Superheavy nuclei and quasi-atoms produced in collisions of transuranium ions

Low energy collisions of very heavy nuclei (${}^{238}\mathrm{U}+{}^{238}\mathrm{U}$, ${}^{232}\mathrm{Th}+{}^{250}\mathrm{Cf}$, and ${}^{238}\mathrm{U}+{}^{248}\mathrm{Cm}$) have been studied within the realistic dynamical model based on multidimensional Langevin equations. Large charge and mass transfer was found to result from the “inverse quasi-fission” process leading to the formation of the surviving superheavy long-lived neutron-rich nuclei. In many events, the lifetime of the composite system consisting of two touching nuclei turns out to be rather long; sufficiently long for the spontaneous formation of positrons to occur from a super-strong electric field—a fundamental QED process.

Figure 1

Potential energy surface for the nuclear system formed by ${}^{232}\mathrm{Th}+{}^{250}\mathrm{Cf}$ as a function of R and α ($\beta =0.22$) for nose-to-nose (a) and side-by-side (b) orientations, and as a function of R and β ($\alpha =0.037$) (c). Typical trajectories are shown by the thick curves with arrows. Before nuclei reach the region of nuclear friction forces, they are in their ground states (zero temperature) and no fluctuations occur. However, when nuclei come in contact, they are already well excited and fluctuations play a significant role [12].

Figure 2

Back and top views (upper and bottom figures) of subsequent shapes of the nuclear system evolving from the configuration of two equatorially touching statically deformed nuclei to the configuration of spherical CN.

Figure 3

Mass (a) and charge (b) distributions of primary (solid histograms) and survived (dashed histograms) fragments in the ${}^{232}\mathrm{Th}+{}^{250}\mathrm{Cf}$ collision at c.m. energy of 800 MeV. Thin solid histogram in (b) shows the primary fragment distribution in the hypothetical reaction ${}^{248}\mathrm{Cm}+{}^{250}\mathrm{Cf}$.

Figure 4

Isotopic yield of the elements 98–101 in the reactions ${}^{238}\mathrm{U}+{}^{238}\mathrm{U}$ (crosses) [14] and ${}^{238}\mathrm{U}+{}^{248}\mathrm{Cm}$ (circles and squares) [3]. Curves show the results of our calculations.

Figure 5

Excitation functions for the yields of Es isotopes (numbers near the curves) for the ${}^{238}\mathrm{U}+{}^{238}\mathrm{U}$ reactions. The arrows indicate potential energy of two nuclei in contact ($r_0=1.16$ fm) for tip (left) and side (right) configurations.

Figure 6

Yield of superheavy nuclei in collisions of ${}^{238}\mathrm{U}+{}^{238}\mathrm{U}$ (dashed), ${}^{238}\mathrm{U}+{}^{248}\mathrm{Cm}$ (dotted), and ${}^{232}\mathrm{Th}+{}^{250}\mathrm{Cf}$ (solid lines) at 800 MeV c.m. energy. Solid curves in upper part show isotopic distribution of primary fragments in the Th+Cf reaction. In the case of U+Cm, only the upper curve is marked by Z number $(Z=98)$; curves for the other reactions are marked one by one up to $Z=107$.

Figure 7

(a) Reaction time distribution for the ${}^{238}\mathrm{U}+{}^{248}\mathrm{Cm}$ collision at 800 MeV c.m. energy. Dashed and dotted histograms show the effect of switching off dynamic deformations or mass transfer, respectively. (b) Cross section for events with interaction time longer than ${10}^{-20}$ s depending on beam energy.

Figure 8

Energy-mass (a) and energy-angular (b) distributions of primary fragments in the ${}^{238}\mathrm{U}+{}^{248}\mathrm{Cm}$ collision at 800 MeV ($E_{\mathrm{loss}}>$ 15 MeV) and logarithmic landscape of the corresponding double differential cross sections. Lines are drawn over each half order of magnitude. Dashed rectangles show regions of longest events.