Search

One-particle-inclusive measurements have been performed for the charge, kinetic energy and angular distributions of reaction products from238U +238U at 1 766MeV (7.42MeV/u) incident energy. The deep inelastic products exhibit features similar to those seen in reactions induced by medium heavy nuclei: increasing particle transfer is observed with increasing energy damping, the angular distributions are peaked near the grazing angle, they broaden significantly with increasing energy loss and/or charge transfer. The dominant reaction mechanism, however, is found to be sequential fission of one or both primary reaction products. The reconstructed primaryZ- andQ-value distributions show more particle transfer at a given energy loss than in other systems, i.e. the diffusion process seems to proceed colder in this system. This is confirmed by relatively large cross sections for surviving deep inelastic reaction products belowZ=92. A direct search fora-decay or fission of superheavy nuclei being produced in a deep inelastic reaction and being implanted in a surface barrier detector resulted in an upper cross section limit of 2 ×10-32cm2.

Angular distributions of the proton and neutron transfer reactions14C(16O,15N)15N and14C(16O,17O)13C leading to the ground states of the final nuclei were measured atElab=20, 25 and 30 MeV. A DWBA analysis was performed using the no-recoil approximation of Buttle and Goldfarb. All angular distributions, including the pronounced structures of the proton transfer arising from the fact that the final nuclei are identical, are well reproduced. The spectroscopic factor for the neutron transfer is in agreement with shell model calculations whereas the proton transfer into aj<-state yields a value which is too high. Exact finite-range calculations do not show this discrepancy, indicating that recoil effects are important even for light targets and lower energies. Contributions of the nonnormall-transfer, however, are small.

Protons and a-particles were measured in coincidence with fusion residues for the reaction48Ti+45Sc at 16 MeV/u beam energy. Their velocity distributions show a drastic broadening to higher as well as to lower velocities as compared to the calculated equilibrium emission pattern. This is interpreted as a kinematical effect of variations of the initial velocity of the emitting system, which are attributed to the preequilibrium emission of heavier clusters from the projectile or the target nucleus in incomplete fusion processes. The measured widths of the velocity distributions of the fusion residues are consistent with this mechanism. A peak in the product mass spectrum at masses between 6 and 18 is tentatively identified with the postulated cluster emission.

All reaction products in the range from target- and projectile-like to fission-like fragments were measured for the system84Kr+27Al at 5.9 MeV/u beam energy. They are assigned to the various reaction mechanisms on the basis of experimental signatures (energy dissipation, mass and angular distribution). The sum of the measured partial cross sections, including the evaporation residue yield obtained previously, agrees with the total reaction cross section derived from elastic scattering. A small fast-fission component was found, discernible from deep-inelastic reactions by its 1/sin ? angular distribution, and distinguished from compound-nucleus fission by an incomplete mass asymmetry relaxation.

Angular distributions for inelastic scattering, for 1 and 2-nucleon transfer reactions were studied at 18.2 MeV/u beam energy. Owing to sequential decay of excited reaction products the nuclide yield measured at various angles and in different regions of total kinetic energy loss (TKEL) does not represent the yield of primordial nuclidesAZ. Monte Carlo calculations were applied in order to reconstruct the primary double-differential cross-sections d2s/dOdTKEL for values of TKEL up to 80 MeV. By comparing the probability of inelastic scattering and that of transfer reactions it is found that the former dominates for TKEL< 10 MeV, whereas the situation is reversed above 20 MeV. No evidence is found for the excitation of giant resonances with significant cross section.

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.

A search fora-decay of high spin isomers in the mass range 140132Xe and50Ti have been used to provide high velocity recoils with angular momenta beyond 50?. Recoil distance experiments sensitive to half lives longer than 10 ps and shorter than about 1 ns have been performed for 31 different compound nuclei. Within a sensitivity of 1–30µb per event no delayeda-emission has been observed.

Mass andZ-distributions of fusion products from the reaction 5.8 MeV/u46Ti+12C were measured in the angular range between?lab=1.5° and 8° using a time-of-flight?E-E telescope. The results are compared with an earlier measurement of the reaction32S+26Mg, in which the same compound nucleus was produced at higher excitation energy. The residual nuclide distributions are interpreted with the aid of evaporation calculations and the influence of shell effects in the level densities is discussed. The data are consistent with the assumption that the shell effects vanish above 15 to 20 MeV excitation energy, that is, that they are essentially only important for the last evaporation step.

We present data for the132Xe+56Fe-system at 5.73 MeV/u laboratory energy. Due to inverted kinematics, where Xe is the projectile, we were able to measure energy spectra as well as angular distributions for reaction products with 20?Z?60 with unitZ-resolution; i.e. target- and projectile-like fragments have been investigated. The reaction shows a well focussed quasielastic component, where charge transfer from the light to the heavy collision partner dominates. This apparent tendency towards more asymmetric fragmentation is explained by a potential energy surface which favours such charge transfer in order to minimize the asymmetry energy of the liquid drop. The strongly damped component which constitutes the major part of the reaction cross section exhibits characteristics of a fusion-fission reaction with typical fission fragment kinetic energies and 1/sinTc.m. angular distributions.