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Abstract.: The mass distributions of baryon resonances populated in near-central collisions of Au on Au and Ni on Ni are deduced by defolding the pt spectra of charged pions by a method which does not depend on a specific resonance shape. In addition the mass distributions of resonances are obtained from the invariant masses of (p, π±) pairs. With both methods the deduced mass distributions are shifted by an average value of −60 MeV/c2 relative to the mass distribution of the free Δ(1232) resonance, the distributions descent almost exponentially towards mass values of 2000 MeV/c2. The observed differences between (p, π−) and (p, π+) pairs indicate a contribution of isospin I = 1/2 resonances. The attempt to consistently describe the deduced mass distributions and the reconstructed kinetic energy spectra of the resonances leads to new insights about the freeze out conditions, i.e. to rather low temperatures and large expansion velocities.

Charged pions are measured with the 4π detector FOPIat GSIusing the Au on Au reaction at 1.06 AGeV bombarding energy. The pion multiplicities ngpincrease with the number of participants Apart. The average pion multiplicities per participant are #### and #### These values are only half as large as extrapolated from the low-mass systems studied by Harris et al. The ratio #### increases with Apartand decreases with the pion kinetic energies. The pion kinetic energy spectra have concave shapes, their parametrization in terms of thermal Boltzmann distributions yields a low (Tl,π) and high (Th,π) temperature which change with the cm emission angle Θ of the pions. In the angular range 45dg < Θ< 135° the low temperature < Tl,π−> is larger than < Tl,π+>, the high temperatures < Th,π−>, < Th,π+> are, within experimental uncertainties, the same. The inclusive polar angular distributions of pions are anisotropic, dσ/dΩincreases for forward and backward angles. The forward — backward enhancements are independent of the pion kinetic energies or the number of participants. In addition to the preferred forward — backward emission, also the enhanced emission into the transverse direction Θ =90° is observed for pions with high energies or for pions from near-central collisions. These observations and the shape of the rapidity spectra suggest that pions, emitted from the central rapidity region, are partly rescattered by spectator matter. The strength of the rescattering process depends only weakly on the number of participants. The experimental data are compared to the results of IQMD/GEANTcalculations using momentum dependent NNinteractions and a hard equation of state. The calculated pion multiplicities are approximately 50% larger than experimentally determined; the existence of secondary pion sources is reproduced by the calculation, but their predicted strengths are larger than experimentally observed.

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.

Using on-line mass separation of evaporation residues from the reaction58Ni +92Mo?150Yb*, a proton line of 1,055±6 keV energy and 0.42±0.10 s half-life was observed at mass number 147. The origin of this activity is very likely the direct proton decay of147Tm. Beta-delayed protons registered at the same mass position show a pronounced peak structure in their energy distribution. A lower limit of their half-life was set to 1 s.

A (1231±3) keV proton activity has been observed in the fusion reaction58Ni +96Ru?154Hf*. The production cross section peaks at 50 MeV of excitation energy with a value of about 70 µb. No coincidences with annihilation radiation or withK X-rays could be observed. The activity is assigned to direct proton decay of the new isotope151Lu. The measured half life is (85±10) ms.

The cross sections for production of evaporation residues (ser) and for fusion-fission (sff) have been measured for40Ar+144, 148, 154Sm at sub-barrier energies by observation of x-ray emission from radioactive products and by direct,?E-E identification of fission fragments, respectively. These isotopes span the transition region from spherical (144Sm) to strongly deformed (154Sm) equilibrium shapes. The cross section for fusion,sfus=ser+sff, is found to vary markedly at low energies with the isotope number and, hence, with the quadrupole collectivity of the target. The thresholds for fusion of148Sm and144Sm are, respectively, ~3.5 MeV and ~7 MeV (c.m.) higher than for fusion with154Sm. These differences and the energy dependence of the fusion cross sections are discussed in terms of the effect of nuclear deformation on heavy-ion fusion. A comparative analysis of results for16O+Sm and40Ar+Sm in terms of static deformation indicates thatsfus for the Ar+Sm system at very low energies is enhanced relative to the prediction for a one-dimensional barrier based on a fit tosfus for16O+Sm. This may be an indication that additional degrees of freedom (such as formation of a neck or fragment elongation) may be important for fusion with the larger projectile. At energies above the fusion barrier, values ofsfus for144, 148Sm are nearly equal, but are significantly smaller than for154Sm. This is in contrast to the results of previous experiments with16O projectiles in whichsfus (16O+148Sm) andsfus (16O+154Sm) were nearly equal above the barrier. These differences, observed for144, 148Sm and154Sm at energies above the barrier may reflect a new mechanism which is not encompassed by a static theory.

