Fusion and quasifission studies in reactions forming Rn via evaporation residue measurements

Background: Formation of the compound nucleus (CN) is highly suppressed by quasifission in heavy-ion collisions involving massive nuclei. Though considerable progress has been made in the understanding of fusion-fission and quasifission, the exact dependence of fusion probability on various entrance channel variables is not completely clear, which is very important for the synthesis of new heavy and superheavy elements.

Purpose: To study the interplay between fusion and quasifission in reactions forming CN in the boundary region where the fusion probability starts to deviate from unity.

Methods: Fusion evaporation residue cross sections were measured for the ${}^{28,30}\mathrm{Si}+{}^{180}\mathrm{Hf}$ reactions using the Hybrid Recoil Mass Analyser at IUAC, New Delhi. Experimental data were compared with data from other reactions forming the same CN or isotopes of the CN. Theoretical calculations were performed using the dinuclear system and statistical models.

Results: Reduced evaporation residue cross sections were observed for the reactions studied compared with the asymmetric reaction forming the same CN, indicating fusion suppression in more symmetric systems. The observations are consistent with fission fragment measurements performed in the same or similar systems. Larger ER cross sections are observed with increase in mass in the isotopic chain of the CN.

Conclusions: Fusion probability varies significantly with the entrance channels in reactions forming the same CN. While complete fusion occurs for the ${}^{16}\mathrm{O}+{}^{194}\mathrm{Pt}$ reaction, the fusion probability drops to approximately $60–70\%$ for the ${}^{30}\mathrm{Si}+{}^{180}\mathrm{Hf}$ and less than $20\%$ for the ${}^{50}\mathrm{Ti}+{}^{160}\mathrm{Gd}$ reactions, respectively, forming the same CN at similar excitation energies.

Figure 1

Two-dimensional plot of $\mathrm{\Delta }E$ vs TOF for the reaction ${}^{28}\mathrm{Si}+{}^{180}\mathrm{Hf}$ at 130.6 MeV energy.

Figure 2

The experimental ER cross sections for the ${}^{28,30}\mathrm{Si}+{}^{180}\mathrm{Hf}$ reactions as a function of center-of-mass energy.

Figure 3

The reduced ER cross sections for the ${}^{16}\mathrm{O}+{}^{194}\mathrm{Pt}$ and ${}^{30}\mathrm{Si}+{}^{180}\mathrm{Hf}$ reactions populating the CN ${}^{210}\mathrm{Rn}$ as a function of CN excitation energy.

Figure 4

Experimental total ER [28] and fusion excitation functions for the ${}^{16}\mathrm{O}+{}^{194}\mathrm{Pt}$ reaction. Theoretical calculations for different neutron evaporation channels, sum of neutron evaporation channels, total ER, fusion, and capture cross sections are also shown.

Figure 5

Calculated values of fusion probability ($P_{\mathrm{CN}}$) for different reactions forming the CN ${}^{210}\mathrm{Rn}$.

Figure 6

Experimental total ER excitation function for the ${}^{30}\mathrm{Si}+{}^{180}\mathrm{Hf}$ reaction compared with the theoretical calculations. The results for different neutron evaporation channels, sum of neutron channels, fusion, and capture cross sections are also shown.

Figure 7

Experimental ER cross sections for $4n+5n$ channels [60] for the ${}^{50}\mathrm{Ti}+{}^{160}\mathrm{Gd}$ reaction compared with the model calculations. The results for different neutron evaporation channels, sum of neutron channels, fusion, and capture cross sections are also shown.

Figure 8

Same as Fig. 6, but for the ${}^{28}\mathrm{Si}+{}^{180}\mathrm{Hf}$ reaction.

Figure 9

(a) Total ER cross section for different Si+Hf reactions populating isotopes of radon as CN, as a function of excitation energy. (b) ER formation probability ($P_{\mathrm{CN}}\times {\mathrm{W}}_{\mathrm{sur}}$) for different reactions populating ${}^{206,208,210}\mathrm{Rn}$.