Fission and quasifission of composite systems with $Z=108-120$: Transition from heavy-ion reactions involving S and Ca to Ti and Ni ions

Background: Suppression of compound nucleus formation in the reactions with heavy ions by a quasifission process in dependence on the reaction entrance channel.

Purpose: Investigation of fission and quasifission processes in the reactions ${}^{36}\mathrm{S},{}^{48}\mathrm{Ca},{}^{48}\mathrm{Ti}$, and ${}^{64}\mathrm{Ni}+{}^{238}\mathrm{U}$ at energies around the Coulomb barrier.

Methods: Mass-energy distributions of fissionlike fragments formed in the reaction ${}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}$ at energies of 247, 258, and 271 MeV have been measured using the double-arm time-of-flight spectrometer CORSET at the U400 cyclotron of the Flerov Laboratory of Nuclear Reactions and compared with mass-energy distributions for the reactions ${}^{36}\mathrm{S},{}^{48}\mathrm{Ca},{}^{64}\mathrm{Ni}+{}^{238}\mathrm{U}$.

Results: The most probable fragment masses as well as total kinetic energies and their dispersions in dependence on the interaction energies have been investigated for asymmetric and symmetric fragments for the studied reactions. The fusion probabilities have been deduced from the analysis of mass-energy distributions.

Conclusion: The estimated fusion probability for the reactions S, Ca, Ti, and Ni ions with actinide nuclei shows that it depends exponentially on the mean fissility parameter of the system. For the reactions with actinide nuclei leading to the formation of superheavy elements the fusion probabilities are of several orders of magnitude higher than in the case of cold fusion reactions.

Figure 1

The mass and energy distributions of binary fragments obtained in the ${}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}$ reaction at energies close to the Coulomb barrier [from top to bottom: the (TKE, $M$) matrices; the mass yields; the average TKEs as a function of mass for fissionlike events inside the outlined contours on the (TKE, $M$) matrices]. Solid lines in the average TKE distributions delineate the expectation from the liquid drop model. The arrows in the mass distributions indicate the positions of neutron and proton shells.

Figure 2

Mass distributions of fissionlike fragments for the reactions ${}^{36}\mathrm{S},{}^{48}\mathrm{Ca},{}^{48}\mathrm{Ti}$, and ${}^{64}\mathrm{Ni}+{}^{238}\mathrm{U}$ at energies near (the solid triangles) and above (the open circles) the Coulomb barrier together with potential energies of the systems at contact configuration (the thick lines). The incident energies $\left(E_{\mathrm{c}.\mathrm{m}.}/E_{\mathrm{Bass}}\right)$ are indicated at each plot.

Figure 3

Contributions of the symmetric fragments to the capture cross sections for the studied systems as a function of energy above the barrier.

Figure 4

TKE distributions of fragments with masses $A_{\mathrm{CN}}/2\pm 20\mathrm{u}$ for the reactions ${}^{36}\mathrm{S},{}^{48}\mathrm{Ca},{}^{48}\mathrm{Ti}$, and ${}^{64}\mathrm{Ni}+{}^{238}\mathrm{U}$ at energies above the Coulomb barrier. The filled region corresponds to the TKE distribution for CN fission. The dashed and dashed-dotted curves are associated with asymmetric and symmetric QF, respectively.

Figure 5

The fusion probabilities obtained from the analysis of mass-energy distributions of fissionlike fragments for the reactions with strongly deformed target nuclei: ${}^{48}\mathrm{Ca}+{}^{154}\mathrm{Sm},{}^{36}\mathrm{S}+{}^{238}\mathrm{U},{}^{48}\mathrm{Ca}+{}^{238}\mathrm{U},{}^{244}\mathrm{Pu},{}^{248}\mathrm{Cm}$, and ${}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}$.

Figure 6

The solid triangles are the cross sections for fissionlike fragments in the reactions ${}^{36}\mathrm{S},{}^{48}\mathrm{Ca},{}^{48}\mathrm{Ti}$, and ${}^{64}\mathrm{Ni}+{}^{238}\mathrm{U}$ measured with the CORSET spectrometer, and the open triangles are the cross sections for fissionlike fragments from Refs. [8, 37, 41]. The solid lines are the empirical model calculations [32]. The circles are the cross sections for fragments with masses $A_{\mathrm{CN}}/2\pm 20\mathrm{u}$, and the dashed-dotted lines are the estimated CN-fission cross sections ${\sigma }_{\mathrm{ff}}$.

Figure 7

The capture cross section as a function of the effective fissility parameter for the reactions with ${}^{208}\mathrm{Pb}$ and ${}^{238}\mathrm{U}$ nuclei at interaction energies of $E_{\mathrm{c}.\mathrm{m}.}/E_{\mathrm{Bass}}=1.1-1.2$. The lines are a guide for the eye.

Figure 8

Fusion probability in cold (spherical target nuclei) and hot fusion (strongly deformed target nuclei) reactions at energies above the Coulomb barrier in dependence on the mean fissility parameter of the reaction. The deduced fusion probabilities for the reactions with well-deformed target nuclei are shown by the squares, whereas for the reactions of Ca, Ti, and Fe ions with lead — by the stars. The circles are the fusion probabilities calculated for cold fusion reactions [44].