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The properties of deexcitation of the same $^{220}$Th compound nucleus (CN) formed by different mass (charge) asymmetric reactions are investigated. It is demonstrated that the effective fission barrier $$ value being a function of the excitation energy $E^*_{\rm CN}$ is strongly sensitive to the various orbital angular momentum $L=\ell\hbar$ distributions of CN formed with the same excitation energy $E^*_{CN}$ by the very different entrance channels $^{16}$O+$^{204}$Pb, $^{40}$Ar+$^{180}$Hf, $^{82}$Se+$^{138}$Ba and $^{96}$Zr+$^{124}$Sn. Consequently, the competition between the fission and evaporation of light particles (neutron, proton, and $\alpha$-particle) processes along the deexcitation cascade of CN depends on the orbital angular momentum distribution of CN. Therefore, the ratio between the evaporation residue cross sections obtained after emission of neutral and charged particles and neutrons only for the same CN with a given excitation energy $E^*_{CN}$ is sensitive to the mass (charge) asymmetry of reactants in the entrance channel., Comment: 9 pages, 6 figures, 2 tables, Accepted for publication on Phys. Rev. C

Quasielastic (QE) excitation function measurements have been carried out for the ${}^{12}\mathrm{C}{+}^{232}\mathrm{Th}$ fissile system at ${\ensuremath{\theta}}_{\mathrm{lab}}=170\ifmmode^\circ\else\textdegree\fi{}$ in the energy range of ${E}_{\mathrm{lab}}$ = 51\char21{}79 MeV. The data has been analyzed to obtain a representation of the fusion barrier distribution, which has been compared with that obtained from fusion-fission excitation function measurements available from literature. The QE data have also been analyzed in the framework of generalized elastic scattering model to obtain the mean-square average compound nuclear spin $(〈{l}^{2}〉)$ values, which has been compared with the prediction of standard fusion model (CCDEF) calculations and also with that obtained from fission fragment angular anisotropy measurements. The results show that the barrier distributions obtained from QE and fission excitation function measurement are consistent with each other. The $〈{l}^{2}〉$ values are also consistent with prediction of CCDEF calculations, but are in disagreement with the experimental $〈{l}^{2}〉$ values obtained from fission fragment anisotropy measurements.

Nonfusion, fissionlike reactions in collisions of four heavy systems (well below the fusion extra-push energy threshold), Mr which Hinde and co-workers had measured the prescission neutron multiplicities, have been analyzed in terms of the deterministic dynamic model of Feldmeier coupled to a time-dependent statistical cascade calculation. In order to reproduce the measured prescission multiplicities and the observed (nearly symmetric) mass divisions, the energy dissipation must be dramatically changed with regard to the standard one-body dissipation: In the entrance channel, in the process of forming a composite system, the energy dissipation has to be reduced to at least half of the one-body dissipation strength (k(s)(in) less than or equal to 0.5), and in the exit channel (from a mononucleus shape to scission) it must be increased by a factor ranging for the studied reactions from k(s)(out) = 4 to k(s)(out) = 12. These results are compared with the temperature dependence of the friction coefficient, recently deduced by Hofman, Back, and Paul from data on the prescission giant dipole resonance emission in fusion-fission reactions. The combined picture of the temperature dependence of the friction coefficient, for both fusion-fission and nonfusion reactions, may indicate the onset of strong two-body dissipation already at a nuclear temperature of about 2 MeV.

Nonfusion, fissionlike reactions in collisions of four heavy systems (well below the fusion extra-push energy threshold), Mr which Hinde and co-workers had measured the prescission neutron multiplicities, have been analyzed in terms of the deterministic dynamic model of Feldmeier coupled to a time-dependent statistical cascade calculation. In order to reproduce the measured prescission multiplicities and the observed (nearly symmetric) mass divisions, the energy dissipation must be dramatically changed with regard to the standard one-body dissipation: In the entrance channel, in the process of forming a composite system, the energy dissipation has to be reduced to at least half of the one-body dissipation strength (k(s)(in) less than or equal to 0.5), and in the exit channel (from a mononucleus shape to scission) it must be increased by a factor ranging for the studied reactions from k(s)(out) = 4 to k(s)(out) = 12. These results are compared with the temperature dependence of the friction coefficient, recently deduced by Hofman, Back, and Paul from data on the prescission giant dipole resonance emission in fusion-fission reactions. The combined picture of the temperature dependence of the friction coefficient, for both fusion-fission and nonfusion reactions, may indicate the onset of strong two-body dissipation already at a nuclear temperature of about 2 MeV.

