Enhanced nucleon transfer in tip collisions of ${}^{238}\mathrm{U}+{}^{124}\mathrm{Sn}$

Multinucleon transfer processes in low-energy heavy ion reactions have attracted increasing interest in recent years aiming at the production of new neutron-rich isotopes. Clearly, it is an imperative task to further develop understanding of underlying reaction mechanisms to lead experiments to success. In this paper, from systematic time-dependent Hartree-Fock calculations for the ${}^{238}\mathrm{U}+{}^{124}\mathrm{Sn}$ reaction, it is demonstrated that transfer dynamics depend strongly on the orientations of ${}^{238}\mathrm{U}$, quantum shells, and collision energies. Two important conclusions are obtained: (i) Experimentally observed many-proton transfer from ${}^{238}\mathrm{U}$ to ${}^{124}\mathrm{Sn}$ can be explained by a multinucleon transfer mechanism governed by enhanced neck evolution in tip collisions; (ii) novel reaction dynamics are observed in tip collisions at energies substantially above the Coulomb barrier, where a number of nucleons are transferred from ${}^{124}\mathrm{Sn}$ to ${}^{238}\mathrm{U}$, producing transuranium nuclei as primary reaction products, which could be a means to synthesize superheavy nuclei. Both results indicate the importance of the neck (shape) evolution dynamics, which are sensitive to orientations, shell effects, and collision energies, for exploring possible pathways to produce new unstable nuclei.

Figure 1

Comparison of production cross sections for the lighter fragments in the ${}^{238}\mathrm{U}+{}^{124}\mathrm{Sn}$ reaction at $E_{\mathrm{c}.\mathrm{m}.}$ $\simeq 465$ MeV. (a)–(c): Cross sections for primary reaction products from TDHF. (d)–(f): Cross sections for secondary reaction products from TDHF+GEMINI. (g): Experimental data (for reaction products with energy losses $\ge 25$ MeV) reported in Ref. [69]. The magic numbers, $Z=50$ and $N=82$, are indicated by solid lines. The contour lines for the theoretical results in (a)–(f) correspond to 0.001, 0.01, 0.1, 1, 10, and 100 mb. At the top of the figure, the collision geometries examined are depicted, showing the $x$-direction case [for (a), (d)], the $y$-direction case [for (b), (e)], and the $z$-direction case [for (c), (f)]. The figure shown in (g) was taken from Ref. [69] with permission.

Figure 2

Snapshots of the density in ${}^{238}\mathrm{U}+{}^{124}\mathrm{Sn}$ head-on collisions at $E_{\mathrm{c}.\mathrm{m}.}$ $\simeq 465$ MeV (a) and 736 MeV (b).

Figure 3

TDHF results for head-on collisions of ${}^{238}\mathrm{U}+{}^{124}\mathrm{Sn}$ at various center-of-mass energies $E_{\mathrm{c}.\mathrm{m}.}$. Average numbers of neutrons and protons of the heavier fragments $(N_{\mathrm{H}}$ and $Z_{\mathrm{H}})$ are shown in (a) and (b), respectively, while those of the lighter fragments $(N_{\mathrm{L}}$ and $Z_{\mathrm{L}})$ are shown in (c) and (d), respectively. The results associated with tip (side) collisions are shown by read open circles (blue crosses). The neutron and proton numbers of the projectile and the target are indicated by horizontal dotted lines. The frozen Hartree-Fock treatment provides the Coulomb barrier height of $V_{\mathrm{B}}^{\mathrm{tip}}$ $\simeq 410$ MeV and $V_{\mathrm{B}}^{\mathrm{side}}$ $\simeq 448$ MeV for the tip and side collisions, respectively, for this system.