Fission and binary fragmentation reactions in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">Se</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>80</mml:mn></mml:mrow></mml:mmultiscripts><mml:mo>+</mml:mo><mml:mmultiscripts><mml:mi mathvariant="normal">Pb</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>208</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">Se</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>80</mml:mn></mml:mrow></mml:mmultiscripts><mml:mo>+</mml:mo><mml:mmultiscripts><mml:mi mathvariant="normal">Th</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>232</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>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 ${\ensuremath{\nu}}_{\mathrm{tot}}^{\mathrm{sf}}=10\ifmmode\pm\else\textpm\fi{}2$, 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 ${\ensuremath{\nu}}_{\mathrm{tot}}^{\mathrm{sf}}=15\ifmmode\pm\else\textpm\fi{}2$ 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.

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