Fission and quasifission of the composite system $Z=114$ formed in heavy-ion reactions at energies near the Coulomb barrier

Background: Study of competition between compound nucleus fission and quasifission in heavy-ion-induced reactions and its dependence on the reaction entrance channel are important for picking up the right target-projectile combination for the synthesis of superheavy elements.

Purpose: We investigate the decrease of fusion probability in the ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$ reaction in comparison with that in the reactions ${}^{48}\mathrm{Ca}+{}^{244}\mathrm{Pu}$ and ${}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}$. All reactions lead to the formation of composite systems with $Z=114$.

Methods: Mass-energy distributions of binary fragments formed in the reaction ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$ leading to the ${}^{284}\mathrm{Fl}$ composite system at energies of 265, 288, 302, and 320 MeV have been measured using the double-arm time-of-flight spectrometer CORSET. To study the properties and entrance channel dependence of quasifission in more detail, the mass and energy distributions of fragments formed in the ${}^{86}\mathrm{Kr}+{}^{198}\mathrm{Pt}$ reaction, leading to the same composite system ${}^{284}\mathrm{Fl}$, have also been measured.

Results: The contribution of symmetric fragments in all fissionlike events is nearly the same for both the reactions ${}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}$ and ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$ at energies above the barrier, while it is greater for ${}^{48}\mathrm{Ca}+{}^{244}\mathrm{Pu}$. The analysis of total kinetic energy distribution of symmetric fragments formed in the reaction ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$ shows that at an energy 15% over the Bass barrier the contribution of compound nucleus fission in the capture cross section is less than 0.4%.

Conclusions: The fusion probability drops down by about a factor of 4 at the transition from the ${}^{48}\mathrm{Ca}+{}^{244}\mathrm{Pu}$ reaction to ${}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}$ and about a factor of 25 at the transition to ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$ at energies above the barrier. It agrees with the trend of fusion probability to decrease exponentially with increasing the mean fissility parameter of the composite system.

Figure 1

Potential energy surface for the nuclear system ${}^{284}\mathrm{Fl}$ as a function of elongation and mass asymmetry. Injection configurations (contact points) for ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$ and ${}^{86}\mathrm{Kr}+{}^{198}\mathrm{Pt}$ are shown by the circles. Arrows show schematically the QF and CN-formation trajectories.

Figure 2

The mass and energy distributions of binary fragments formed in the reactions ${}^{48}\mathrm{Ca}+{}^{244}\mathrm{Pu}, {}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}, {}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$, and ${}^{86}\mathrm{Kr}+{}^{198}\mathrm{Pt}$ leading to the formation of composite systems with $Z=114$ at energies above the Bass barrier. From top to bottom: The M-TKE matrices of binary reaction products, the mass distributions, and the average total kinetic energy as a function of mass of fissionlike fragments inside the contour line on M-TKE matrices.

Figure 3

The contribution of symmetric fragments in all fissionlike events formed in the reactions ${}^{48}\mathrm{Ca}+{}^{238}\mathrm{U}, {}^{48}\mathrm{Ca}+{}^{244}\mathrm{Pu}, {}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}, {}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$, and ${}^{86}\mathrm{Kr}+{}^{198}\mathrm{Pt}$ in dependence on the interaction energy.

Figure 4

The capture cross sections (open symbols) and the cross sections of symmetric fragments with masses ${\mathrm{A}}_{\mathrm{CN}}/2\pm 20\mathrm{u}$ (filled symbols) for the reactions ${}^{48}\mathrm{Ca}+{}^{238}\mathrm{U}$ [21], ${}^{48}\mathrm{Ca}+{}^{244}\mathrm{Pu}$ [22], ${}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}$ [23], and ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$ in dependence on the interaction energy.

Figure 5

The deviation of average TKE from the Viola systematics for the symmetric fragments formed in the reactions ${}^{48}\mathrm{Ca}+{}^{238}\mathrm{U}$ [21], ${}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}$ [23], ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$, and ${}^{86}\mathrm{Kr}+{}^{198}\mathrm{Pt}$ in dependence on the interaction energy.

Figure 6

The standard deviation of TKE distributions of symmetric fragments with masses ${\mathrm{A}}_{\mathrm{CN}}/2\pm 20\mathrm{u}$ (filled symbols) and asymmetric fragments with masses 200 ± 10 u (open symbols) formed in the reactions ${}^{48}\mathrm{Ti}+{}^{238}\mathrm{U}$ [23], ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$, and ${}^{86}\mathrm{Kr}+{}^{198}\mathrm{Pt}$ in dependence on the interaction energy.

Figure 7

TKE distributions of fragments with masses $A_{\mathrm{CN}}/2\pm 20\mathrm{u}$ for the reactions ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$ (squares) and ${}^{86}\mathrm{Kr}+{}^{198}\mathrm{Pt}$ (stars) leading to the formation of the same composite system ${}^{284}\mathrm{Fl}$ at energies above the Coulomb barrier. The filled region corresponds to the TKE distribution for CN fission for ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$. The dashed and dash-dotted curves are associated with asymmetric and symmetric QF, respectively.

Figure 8

Fusion probability for the reaction ${}^{52}\mathrm{Cr}+{}^{232}\mathrm{Th}$ in comparison with fusion probabilities in hot fusion reactions [23] at energies above the Coulomb barrier in dependence on the mean fissility parameter.