Asymmetric and symmetric fission of excited nuclei of ${}^{180,190}\mathrm{Hg}$ and ${}^{184,192,202}\mathrm{Pb}$ formed in the reactions with ${}^{36}\mathrm{Ar}$ and ${}^{40,48}\mathrm{Ca}$ ions

Background: Observation of asymmetric fission of ${}^{180}\mathrm{Hg}$ has led to intensive theoretical and experimental studies of fission of neutron-deficient nuclei in the lead region.

Purpose: The study of asymmetric and symmetric fission modes of ${}^{180,190}\mathrm{Hg}$ and ${}^{184,192,202}\mathrm{Pb}$ nuclei.

Methods: Mass-energy distributions of fission fragments of ${}^{180,190}\mathrm{Hg}$ and ${}^{184}\mathrm{Pb}$ formed in the ${}^{36}\mathrm{Ar}+{}^{144,154}\mathrm{Sm}$ and ${}^{40}\mathrm{Ca}+{}^{144}\mathrm{Sm}$ reactions, respectively, at energies near the Coulomb barrier have been measured using the double-arm time-of-flight spectrometer CORSET and compared with previously measured ${}^{192,202}\mathrm{Pb}$ isotopes produced in the ${}^{48}\mathrm{Ca}+{}^{144,154}\mathrm{Sm}$ reactions. The mass distributions for ${}^{180,190}\mathrm{Hg}$ and ${}^{184,192,202}\mathrm{Pb}$ together with old data for ${}^{187}\mathrm{Ir}, {}^{195}\mathrm{Au}, {}^{198}\mathrm{Hg}, {}^{201}\mathrm{Tl}, {}^{205,207}\mathrm{Bi}, {}^{210}\mathrm{Po}$, and ${}^{213}\mathrm{At}$ [J. Nucl. Phys. 53, 1225 (1991)] have been decomposed into symmetric and asymmetric fission modes. The total kinetic-energy distributions for different fission fragment mass regions have been analyzed for ${}^{180,190}\mathrm{Hg}$ and ${}^{184}\mathrm{Pb}$.

Results: The stabilization role of proton numbers at $Z\approx 36$, 38, $Z\approx 45$, 46, and $Z=28/50$ in asymmetric fission of excited preactinide nuclei has been observed. The high ($\approx 145\text{−}\mathrm{MeV}$) and the low ($\approx 128\text{−}\mathrm{MeV}$) energy components have been found in the total kinetic-energy distributions of ${}^{180,190}\mathrm{Hg}$ fission fragments corresponding to the fragments with proton numbers near $Z\approx 46$ and $Z\approx 36$, respectively. In the case of fission of ${}^{184}\mathrm{Pb}$ only the low-energy component ($\approx 135\mathrm{MeV}$) for the fragments with masses corresponding to the proton numbers $Z\approx 36$ and 46 has been found.

Conclusions: The studied properties of asymmetric fission of ${}^{180,190}\mathrm{Hg}$ and ${}^{184,192,202}\mathrm{Pb}$ nuclei point out the existence of well deformed proton shell at $Z\approx 36$ and less deformed proton shell at $Z\approx 46$.

Figure 1

The mass-energy distributions of binary fragments formed in the ${}^{36}\mathrm{Ar}+{}^{144,154}\mathrm{Sm}$ reactions leading to the formation of mercury ($Z=80$) isotopes at energies near the Coulomb barrier.

Figure 2

The mass-energy distributions of binary fragments formed in the ${}^{40}\mathrm{Ca}+{}^{144}\mathrm{Sm}$ and ${}^{48}\mathrm{Ca}+{}^{144,154}\mathrm{Sm}$ reactions leading to the formation of lead ($Z=82$) isotopes at energies near the Coulomb barrier.

Figure 3

Left and middle panels: the experimental mass distributions of fission fragments of ${}^{180,190}\mathrm{Hg}$, formed in the ${}^{36}\mathrm{Ar}+{}^{144,154}\mathrm{Sm}$ reactions (open circles), left bottom panel: the experimental mass distribution of β-delayed fission of ${}^{180}\mathrm{Tl}$ [20]; right panels: the mass distributions of the ${}^{198}\mathrm{Hg}$ fission fragments [10] formed in the ${}^{197}\mathrm{Au}$ ($p, f$) and ${}^{194}\mathrm{Pt}$ (α,$f$) reactions. The lines correspond to the decompositions of mass distributions into the symmetric $S$ (dashed-dot-dot lines), the asymmetric $A1$ (diagonally hatched region), the asymmetric $A2$ (vertically hatched region), and the asymmetric $A3$ (crosswise hatched region) modes.

Figure 4

The same as in Fig. 3 but for Pb isotopes formed in the ${}^{40}\mathrm{Ca}+{}^{144}\mathrm{Sm}$ and ${}^{48}\mathrm{Ca}+{}^{144,154}\mathrm{Sm}$ reactions.

Figure 5

The same as in Fig. 3 but for the ${}^{187}\mathrm{Ir}, {}^{195}\mathrm{Au}, {}^{201}\mathrm{Tl}, {}^{207}\mathrm{Bi}, {}^{210}\mathrm{Po}$, and ${}^{213}\mathrm{At}$ nuclei obtained in the reactions with protons and α particles.

Figure 6

TKE distributions of fragments for different mass regions for fission of ${}^{180,190}\mathrm{Hg}$ and ${}^{184}\mathrm{Pb}$. The lines correspond to the decompositions of the TKE distributions into the symmetric (dashed-dot-dot lines), the asymmetric $A1$ (diagonally hatched region), the asymmetric $A2$ (vertically hatched region), and the asymmetric $A3$ (crosswise hatched region).

Figure 7

The proton numbers for the $A1$ (top panel) and the $A2$ (bottom panel) modes in dependence on the neutron numbers of formed fragments.

Figure 8

The contribution of symmetric fission for ${}^{180,190,198}\mathrm{Hg}$ and ${}^{184,192,202}\mathrm{Pb}$ in dependence on excitation energy at the saddle point.