Fusion reactions between58Ni projectiles and58Ni target nuclei were used to produce114Cs activity, which was studied by on-line mass separation and decay spectroscopy. The obtained half-life of 0.57±0.02 s and the probability ratio of 44±3 between betadelayed proton and beta-delayed alpha emission represent data with improved precision. The absolute branching ratios for both particle decay modes were determined to be (7 ±2)×10-2 and (1.6±0.6)×10-3, respectively. These branching ratios and the shapes of particle spectra are discussed with reference to the statistical model calculations. Groundstate alpha decay of114Cs was observed with an alpha energy of 3,226±30 keV and a branching ratio of (1.8±0.6)×10-4. An upper limit of the branching ratio for a possible ground-state proton decay was set at 5×10-4. From?-ray singles and?-? coincidence measurements, excited states in114Xe are proposed.

Very neutron deficient isotopes in the Hf-Re region were produced by bombarding targets of107, 109Ag,nat, 108, 110Pd, and103Rh with58Ni accelerated by the linear accelerator UNILAC at GSI. After separation from the projectile beam by the velocity filter SHIP, the fusion products were implanted with their full recoil energy into a silicon surface barrier detector. The subsequent alpha decay of the implanted ions was measured with the same detector. In some of the experiments the evaporation residues were implanted into a position sensitive silicon surface barrier detector. With a newly developed position and time correlation technique parent daughter relationships, half lives, anda branching ratios of a large number of isotopes could be determined. In the investigated reactions the eleven new isotopes161–164Re,160W,157–161Ta, and156Hf could be identified. The measuredQa values, half lives, and a branching ratios are discussed. Two new high energya. transitions of (7,408±10) keV and (7,804±15) keV were also found with half lives of 2.7 ms and 0.52 ms, respectively.A tentative mass and atomic number assignment givesA=153–156 andZ=70–72. The energies of the excited isomeric states are between 2 MeV and 3 MeV. From the systematics of alpha ground state transitions in even nuclei a hindrance of 105 can be deduced for both transitions, possibly indicating orbital angular momenta on the order of 10h for the alpha emitting states. The isomers are proposed to belong to shell model isomers beyond146Gd.

Very neutron-deficient isotopes in the trans-tin region were produced by reactions of 290 MeV 58Ni ions on 58Ni targets, and studied by on-line mass separation at GSI. Twelve α-emitters — 107–110Te, 110–113I, 111–113Xe, 114Cs — were identified. Systematics of Qαvalues and reduced α-widths for this new island of α-emission are discussed.

The experiment having led to the discovery of first isotopes of element 108, as published previously in Short Notes to this Journal [1, 2] are described and discussed in a final paper.

The height and position of the fusion barrier for the real part of the interaction potential between two colliding nuclei are studied as a function of the Coulomb strength. The calculations are performed in the framework of the spherical constrained HF + BCS formalism using the frozen approximation. The effective Skyrme type force SKa is used. The model contains no free parameters. For the height of the fusion barrier good agreement is found with the values obtained using the empirical Bass potential that is known to reproduce well the data for many nuclear systems. In contrast, the recently observed apparent rise of the fusion barrier (‘extra-extra-push’) for very massive nuclear systems with a product of nuclear chargesZ1 xZ2 above 1600 cannot be reproduced. We also give estimations for the overlap of the mass densities of both colliding nuclei at the fusion barrier distance.

In irradiations of207Pb and208Pb, respectively, with54Cr thea-decay of the isotopes259106,260106, and261106 could be observed for the first time. For260106 a spontaneous fission branch of (50-20+30)% was observed. The isotopes were identified by genetic relationships of a-decay after separation in-flight with the velocity filter SHIP and implantation into a position-sensitive silicon surface-barrier detector.

