Fusion-fission and quasifission of superheavy systems with $Z=110–116$ formed in ${}^{48}\mathrm{Ca}$-induced reactions

Background: In heavy-ion-induced reactions the mechanism leading to the formation of the compound nucleus and the role of quasifission is still not clear.

Purpose: Investigation of the quasifission process of superheavy composite systems with $Z=110\text{−}116$ and comparison with properties of fusion-fission and quasifission of lighter composite systems.

Method: Mass and energy distributions of fissionlike fragments formed in the reactions ${}^{48}\mathrm{Ca}+{}^{232}\mathrm{Th}$, ${}^{238}\mathrm{U}$, ${}^{244}\mathrm{Pu}$, and ${}^{248}\mathrm{Cm}$ at energies near the Coulomb barrier have been measured using the double-arm time-of-flight spectrometer CORSET at the U-400 cyclotron of the FLNR JINR.

Results: The most probable fragment masses as well as total kinetic energies and their dispersions in dependence on the interaction energies and ion-target combinations have been studied for asymmetric and symmetric fragments formed in the reactions. The capture cross sections were obtained for the reactions ${}^{48}\mathrm{Ca}+{}^{244}\mathrm{Pu}$ and ${}^{248}\mathrm{Cm}$. The lower limits for fission barriers of ${}^{283-286}\mathrm{Cn}$, ${}^{289-292}\mathrm{Fl}$, and ${}^{293-296}\mathrm{Lv}$ compound nuclei were estimated.

Conclusions: Analysis of the properties of symmetric fragments has shown that a significant part of these fragments may be attributed to fusion-fission process for the reactions ${}^{48}\mathrm{Ca}{+}^{238}\mathrm{U}$, ${}^{244}\mathrm{Pu}$, and ${}^{248}\mathrm{Cm}$.

Figure 1

Mass-energy distributions of fragments obtained in the reactions ${}^{48}\mathrm{Ca}+{}^{232}\mathrm{Th},{}^{238}\mathrm{U},{}^{244}\mathrm{Pu},{}^{248}\mathrm{Cm}$ at energies above (top) and below (bottom) the Bass barrier. The red rectangles indicate the gates used to select fissionlike events.

Figure 2

Mass-energy distributions of fragments obtained in the reactions ${}^{48}\mathrm{Ca}+{}^{144}\mathrm{Sm}$ (a) and ${}^{48}\mathrm{Ca}+{}^{208}\mathrm{Pb}$ (c) (target and projectile are spherical) in comparison with the reactions ${}^{48}\mathrm{Ca}+{}^{154}\mathrm{Sm}$ (b) and ${}^{48}\mathrm{Ca}+{}^{238}\mathrm{U}$ (d) (targets are strongly deformed) at energy near the Coulomb barrier.

Figure 3

Potential energies at contact configuration (top panel) and experimental mass distributions of fissionlike fragments formed in the reactions ${}^{48}\mathrm{Ca}+{}^{232}\mathrm{Th},{}^{238}\mathrm{U},{}^{244}\mathrm{Pu},{}^{248}\mathrm{Cm}$: at energy above (middle) and below (bottom) the Bass barrier.

Figure 4

The widths of QFasym mass distributions as a function of the energy above the Bass barrier for the reactions ${}^{48}\mathrm{Ca}+{}^{232}\mathrm{Th},{}^{238}\mathrm{U},{}^{244}\mathrm{Pu}$, and ${}^{248}\mathrm{Cm}$. The solid circles are taken from Ref. [22] for the ${}^{48}\mathrm{Ca}+{}^{238}\mathrm{U}$ reaction.

Figure 5

The contributions of symmetric fragments to the capture cross sections for the studied systems as a function of energy above the barrier. The solid circles are taken from Ref. [22] for the ${}^{48}\mathrm{Ca}+{}^{238}\mathrm{U}$ reaction.

Figure 6

Average total kinetic energy (top) and its dispersion (bottom) as a function of mass for fissionlike fragments formed in the reactions ${}^{48}\mathrm{Ca}+{}^{238}\mathrm{U},{}^{244}\mathrm{Pu}$, and ${}^{248}\mathrm{Cm}$ at energies slightly above the Bass barrier.

Figure 7

The average TKEs for asymmetric fragments corresponding to the fragments with the maximum yield (solid symbols) and symmetric fragments with masses $A_{\text{CN}}/2\pm 20$ u (open symbols) as a function of the collision energy for the reaction ${}^{48}\mathrm{Ca}+{}^{238}\mathrm{U}$. Circles are the data from present work; triangles are from Ref. [22].

Figure 8

Average TKEs for asymmetric QF (corresponding to the QFasym fragments with the maximum yield) as a function of the $Z^2/A^{1/3}$ of composite systems with $Z=82-120$. Circles are the data from present work; stars are the data from Ref. [20, 22, 30, 32, 33].

Figure 9

TKE distributions of fragments with masses $A_{\text{CN}}/2\pm 20$ u for the reactions ${}^{48}\mathrm{Ca}+{}^{238}\mathrm{U},{}^{244}\mathrm{Pu}$, and ${}^{248}\mathrm{Cm}$. The filled region corresponds to the TKE distribution for the CN fission. The dashed and dotted curves are associated with asymmetric and symmetric QF, respectively.

Figure 10

The fusion probabilities in the reactions ${}^{48}\mathrm{Ca}{+}^{238}\mathrm{U},{}^{244}\mathrm{Pu}$ and ${}^{248}\mathrm{Cm}$ obtained from the present analysis of mass-energy distributions of fissionlike reaction products in comparison with the fusion probabilities in the reactions ${}^{36}\mathrm{S}{+}^{238}\mathrm{U}$ from Ref. [24] and ${}^{48}\mathrm{Ca}{+}^{154}\mathrm{Sm}$ from Ref. [20]. The solid lines are the results of the fitting of the fusion probabilities with Eq. (3).

Figure 11

Top: Triangles are the cross sections for fissionlike fragments of the reactions ${}^{48}\mathrm{Ca}{+}^{238}\mathrm{U}$ [22, 27],${}^{48}\mathrm{Ca}{+}^{244}\mathrm{Pu}$ and ${}^{248}\mathrm{Cm}$. Solid lines are the coupled channel calculations. Circles are the cross sections for fragments with masses $A_{\text{CN}}/2\pm 20$ u, the dashed-dotted lines are estimated CN-fission cross sections ${\sigma }_{\text{ff}}$. Bottom: excitation functions for $3n$ (squares) and $4n$ (circles) evaporation channels taken from Ref. [1]. Solid and dotted curves are the ER cross sections for the $3n$ and $4n$ channels, respectively, evaluated using the values of ${\sigma }_{\text{ff}}$ obtained in the present work.