Production mechanism of neutron-rich transuranium nuclei in ${}^{238}\mathrm{U}+{}^{238}\mathrm{U}$ collisions at near-barrier energies

The improved quantum molecular dynamics (ImQMD) model incorporated with the statistical evaporation model is applied to study the production mechanism of transuranium nuclei in the reaction of ${}^{238}\mathrm{U}+{}^{238}\mathrm{U}$ at 7.0 MeV/nucleon. The production of primary fragments in the dynamical process is simulated by the ImQMD model, and the decays of them are described by the statistical evaporation model (hivap code). The calculated isotope distributions of the residual fragments and the most probable mass number of fragments are generally in agreement with experimental data. By tracking residual fragments back to their original primary fragments with this approach, we find different mechanisms for the production of the residues: the most probable light uraniumlike residues mainly come from the decay of the most probable primary fragments, while the most probable transuranium residues mainly originate from the decay of more neutron-rich primary fragments rather than from the most probable primary ones. For neutron-rich transuranium isotopes ${}^{254-256}\mathrm{Cf}$, the decay channel of neutron evaporation is suppressed due to the quick drop of the fission barrier height with the increase of neutron number, which leads to the quick drop of the production cross sections for these residues.

Figure 1

Time evolution of the production probability for fragments with charge number $Z>92$ from the ImQMD calculation.

Figure 2

Isotopic production cross sections for transuranium elements from $Z=94$ to 101. The cross sections for primary fragments and residual fragments are denoted by red open and solid circles, respectively. The experimental data and calculation results from Ref. [24] for Cf, Es, and Fm are shown by black solid stars and blue solid lines, respectively.

Figure 3

(a) The production cross sections and (b) the most probable mass numbers of primary fragments and residual fragments for complementary elements. The horizontal axis denotes the atomic numbers of complementary elements. Primary and residual fragments are denoted by open and solid symbols, respectively. The transuranium and light uraniumlike nuclei are denoted by red circles and blue squares, respectively. The experimental data and calculation results from Ref. [24] are denoted by black solid stars and cross symbols, respectively.

Figure 4

(a) The isotope distributions for primary fragments of Rn, Fr, and Ra and (b) the production cross sections for the residue ${}^{214}\mathrm{Rn}$ from the decay of primary fragments of Rn, Fr, and Ra, respectively. Here ${}_Z^{A}X$ denotes the primary fragments $X$ with charge number $Z$ and mass number $A$. $Y$ represents the evaporation residue.

Figure 5

The isotope distributions for primary fragments of Cf, Es, and Fm are shown in panel (a) and the cross sections for the residues ${}^{249}\mathrm{Cf}, {}^{254}\mathrm{Cf}, {}^{255}\mathrm{Cf}$, and ${}^{256}\mathrm{Cf}$ contributed from the decay of primary fragments of Cf, Es, and Fm are shown in panels (b)–(e), respectively. The meanings of ${}_Z^{A}X$ and $Y$ are the same as in Fig. 4.

Figure 6

The isotope dependence of the average excitation energies of primary fragments (upper panels) and the fission barrier heights (lower panels) for Rn, Cf, Es, and Fm, respectively. The most probable primary fragments are denoted by larger size symbols in panels (a)–(d). In panels (e)–(h), the most probable residual fragments are denoted by solid symbols, and the primary fragments decaying to the most probable residual fragments are denoted by open symbols in the corresponding panels.