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We investigate the beam energy dependence of neutron and proton squeeze-out in collisions of197Au+197Au atE/A=400—800 MeV. The azimuthal anisotropy that describes the enhanced emission of mid-rapidity neutrons perpendicular to the reaction plane rises strongly with the transverse momentum of the neutrons. This dependence of the azimuthal anisotropy follows a universal curve — independent of beam energy — if the neutron momenta are measured in fractions of the projectile momentum per mass unit. Analogously, the kinetic energy spectra of mid-rapidity neutrons exhibit a universal behaviour as a function of the kinetic energy of the projectile.

We present data on collective flow of neutrons in collisons of [sup 197]Au + [sup 197]Au at 400 MeV/nucleon. The azimuthal distribution about the beam axis is investigated with respect to the reaction plane as determined from light charged particles. The squeezeout'' of neutrons, perpendicular to the reaction plane, is observed for the first time. Quantitative agreement to a high level of accuracy is found between the behavior of neutrons and hydrogen ions.

Excitation functions in the vicinity of the Coulomb barrier have been measured for the formation of evaporation residues in100Mo-induced fusion reactions with90, 92, 96Zr,92, 96, 98, 100Mo,104Ru and110Pd as well as for the system96Zr+96Zr. From these data the fusion probability in central collisions was extracted covering a range of 4 orders of magnitude. At the fusion barriers expected from systematics we find that the fusion probability is suppressed by one to three orders of magnitude. It is rising very gradually at higher energies and reaches for the heaviest systems saturation only at energies as high as 30 MeV above the barrier. The observed hindrance of the fusion process increases roughly with the growing Coulomb repulsion between the collision partners, but there is also a distinct influence of their individual nuclear structure. The data are compared to the extra-push model, the surface-friction model and the diabatic fusion model. A parameterisation of the extra-push energy and its fluctuation in terms of a macroscopic quantity like the Coulomb repulsion combined with a microscopic quantity characterizing the nuclear structure is proposed. As a byproduct of this work a new alpha emitter,191Po, could be identified. Its half-life is (15.5 −2.5 +6 ) ms, the alpha energy is (7314±20) keV.

The nuclear response of a medium-mass nucleus ([sup 136]Xe) to electromagnetic excitation in a near-relativistic heavy-ion collision was investigated in the reaction[sup 136]Xe(0.7[ital A] GeV)+Pb. From an exclusive measurement of the neutron decay of the excited [sup 136]Xe projectiles, strong excitations of giant resonances and, in particular, of the double isovector giant dipole resonance were identified. A resonance energy of 28.3[plus minus]0.7 MeV, a width of 6.3[plus minus]1.6 MeV, and a total cross section of 215[plus minus]50 mb were found for the double giant dipole resonance.

In bombardments of 90Zr, 92Zr and 94Mo with 90Zr ions at energies near the Coulomb barrier, radiative fusion has been observed where the compound nuclei do not evaporate nucleons but emit only γ radiation. The deexcitation by the emission of γ radiation only is probably favoured by the very low excitation energy of the compound nuclei. In addition a substantial enhancement of the El γ strength in the deexcitation process seems to be necessary to reproduce the experimental cross sections. To obtain further information on the fusion process, the spectral distributions of the deexcitation γ transitions as well as their multiplicities have been measured in the reactions 90Zr, 96Zr + 90Zr.

The entrance-channel dependence of the distribution of reaction strength has been studied for three systems, namely $^{32}\mathrm{S}$${+}^{182}$W, $^{48}\mathrm{Ti}$${+}^{166}$Er, and $^{60}\mathrm{Ni}$${+}^{154}$Sm, which all lead to the compound system $^{214}\mathrm{Th}$ in complete fusion reactions. The cross sections for elastic/quasielastic scattering, deeply inelastic, and fissionlike processes were measured at beam energies of ${\mathit{E}}_{\mathrm{lab}}$ =166, 177, 222, 260 MeV for $^{32}\mathrm{S}$${+}^{182}$W, ${\mathit{E}}_{\mathrm{lab}}$=220, 240, 270, 298 MeV for $^{48}\mathrm{Ti}$${+}^{166}$Er, and ${\mathit{E}}_{\mathrm{lab}}$=339, 390, 421 MeV for $^{60}\mathrm{Ni}$${+}^{154}$Sm, respectively. The maximum contribution of complete-fusion fission processes to the fissionlike cross section is estimated on the basis of expected angle-mass correlations for such reactions. The results show a strong entrance-channel dependence as predicted by the extra-push model. \textcopyright{} 1996 The American Physical Society.

Neutron and He-6 momentum distributions from He-8 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(-2n) = 0.27 +/- 0.03 b. The nature of the momentum distributions is interpreted in some simple reaction scenarios.