In fusion evaporation reactions of a92Mo beam with targets of neutron deficient Rb — Mo isotopes very neutron deficient isotopes of elements between Au and Po have been produced. The new isotopes173, 174Au,175, 176Hg, and179Tl were identified by alpha spectroscopy. The mass excess value of176Hg could be linked to known values of theN-Z=16 chain. The location of the new isotopes with respect to the proton drip line is discussed. A new high energya transition of (7.20±0.02) MeV andT1/2=(1.4±0.5) ms has been found in the reaction92Mo+89Y?181Tl* at an excitation energy of 37 MeV. It is tentatively assigned to isotopes produced in 2-particle evaporation channels.

Experimental cross sections to synthesize heavy ion induced fusion reactions are confronted with calculations using the conventional picture of complete fusion after passing the barrier plus statistical deexcitation after complete equilibration. The sensitivity of the calculations to some of the parameters is demonstrated. Despite this sensitivity, a single parameter set is found that reproduces well the large body of data from experiments using actinide targets. By comparison, for the more symmetric entrance channels using targets around208Pb, we find a fusion hindrance that increases steeply with increasing compound nuclear charge. Predictions for a few reactions of future interest for the synthesis of heavy elements are also presented.

We identified twoa-emitting isotopes of element 107 with masses of 261 and 262 respectively by parent-daughter correlations. For the isotope with mass 262 we found two transitions with (102±26) ms and (8.0±2.1) ms half-life, which we assign to the groundstate and to an isomeric transition, respectively. The half-life of the isotope with mass 261 is (11.8-2.8+5.3) ms. Spontaneous fission, which could be assigned to the decay of an isotope of element 107, was not detected in our experiments. This observation is in-line with our previous results, indicating a region of nuclei with strong microscopic stabilisation in the trans-actinides. Both new isotopes were produced by complete fusion of209Bi with54Cr. The production cross sections are (163±34) pb for262107 and (36-14+22) pb for261107.

Evaporation residues produced in heavy-ion fusion-reactions of50Ti and207Pb,208pb have been investigated. They were separated from the projectile beam by the velocity filter SHIP and identified after implantation into position-sensitive surface-barrier detectors, where their a decay or their decay by spontaneous fission was measured. The production of weakly excited compound nuclei in reactions of projectiles and targets close to the doubly magic48Ca and208Pb was confirmed by the observation of the 1n deexcitation channel in the reaction50Ti +208Pb.

The energy dependence of the fusion probabilities for central collisions of the systems124Sn +92,94,96Zr and86Kr +123Sb was determined from neutron-evaporation-residue cross sections. Near the barrier as calculated from one-dimensional barrier-penetration models, the fusion probability was found to be reduced by several orders of magnitude.

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.

Evaporation residues from the heavy-ion fusion reaction50Ti on209Bi were investigated. They were separated from the projectile beam by the velocity filter SHIP and identified after implantation into an array of position-sensitive surface-barrier detectors by analyzing theira-decay chains. Spontaneous fission was also observed.

An experiment to synthesize element 109 is presented. Decay patterns characteristic of complete fusion products were searched for in an irradiation of209Bi targets with58Fe projectiles at specific incident energies of 4.95, 5.05, and 5.15 MeV/u. A total dose of 7 ×1017 particles was obtained. The experimental method involves in-flight separation of forward peaked reaction products with a static-field velocity filter, their passage through a time-of-flight device and their final implantation into position sensitive solid state detectors to measure their kinetic energy, approximate mass and their time and position of incidence. The subsequent decay of the narrowly localised reaction products by cascades of alpha particles and/or spontaneous fission is also registered in terms of the energies and times of all the emitted particles. One outstanding decay sequence that started with the emission of two alpha particles within subsequent time intervals of 5 ms and 22 ms and ended with spontaneous fission after 13 s was found at 5.15 MeV/u. The first alpha particle had a kinetic energy of (11.10±0.04) MeV. A detailed analysis of all the alternative interpretations of this observation, such as a purely random correlation of signals, the decay of a product from a transfer reaction or of any of the various energetically possible evaporation residues, shows that the isotope with mass 266 of element 109, i.e. the one neutron evaporation channel after complete fusion, is the statistically most significant assignment. The outlook for new element synthesis is also briefly discussed.

