinetic energies and modes of fragment formation

Kinetic energies of light fragments (A #<=# 10) from the decay of target spectators in "1"9"7Au + "1"9"7Au collisions at 1000 MeV per nucleon have been measured with high-resolution telescopes at backward angles. Except for protons and apart from the observed evaporation components, the kinetic-energy spectra exhibit slope temperatures of about 17 MeV, independent of the particle species, but not corresponding to the thermal or chemical degrees of freedom at breakup. It is suggested that these slope temperatures may reflect the intrinsic Fermi motion and thus the bulk density of the spectator system at the instant of becoming unstable. (orig.)39 refs.Available from TIB Hannover: RO 801(2000-10) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Breakup conditions of projectile spectators from dynamical observables

Momenta and masses of heavy projectile fragments (Z #>=# 8), produced in collisions of "1"9"7Au with C, Al, Cu and Pb targets at E/A=600 MeV, were determined with the ALADIN magnetic spectrometer at SIS. Using these informations, an analysis of kinematic correlations between the two and three heaviest projectile fragments in their rest frame was performed. The sensitivity of these correlations to the conditions at breakup was verified within the schematic SOS-model. For a quantitative investigation, the data were compared to calculations with statistical multifragmentation models and to classical three-body calculations. With classical trajectory calculations, where the charges and masses of the fragments are taken from a Monte Carlo sampling of the experimental events, the dynamical observables can be reproduced. The deduced breakup parameters, however, differ considerably from those assumed in the statistical multifragmentation models which describe the charge correlations. If, on the other hand, the analysis of kinematic and charge correlations is performed for events with two and three heavy fragments produced by statistical multifragmentation codes, a good agreement with the data is found with the exception that the fluctuation widths of the intrinsic fragment energies are significantly underestimated. A new version of the multifragmentation code MCFRAG was therefore used to investigate the potential role of angular momentum at the breakup stage. If a mean angular momentum of 0.75 #Planck constant#/nucleon is added to the system, the energy fluctuations can be reproduced, but at the same time the charge partitions are modified and deviate from the data. (orig.)Available from TIB Hannover: RO 801(98-15) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Breakup temperature of target spectators in "1"9"7Au + "1"9"7Au collisions at E/A=1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction "1"9"7Au+"1"9"7Au at 1000 MeV per nucleon. Pairs of "3","4He and "6","7Li isotopes and pairs of "3","4He and H isotopes (p,d and d,t) yield consistent temperatures after feeding corrections, based on the quantum statistical model, are applied. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions. The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the observed universality of the spectator decay at relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles, measured at #theta#_l_a_b=150 , exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Inclusive fragment production in

Inclusive cross sections of intermediate mass fragments from the reaction "8"4Kr+"1"9"7Au at E/A=35 MeV were measured over the range 8 #<=##THETA#_l_a_b#<=#70 with a low detection threshold. A moving-source parameterization was used to fit the double-differential cross sections. The integrated cross section for fragment production exceeds the total reaction cross section thus indicating a large probability for multi-fragment processes. The deduced large temperature parameters can be explained by assuming emission from a rotating source. From the comparison to reactions with "1"2C and "4"0Ar projectiles at E/A=30 MeV a systematics of inclusive fragment production as a function of the projectile mass is obtained. (orig.)Available from TIB Hannover: RO 801(93-30) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Total and nuclear fission cross sections of "2"3"8U at relativistic energies

Total fission cross sections of "2"3"8U projectiles were measured at bombarding energies of 0.6 and 1 GeV per nucleon for seven different targets (Be, C, Al, Cu, In, Au and U). It is found that all data points fall onto one curve, independent of bombarding energy, once the electromagnetic contribution to the total fission cross sections is subtracted. The abrasion-ablation model predicts a significantly weaker target dependence than observed, and underestimates the nuclear fission cross sections for the heavier targets. (orig.)SIGLEAvailable from TIB Hannover: RO 801(96-11) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Multifragmentation of spectators in relativistic heavy-ion reactions