Nonfusion, fissionlike reactions in collisions of four heavy systems (well below the fusion extra-push energy threshold), for which Hinde and co-workers had measured the prescission neutron multiplicities, have been analyzed in terms of the deterministic dynamic model of Feldmeier coupled to a time-dependent statistical cascade calculation. In order to reproduce the measured prescission multiplicities and the observed (nearly symmetric) mass divisions, the energy dissipation must be dramatically changed with regard to the standard one-body dissipation: In the entrance channel, in the process of forming a composite system, the energy dissipation has to be reduced to at least half of the one-body dissipation strength (${\mathit{k}}_{\mathit{s}}^{\mathrm{in}}$\ensuremath{\le}0.5), and in the exit channel (from a mononucleus shape to scission) it must be increased by a factor ranging for the studied reactions from ${\mathit{k}}_{\mathit{s}}^{\mathrm{out}}$=4 to ${\mathit{k}}_{\mathit{s}}^{\mathrm{out}}$=12. These results are compared with the temperature dependence of the friction coefficient, recently deduced by Hofman, Back, and Paul from data on the prescission giant dipole resonance emission in fusion-fission reactions. The combined picture of the temperature dependence of the friction coefficient, for both fusion-fission and nonfusion reactions, may indicate the onset of strong two-body dissipation already at a nuclear temperature of about 2 MeV. \textcopyright{} 1996 The American Physical Society.

High energy photon emission accompanying the spontaneous fission of Cf-252 is measured for different mass splits. The photon yields up to an energy of 20 MeV are obtained at several angles relative to the fission direction. Statistical model calculations are used to interpret the data. The photon yield is found to be very sensitive to the initial excitation energy sharing among the daughter nuclei and to their level density parameters. Using experimentally extracted level densities obtained from neutron evaporation measurements, the photon yield is well described by calculations for all mass splits.

High energy photon emission accompanying the spontaneous fission of Cf-252 is measured for different mass splits. The photon yields up to an energy of 20 MeV are obtained at several angles relative to the fission direction. Statistical model calculations are used to interpret the data. The photon yield is found to be very sensitive to the initial excitation energy sharing among the daughter nuclei and to their level density parameters. Using experimentally extracted level densities obtained from neutron evaporation measurements, the photon yield is well described by calculations for all mass splits.

The fusion and evaporation residue cross sections for the ${}^{50}$Ti${+}^{249}$Cf and ${}^{54}$Cr${+}^{248}$Cm reactions calculated by the combined dinuclear system and advanced statistical models are compared. These reactions are considered to be used to synthesize the heaviest superheavy element. The ${}^{50}$Ti${+}^{249}$Cf reaction is more mass asymmetric than ${}^{54}$Cr${+}^{248}$Cm, and the fusion excitation function for the former reaction is higher than the one for the latter reaction. The evaporation residue excitation functions for the mass asymmetric reaction is higher in comparison with the one for the ${}^{54}$Cr${+}^{248}$Cm reaction. The use of the mass values of superheavy nuclei calculated in the framework of the macroscopic-microscopic model by the Warsaw Group [Muntian, Z. Patyk, and A. Sobiczewski, Phys. At. Nuclei 66, 1015 (2003)] leads to a smaller evaporation residue cross section for both the reactions in comparison with the case of using the masses calculated by M\"oller and Nix [J. Phys. G: Nucl. Part. Phys. 20, 1681 (1994)]. The ${}^{50}$Ti${+}^{249}$Cf reaction is more favorable in comparison with the ${}^{54}$Cr${+}^{248}$Cm reaction: the maximum values of the excitation function of the 3$n$ channel of the evaporation residue formation for the ${}^{50}$Ti${+}^{249}$Cf and ${}^{54}$Cr${+}^{248}$Cm reactions are about 0.1 and 0.07 pb, respectively, but the yield of the 4$n$ channel for the former reaction is lower (0.004 pb) in comparison with the one (0.01 pb) for the latter reaction.