Reactions between 12C and heavy target nuclei at 84 MeV/u

Using an experimental setup which allows the coincident investigation of the various projectile and target fragments the reactions between 84 MeV/u ¹²C projectiles and different targets were studied. Impact parameters have been deduced from the size and multiplicity of projectile fragments. Using a “minimum bias” triqqer relative cross sections have been determined for the different reaction channels. New results on the production of medium-heavy fragments are presented.     

Direct comparison of phase-space distributions of K- and K+ mesons in heavy-ion collisions at SIS energies -- evidence for in-medium modifications of kaons?

The ratio of K^- to K⁺-meson yields has been measured in the systems ⁹⁶Ru + ⁹⁶Ru at 1.69 A GeV, ⁹⁶Ru+⁹⁶Zr at 1.69 A GeV, and ⁵⁸Ni+⁵⁸Ni at 1.93 A GeV incident beam kinetic energy. The yield ratio is observed to vary across the measured phase space. Relativistic transport-model calculations indicate that the data are best understood if in-medium modifications of the kaons are taken into account. Received: 30 November 1998 / Accepted: 20 December 2000     

Alkylidynephosphines - Syntheses and Reactivity

Abstract

In the past, different methods have been utilized for the preparation of alkylidynephosphines. Whereas, however, small amounts of thermally instable derivatives might be obtained from reactions in the gas phase, the synthesis of phosphines which are stable under an inert atmosphere, as for instance those with a tert-butyl or a 1-adamantyl substituent at the carbon atom of the PEC group, is best started with tris (trimethylsilyl) phosphine itself or with the more reactive lithium bis (trimethylsilyl) phosphide.2 tetrahydrofuran complex. Treatment of either compound with acyl halides results in the formation of acylbis (trimethylsilyl) phosphines which, at room temperature, rearrange to the corresponding trimethylsilyl [1-(trimethylsiloxy) alkylidene] isomers. As traces of hydrogen halide accelerate the conversion of tris (trimethylsilyl) phosphine to the triacyl derivatives, the use of the lithium phosphide is strongly recommended in all cases where impure acyl halides are used. In the presence of small amounts of solid sodium hydroxide suspended in an etherial solvent, the thus prepared trimethyl[1-(trimethylsiloxy)-alkylidene]phosphines eliminate hexamethyldisiloxane to yield the required alkylidyne compounds. Running the decomposition without a solvent at a higher temperature, Regitz and coworkers were able to improve this method further.     

Moments of charge distributions in the reaction92Mo+92Mo at 14.7 MeV/u

First and second moments of primary charge distributions were determined as a function of the total kinetic energy loss in the reaction⁹²Mo+⁹²Mo at an incident energy of 14.7 MeV/u. Up to energy losses of 200 MeV no significant drift of the first moments is observed. The second moments are in agreement with predictions of a stochastic model derived at lower bombarding energies.     

Fusability and fissionability in 86Kr-induced reactions near and below the fusion barrier

Evaporation-residue excitation functions for the reactions ⁸⁶Kr + ^70,76Ge, ^92,100 Mo, ^99,102,104 Ru have been measured using activation methods and the velocity filter SHIP. The data span the region from well below the fusion barrier up to, and beyond, the energy where limitation by fission competition takes place. The data are shown to be compatible with the concept of complete fusion followed by the statistical decay of the equilibrated compound nucleus. Information on both the fusion probability at and below the fusion threshold and the fissionability of the compound nuclei formed, is extracted. The model dependence of the extracted fission barriers is discussed in detail. In analogy to studies involving lighter projectiles, strong correlations between the low-energy nuclear-structure properties of the nuclei and the subbarrier fusion probability are found. A relative shift of the fusion barrier to higher energies, that increases with the number of valence neutrons in the target nuclei, is observed.     

The (18O,16O) reaction on even tin isotopes

The two-neutron transfer reaction^ASn(¹⁸O,¹⁶O)^A+2Sn has been measured for the isotopes A=112, 116, 118, 120, 122 and 124 at bombarding energies of 57 and 60 MeV together with the elastic scattering. Angular distributions have been analysed for the transitions to the ground state and to the first excited 2⁺ state. The observed ground state transition is strongly enhanced. The theoretical DWBA analysis is performed with a finite range 2n-transfer form factor including recoil correction. The calculated cross section reproduces the observed systematic change over all isotopes. The absolute cross sections are normalized by a factor of 4.7 and 7.5, depending on the two different sets of 2n-wave functions used in the analysis. The results confirm the prediction of the pairing model that the transition strengths of a neutron pair between the ground states of even tin isotopes are the same.     

Analysis of the sequential fission observed in collisions of100Mo +100Mo and120Sn +120Sn around 20 A·MeV

Events with 2, 3 and 4 heavy fragments (A≥20) detected in the reactions¹⁰⁰Mo +¹⁰⁰Mo at 18.7, 23.7 A·MeV and¹²⁰Sn +¹²⁰Sn at 18.4 A·MeV were analyzed by means of an improved version of the kinematic coincidence method. The phase-space distributions prove that 3- (and possibly 4-) body events predominantly originate from a two-step mechanism and are compatible with the hypothesis of a binary deep-inelastic interaction followed by the further fissionlike decay of one (or both) of the primary fragments. The characteristics of the fission step — mass asymmetry, relative velocity, in-plane and out-of-plane angles — have been reconstructed for the 3-body events and indications are found that nonequilibrium effects at the end of the deep-inelastic phase may influence the fissionlike decay.