Using the ALADIN forward-spectrometer at SIS we have studied multi-fragment decays of "1"9"7Au projectiles after collisions with C, Al, Cu, and Pb targets at a bombarding energy of E/A=600 MeV. The new data presented in this work comprise the measured cross sections of multi-fragment processes, the N/Z ratios of the produced fragments, and the differential distributions of fragment multiplicities and of observables built on the charge correlations within the event. The "1"9"7Au+Cu data are analyzed with the Statistical Multifragmentation Model. It is shown that all observables can be simultaneously reproduced on an absolute scale, apart from an overall normalization constant which relates the number of model events to the measured cross section. A continuous distribution of excited residual nuclei, used as input for the calculations, was derived from the criterion of a best fit to the data. It exhibits a correlation between decreasing mass number A with increasing excitation energy E_x/A and a saturation of the excitation energy at E_x/A#approx#8 MeV. (orig.)Available from TIB Hannover: RO 801(94-36) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Electromagnetic fission of "2"3"8U at 600 and 1000 MeV per nucleon

Electromagnetic fission of "2"3"8U projectiles at E/A=600 and 1000 MeV was studied with the ALADIN spectrometer at the heavy-ion synchrotron SIS. Seven different targets (Be, C, Al, Cu, In, Au and U) were used. By considering only those fission events where the two charges added up to 92, most of the nuclear interactions were excluded. The nuclear contributions to the measured fission cross sections were determined by extrapolating from beryllium to the heavier targets with the concept of factorization. The obtained cross sections for electromagnetic fission are well reproduced by extended Weizsaecker-Williams calculations which include E1 and E2 excitations. The asymmetry of the fission fragments' charge distribution gives evidence for the excitation of the double giant-dipole resonance in uranium. (orig.)SIGLEAvailable from TIB Hannover: RO 801(95-28) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Charge-pickup of "2"3"8U at relativistic energies

Cross sections for the charge-pickup of "2"3"8U projectiles were measured at E/A=600 and 1000 MeV for seven different targets (Be, C, Al, Cu, In, Au and U). Events with two fission fragments with a sum charge of 93 in the exit channel were selected. Due to the significant excitation energy, the dominant part of produced Np nuclei fission instead of decaying to the ground state by evaporation. The observed cross sections can be well reproduced by intranuclear-cascade-plus-evaporation calculations and, therefore, confirm recent results that no exotic processes are needed to explain charge-pickup processes. (orig.)24 refs.Available from TIB Hannover: RO 801(95-60) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Probing the nuclear liquid-gas phase transition

Fragment distributions resulting from Au+Au collisions at an incident energy of E/A=600 MeV are studied. From the measured fragment and neutron distributions the mass and the excitation energy of the decaying pre-fragments were determined. A temperature scale was derived from observed yield ratios of He and Li isotopes. The relation between this isotope temperature and the excitation energy of the system exhibits a behavior which is expected for a phase transition. The nuclear vapor regime takes over at an excitation energy of 10 MeV per nucleon, a temperature of 5 MeV and may be characterized by a density of 0.15-0.3 normal nuclear density. (orig.)55 refs.SIGLEAvailable from TIB Hannover: RO 801(95-13) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Breakup configurations in multiple disintegration of projectile fragments

Kinematic correlations between three heavy projectile fragments produced in Au induced reactions at E/A=600 MeV are presented. The sensitivity of these correlations to the disassembly geometry is confirmed by classical three-body trajectory calculations. The simulations suggest a fast disintegration process of an highly excited system. Within the assumed scenarios the data constrain a possibly existing radial flow to a maximum value of about 1 MeV per nucleon. This radial motion can be provided by the Coulomb repulsion between the fragments if the breakup occurs out of a volume with a radius not larger than 15 fm. (orig.)SIGLEAvailable from TIB Hannover: RO 801(93-54) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman