No abstract

A complete set of existing data on hot fusion reactions leading to synthesis of superheavy nuclei of Z =114-118, obtained in a series of experiments in Dubna and later in GSI Darmstadt and LBNL Berkeley, was analyzed in terms of a new angular-momentum dependent version of the fusionby-diffusion (FBD) model with fission barriers and ground-state masses taken from the Warsaw macroscopic-microscopic model (involving non-axial shapes) of Kowal et al. The only empirically adjustable parameter of the model, the injection-point distance (sinj), has been determined individually for all the reactions. Very regular systematics of this parameter have been established. The regularity of the obtained sinj systematics indirectly points at the internal consistency of the whole set of fission barriers used in the calculations. (In an attempt to fit the same set of data by using the alternative theoretical fission barriers of Möller et al. we did not obtain such a consistent result.) Having fitted all the experimental excitation functions for elements Z = 114-118, the FBD model was used to predict cross sections for synthesis of elements Z = 119 and 120. Regarding prospects to produce the new element Z = 119, our calculations prefer the 252 Es( 48 Ca,xn) 300−x 119 reaction, for which the synthesis cross section of about 0.2 pb in 4n channel at Ec.m. ≈ 220 MeV is expected. The most favorable reaction to synthesize the element Z = 120 turns out to be 249 Cf( 50 Ti,xn) 299−x 120, but the predicted cross section for this reaction is only 6 fb (for 3n and 4n channels).

We present a new experimental method to correlate the isotopic composition of intermediate mass fragments (IMF) emitted at mid-rapidity in semi-peripheral collisions with the emission timescale: IMFs emitted in the early stage of the reaction show larger values of isospin asymmetry, stronger angular anisotropies and reduced odd-even staggering effects in neutron to proton ratio distributions than those produced in sequential statistical emission. All these effects support the concept of isospin "migration", that is sensitive to the density gradient between participant and quasi-spectator nuclear matter, in the so called neck fragmentation mechanism. By comparing the data to a Stochastic Mean Field (SMF) simulation we show that this method gives valuable constraints on the symmetry energy term of nuclear equation of state at subsaturation densities. An indication emerges for a linear density dependence of the symmetry energy.

Predictions based on the "Fusion by Diffusion" model in a version adapted for calculating xn channels, with the ground-state masses, shell effects and barrier heights as given by Muntian, Patyk and Sobiczewski are presented. Sensitivity of the model to uncertainties in determination of the theoretical fission barriers is discussed. Predictions concerning formation of the element Z = 120 include comparison of fusion of the most asymmetric systems, 50 Ti + 249 Cf and 54 Cr + 248 Cm , with less asymmetric combinations 58 Fe + 244 Pu and 64 Ni + 238 U .

A complete set of 27 excitation functions for synthesis of superheavy nuclei produced in cold fusion reactions was analyzed in terms of the "Fusion by Diffusion Model" of Światecki et al., modified to account for the angular momentum dependence of the fusion hindrance factor. The data on cold fusion reactions originate from experiments carried out at GSI Darmstadt, RIKEN Tokyo and LBNL Berkeley in which 208 Pb and 209 Bi targets were bombarded with the variety of projectiles ranging from 48,50 Ti to 70 Zn .