The evaporation residues produced in heavy ion fusion reactions between40Ar and206Pb,208Pb, and209Bi, respectively, were investigated. They were separated in-flight using a velocity filter (SHIP) and identified after implantation into an array of position sensitive surface barrier detectors by measuring their decay characteristics.

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 ofZ1·Z2=2,000, about the highest value where evaporation residues have ever been observed.

A level density formula that takes into account the smoothed volume, surface and curvature dependence of the single particle level density at the Fermi surface using the results of Balian and Bloch, is shown to be compatible with the level spacings found in neutron resonance data if complemented by a simple Ansatz for shell effects (due to Ignatyuk) and pairing effects. The three parameters involved, a scaling parameter, a shell damping energy and a pairing energy shift are compatible, respectively, with known nuclear radii, microscopic level density calculations and odd-even mass fluctuations. At excitation energies on the order of the neutron binding energy no evidence for an absolute level density problem or a different behaviour of level densities (collective contributions) for deformed nuclei as opposed to spherical nuclei is found. The proposed level density formula allows to calculate a priori macroscopic ratios of level densities, e.g. at the groundstate and at the saddle point, removing this important parameter from the analysis of fissionability data. As a first application, the fissionability of a number of actinide nuclei at excitation energies a few MeV above the fission barrier is analysed.

The excitation function for the232Th(p,f) reaction has been determined in the energy range from 4 to 11 MeV. A discrepancy existing in the literature is resolved. The mass yield distribution has been studied at 8 MeV proton energy and the peak-to-valley ratio has been determined to be 9.2. The fission data, together with evaporation residue data from an earlier experiment, are understood rather well within the framework of a statistical decay model which derives its parameters from independent data. The absorption cross sections for protons in the vicinity of and below the Coulomb barrier are also discussed and confronted with optical model predictions using published global proton potentials.

Mass- and charge-yield distributions for 19 < Z< 84 were determined radiochemically for the binary collision products of 51V +197Au collisions at a bombarding energy corresponding exactly to the Bass-model barrier, Ecm= B, and at Ecm= B+25 MeV. The average excitation energies as a function of Zare determined by comparing the centroids of the experimental, secondary mass distributions for given values of Zwith the calculated primary centroids from minimization of the potential energy of the di-nuclear system, i.e. from the missing masses. At the barrier, in striking contrast to a thermal equilibrium, we find an extreme donor-acceptor asymmetry in the excitation-energy division reminiscent of the “sawtooth” phenomenon in low-energy nuclear fission. Here, the excitation energy sharing is apparently dominated by shape fluctuations at scission. At the slightly higher bombarding energy, Ecm= B+25 MeV, we observe a rapid change toward equipartition of the excitation energy indicating that, here, the excitation energy division due to shape fluctuations is already covered up by the dissipative exchange of nucleons. Also, the balance of integral cross sections for fusion fission, deep-inelastic scattering, and quasi fission is investigated and is shown to contain important information about the dynamical evolution of the 51V +197Au system after having passed the entrance channel barrier.

Using the FOPI facility at GSI, charged particles (1 ≤Z≤6) produced in the Au(150 A MeV)+Au reaction have been measured at laboratory angles 1.20< Θlab<300. Highly central collisions have been selected with two criteria, both dealing with the longitudinal and transverse degrees of freedom of the reaction. The relevance of this selection method is supported by QMD calculations which indicate that such criteria are able to select mean impact parameters less than 2 fm. Bias effects introduced by the criteria have been evaluated. The centre-of-mass polar angle distributions of low energy clusters emitted in these central collisions, have been extracted: the intensity ratio deduced for a transverse to longitudinal emission is found to be R= 1.4−0.4+0.2. Model comparisons using QMD are presented. The value of R appears to depend sensitively on the nucleon-nucleon cross section, σnn. Within this model, a value of σ=25+ - 5 mb is derived.