Production of unknown neutron-rich isotopes in ${}^{238}\text{U}+{}^{238}\text{U}$ collisions at near-barrier energy

The production cross sections for primary and residual fragments with charge number from $Z=70$ to 120 produced in the collision of ${}^{238}\text{U}+{}^{238}\text{U}$ at 7.0 MeV/nucleon are calculated by the improved quantum molecular dynamics (ImQMD) model incorporated with the statistical evaporation model (hivap code). The calculation results predict that about 60 unknown neutron-rich isotopes from elements Ra ($Z=88$) to Db ($Z=105$) can be produced with the production cross sections above the lower bound of ${10}^{-8}$ mb in this reaction. And almost all of the unknown neutron-rich isotopes are emitted at the laboratory angles ${\theta }_{\mathrm{lab}}\le {60}^{\circ }$. Two cases, i.e., the production of the unknown uranium isotopes with $A\ge 244$ and that of rutherfordium with $A\ge 269$, are investigated to understand the production mechanism of unknown neutron-rich isotopes. It is found that for the former case the collision time between two uranium nuclei is shorter and the primary fragments producing the residues have smaller excitation energies of $\le 30$ MeV and the outgoing angles of those residues cover a range of ${30}^{\circ }–{60}^{\circ }$. For the latter case, a longer collision time is needed for a large number of nucleons being transferred and thus it results in higher excitation energies and smaller outgoing angles of primary fragments, and eventually results in a very small production cross section for the residues of Rf with $A\ge 269$ which have a small interval of outgoing angles of ${\theta }_{\mathrm{lab}}={40}^{\circ }–{50}^{\circ }$.

Figure 1

Landscape of the cross sections for primary and residual fragments produced in ${}^{238}\text{U}+{}^{238}\text{U}$ at 7.0 MeV/nucleon (logarithmic scale, the black contour lines for primary fragments and colored rectangles for residual fragments). The area of known nuclei are denoted by the thick magenta line.

Figure 2

Landscape of the production cross sections for primary fragments emitted within different laboratory angle ranges in ${}^{238}\text{U}+{}^{238}\text{U}$ at 7.0 MeV/nucleon. The red cross symbols denote the center of the “island of stability” ($Z=114, N=184$).

Figure 3

Landscape of the cross sections for residual fragments emitted within different laboratory angle ranges in ${}^{238}\text{U}+{}^{238}\text{U}$ at 7.0 MeV/nucleon. The area of known nuclei are denoted by the thick magenta line. The dotted lines denote the position of the heaviest actinide element Lr ($Z=103$).

Figure 4

Angular distribution of unknown isotopes of uranium ($A\ge 244$) produced in ${}^{238}\text{U}+{}^{238}\text{U}$ reactions at different impact parameters.

Figure 5

(a) Average excitation energies of primary fragments of unknown isotopes of uranium and (b) average lifetime of composite system from which those primary fragments are produced. (c) and (d) are the same as (a) and (b) but for the unknown isotopes of rutherfordium.