7 pages, 3 figures, 1 table., Transverse and longitudinal momentum distributions of 9Li fragments from 11Li break-up reactions in C, Al and Pb targets have been measured at 280 MeV/u. The two-neutron removal cross-section was measured to be σ(−2n) = 0.26 ± 0.02 b for the carbon target, σ(−2n) = 0.47 ± 0.08 b for the aluminum target and σ(−2n) = 1.9 ± 0.4 b for the lead target. No significant difference is observed between the narrow widths (FWHM ≈ 47 MeV/c) of the transverse and longitudinal momentum distributions of the 9Li fragments. The physical implications of this are discussed., This work was supported by the German Federal Minister for Research and Technology (BMFT) under Contracts 06 DA 641 I, 06 BO 103 and 06 MZ 106 and by GSI via Hochschulzusammenarbeitsvereinbarungen under Contracts DA RICK, BO FRE, F ELE, Mz KRD and partly supported by the Polish Committee of Scientific Research under Contract PB 1158/P3/92/02 and CICYT under contract AEN92-078%CO2-02 (MJGB).

Neutron angular distributions from violent break-up reactions of Li-11 and Be-11 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.

Excitation functions for evaporation residues were measured for the reactions 90 Zr + 89 Y, 90 Zr, 92 Zr, 96 Zr, and 94 Mo. Deexcitation by γ-radiation only was found for the compound nuclei 179 Au, 180 Hg, 182 Hg and 184 Pb. Fusion probabilities as well as fusion-barrier distributions were deduced from the measured cross sections. There are strong nuclear structure effects in subbarrier fusion. For energies far below the fusion barrier the increase of the fusion probabilities with increasing energy is found to be much steeper than predicted by WKB calculations. As a by-product of this work new α-spectroscopic information was obtained for neutron-defi- cient isotopes between Ir and Pb.

The interplay of shell structure and rotational motion in the nuclear level density is discussed. The result of a schematic calculation reveals a washing out of spherical shell effects at much lower excitation energies than expected if only the intrinsic level density in the independent particle model is considered.

Evaporation residues from the fusion of the nearly symmetric systems86Kr+123Sb and124Sn+94Zr have been detected. The compound nucleus218Th could be produced with an excitation energy as low as 21 MeV. The barriers for the formation of a compound nucleus were deduced from the evaporation residue cross sections. There is no evidence for an additional amount of energy needed to fuse these systems, if compared with the extrapolation of fusion barrier systematics or with results of fusion model calculations not including friction phenomena. One of the systems reaches a value ofZ 1·Z 2=2,000, about the highest value where evaporation residues have ever been observed.

The reaction /sup 32/S+ /sup 182/W has been studied at beam energies of 166, 177, 222, and 260 MeV with the goal of accounting for the distribution of the total reaction cross section among the main reaction channels. Cross sections as well as mass and energy distributions were measured over the entire angular range from 10/sup 0/ to 170/sup 0/ for elastic (which includes quasi-elastic), deep-inelastic, and fission-like products. Deep inelastic scattering accounts for a substantial fraction of the total reaction cross section at all energies, in particular near the barrier. This behavior is not predicted by current theories such as the extra push model. The angular distributions of fission-like products are more anisotropic than expected on the basis of the saddle point model at the two lowest energies, indicating a sizable contribution from quasi-fission processes, which is estimated from a quantitative analysis of this deviation.

The γ rays emitted in the radiative fusion reaction 90 Zr+ 90 Zr→ 180 Hg+ xy were measured. No indication was found for a specific direct capture process associated with a high-energy γ ray. The shape of the observed γ spectrum is compatible with a statistical deexcitation of the compound nucleus.

In fusion reactions of40Ar with isotopes of Ho, Tm, Yb, Lu, Hf and Ta, cross sections for the production of proton-rich evaporation-residues near the 126 neutron shell were measured. This first comprehensive study of very fissile spherical residues reveals a surprisingly low stabilizing influence of the sphericalN=126 shell on the survival probability. The experimental results are compared with evaporation calculations. Conclusions for the production of superheavy nuclei are drawn.

Theα-decays of199Rn (E α = (6.989±0.010) MeV,T 1/2>300ms) and of199m Rn (E α = (7.060±0.015) MeVT 1/2=(61 −2 +32 ) ms) were identified by anα-α-correlation method. They proceed via separated decay paths to195Po and195m Po, respectively. The spins of the involved states are discussed by aid of the shell model, and a tentative spin assignment is given.

Cross sections for the two-body channels populated in the /sup 32/S+ /sup 24/Mg reaction at E/sub c.m./ = 60.8 MeV have been measured by use of a coincidence technique which allows correction for secondary light-particle evaporation. The data show reaction yields with full equilibration of energy and mass-asymmetry coordinates. These results suggest an asymmetric fission mechanism and are contrary to what is expected from the previously proposed ''orbiting'' mechanism in light systems.

In the fusion reactions 90Zr+90Zr, 92Zr, and 94Mo the radiative fusion process, that is the deexcitation of the compound nucleus only by γ‐radiation, was detected. The radiative fusion was identified in all three cases by the observation of the ground state α‐decay of the compound nucleus. For the reaction 90Zr (358 MeV) + 90Zr, the γ‐rays were detected in coincidence with evaporation residues. Gamma rays from radiative fusion and from particle evaporation channels were separted by establishing a time correlation between the evaporation residues and their subsequent radioactive α‐decay. The γ‐rays were observed in 20 NaI detectors covering a total solid angle of 90% of 4 π. This set‐up allowed a determination of the γ multiplicity distribution of γ rays. The results do not support the assumption that specific direct processes associated with high energy γ rays are observed for radiative fusion. The low exciation energy of the compound nuclei, which is achievable with these heavy, symmetric systems seems t...