Fission and binary fragmentation reactions in ${}^{80}\mathrm{Se}+{}^{208}\mathrm{Pb}$ and ${}^{80}\mathrm{Se}+{}^{232}\mathrm{Th}$ systems

Fission and binary fragmentation of the excited nuclear systems of $Z=116$ and 124 were investigated using the reactions induced by ${}^{80}\mathrm{Se}$ beams on ${}^{208}\mathrm{Pb}$ and ${}^{232}\mathrm{Th}$ targets at bombarding energies ranging from 470 to 630 MeV. The mass and kinetic energy of the binary reaction products were reconstructed by measuring their velocities by the time-of-flight method and the angles of emission using multiwire proportional chambers. Total neutron multiplicities were measured in coincidence with the fragments, using an array of neutron detectors. The fragment mass-energy correlation was studied for the two systems. The average total kinetic energy (TKE) of fragments for the ${}^{80}\mathrm{Se}+{}^{208}\mathrm{Pb}$ system agrees with earlier measurements and with Viola's systematics in the mass symmetric region for compound nucleus fission, whereas for the ${}^{80}\mathrm{Se}+{}^{232}\mathrm{Th}$ system, the TKE values are significantly lower. This is also consistent with higher values of total neutron multiplicities observed for the case of ${}^{80}\mathrm{Se}+{}^{232}\mathrm{Th}$ at comparable available energies. From an extrapolation of the measured total neutron multiplicities for the mass symmetric region to zero compound nucleus excitation energy, the average number of prompt neutrons expected to be emitted in the spontaneous fission of the superheavy $Z=116$ has been estimated to be ${\nu }_{\mathrm{tot}}^{\mathrm{sf}}=10\pm 2$, which is consistent with the value derived for the same compound nucleus populated in the ${}^{56}\mathrm{Fe}+{}^{232}\mathrm{Th}$ reaction in an earlier work. In the case of the ${}^{80}\mathrm{Se}+{}^{232}\mathrm{Th}$ system, similar analysis was carried out by taking the average TKE from Viola's systematics for estimating the available energy for particle emission corresponding to compound nucleus fission. In this way, by extrapolating the observed neutron multiplicities to zero compound nucleus excitation energy, a value of ${\nu }_{\mathrm{tot}}^{\mathrm{sf}}=15\pm 2$ was obtained for the spontaneous fission of the superheavy $Z=124$ nucleus. The increase in the average number of neutrons emitted in fission as a function of the atomic number of the nucleus in the superheavy mass region was confirmed by comparing the results of the present work with published data.

Figure 1

Schematic view of experimental setup. The angles of the neutron detectors with respect to the beam direction are marked outside the boxes.

Figure 2

Two-dimensional plots of counts vs $V_{\perp }$ and $V_{\left|\right|}/V_{\mathrm{CN}}$ for both systems at $E_{\mathrm{lab}}=470$ MeV.

Figure 3

Mass-TKE scatter plot for binary events for ${}^{80}\mathrm{Se}+{}^{208}\mathrm{Pb}$ and ${}^{80}\mathrm{Se}+{}^{232}\mathrm{Th}$ systems at the bombarding energies of 470 and 630 MeV. The polygonal gate is used to select events around the symmetric region.

Figure 4

Mass distributions (corresponding to the gated region in Fig. 3) for ${}^{80}\mathrm{Se}+{}^{208}\mathrm{Pb}$ and ${}^{80}\mathrm{Se}+{}^{232}\mathrm{Th}$ systems. The fragment masses corresponding to the expected shell closure at $Z=50$ and $N=82$ are indicated by dashed lines.

Figure 5

Product of relative cross section and bombarding energy in center of mass of the near-symmetric events (gated region in Fig. 3) as a function of excess energy for both systems.

Figure 6

Average TKE (for the symmetric masses) as a function of bombarding energy for both systems. Dashed and continuous lines correspond to the average TKE expected from Viola systematics $\langle \mathrm{TKE}\rangle =0.1189\hspace{1em}{Z}^2/A^{1/3}+7.3$ [24] and Itkis systematics $\langle \mathrm{TKE}\rangle =0.104\hspace{1em}{Z}^2/A^{1/3}+24.3$ [25], respectively.

Figure 7

Mass distributions of pure binary ( solid line) and binary in coincidence with neutrons (dashed line). Total neutron multiplicities as a function of mass (solid points) were obtained by dividing the two distributions and correcting for neutron detection efficiencies.

Figure 8

${\nu }_{\mathrm{tot}}$ (after correcting for fragment-neutron correlations) as a function of compound nucleus excitation energy for ${}^{80}\mathrm{Se}+{}^{208}\mathrm{Pb}$ and ${}^{80}\mathrm{Se}+{}^{232}\mathrm{Th}$. ${\nu }_{\mathrm{tot}}$ for ${}^{56}\mathrm{Fe}+{}^{232}\mathrm{Th}$ from earlier work [19] is shown as an open square in (a). Lines correspond to energy cost for neutron emission.

Figure 9

Variation of ${\nu }_{\mathrm{tot}}$ with atomic number of compound nucleus at the excitation energy of 35 MeV.

Figure 10

Variation of ${\nu }_{\mathrm{tot}}$ with atomic number of composite nucleus in the excitation energy range 74–85 MeV.