Compound Nucleus Fission Research Articles

Measurement of mass-angle and mass-total kinetic energy distributions from the fission of 190Pt compound nucleus

We have measured the fission fragment mass-angle and mass-total kinetic energy (TKE) distributions for the neutron-deficient 190Pt compound nucleus (CN) populated via 12C + 178Hf reaction, at around and above barrier energies. No mass-angle correlation was observed in the fission of 190Pt signifying the absence of quasi-fission events in the studied reaction. The observed mass-TKE distributions have expected triangular shape and TKE distributions are well described with the single Gaussian fits, and mean TKE shows parabolic dependence on fragment mass as predicted based on liquid drop fission behaviour. The widths of measured TKE distributions agree well with the observed systematics for CN fission in this mass region. Though the CN is relatively neutron deficient, these observations suggest a clear picture of true CN fission behaviour for the chosen reaction in the studied energy domain.

Consideration of memory of spin and parity in the fissioning compound nucleus by applying the Hauser-Feshbach fission fragment decay model to photonuclear reactions

Prompt and $\beta$-delayed fission observables, such as the average number of prompt and delayed neutrons, the independent and cumulative fission product yields, and the prompt $\gamma$-ray energy spectra for the photonuclear reactions on $^{235,238}$U and $^{239}$Pu are calculated with the Hauser-Feshbach Fission Fragment Decay (HF$^3$D) model and compared with available experimental data. In the analysis of neutron-induced fission reactions to the case of photo-induced fission, an excellent reproduction of the delayed neutron yields supports a traditional assumption that the photo-fission might be similar to the neutron-induced fission at the same excitation energies regardless of the spin and parity of the fissioning systems.

Contributions of quasifission and fusion-fission in the Mg24 + Hf178 reaction at 145 MeV laboratory beam energy using the Boltzmann-Uehling-Uhlenbeck model

Background: Understanding the mechanism of the quasifission reaction is important, because it is an essential competitor to the fusion reaction leading to superheavy elements. However, it is a challenge to separate the quasifission and fusion-fission components.Purpose: This paper provides a dynamics description of the $^{24}\mathrm{Mg}+^{178}\mathrm{Hf}$ reaction at laboratory beam energy of 145 MeV, and studies contributions of quasifission and fusion-fission in the reaction.Method: The Boltzmann-Uehling-Uhlenbeck model is improved to study the heavy-ion collision at incident energy near the Coulomb barrier. A method is developed to determine the window of the dinuclear system during the evolution.Results: It is shown that the fusion occurs with a large percentage in the central collision, and the quasiinelastic scattering occurs mainly in the peripheral collision. The quasifission competes with the fusion in collisions with small impact parameters. It is found that the cross sections of quasifission decrease and those of fusion increase with increasing surface energy or incompressibility. The mass-angular correlation of the quasifission events shows the mass asymmetry. The fragments emitted at the front angle are targetlike fragments and those at the back angle are projectilelike fragments. Furthermore, the differential cross sections at the angles ${0}^{\ensuremath{\circ}}$ and ${180}^{\ensuremath{\circ}}$ are larger than that at ${90}^{\ensuremath{\circ}}$.Conclusions: By comparing the calculations of the mass distribution and mass-angular correlation to the data, it is deduced that the compound nucleus fission plays a main role and accounts for the characteristic of fragment observables in the $^{24}\mathrm{Mg}+^{178}\mathrm{Hf}$ reaction at laboratory beam energy of 145 MeV.

Statistical Model Analysis of Neutron Multiplicities from Fission of Compound Nuclei in ~ 200 and 250 Mass Region

Statistical Model Analysis of Neutron Multiplicities from Fission of Compound Nuclei in ~ 200 and 250 Mass Region

Fusion-fission analysis of C12+Cm248 and O16+Pu244 nuclear reactions across the Coulomb barrier

By using the symmetric-asymmetric Gaussian barrier distribution (SAGBD) model, Wong formula, and coupled channel approach, heavy ion fusion dynamics is investigated for $^{12}\mathrm{C}+^{248}\mathrm{Cm}$ and $^{16}\mathrm{O}+^{244}\mathrm{Pu}$ reactions at energies lying below and near the Coulomb barrier. Coupling of various channels linked with the structure of participants to the relative motion of the collision partners is done by considering a Gaussian type of weight function in the SAGBD model and cross sections are found to be enhanced relative to the calculations obtained by the simple barrier penetration model. In the SAGBD model, the channel coupling effects are calculated in terms of channel coupling parameter ($\ensuremath{\lambda}$) and percentage reduction in the height of apparent fusion barrier with respect to the Coulomb barrier (${V}_{\mathrm{CBRED}}$). The channel coupling parameter estimates the cumulative influence of dominant intrinsic channels, which are responsible for the sub-barrier fusion enhancement. The SAGBD calculations appropriately explain the dynamics of $^{12}\mathrm{C}+^{248}\mathrm{Cm}$ and $^{16}\mathrm{O}+^{244}\mathrm{Pu}$ reactions at energies lying around the Coulomb barrier. The coupled channel analysis of the present reactions is done by using the code ccfull and the coupled channel calculations unambiguously identify the dominant influences of the rotational states up to ${10}^{+}$ spin states of the ground state rotational band of target isotopes in both reactions. In addition, the couplings to higher order deformation, such as ${\ensuremath{\beta}}_{4}$ for target and low lying quantum states of the projectile, are necessarily required to reproduce the experimental data of $^{12}\mathrm{C}+^{248}\mathrm{Cm}$ and $^{16}\mathrm{O}+^{244}\mathrm{Pu}$ reactions. Apart from the fusion analysis, the dynamical cluster-decay model (DCM) is applied to understand the fission dynamics of the $^{260}\mathrm{No}^{*}$ nucleus formed via the above-mentioned reactions. The decay study is carried out at the center-of-mass energies spread, ${E}_{\mathrm{c}.\mathrm{m}.}$ ($\ensuremath{\approx}79$ to 109 MeV) by including the quadrupole deformations (${\ensuremath{\beta}}_{2}$) and optimum orientations (${\ensuremath{\theta}}_{i}^{\mathrm{opt}}$) of the decaying fragments. According to the experimental observation, the noncompound nucleus (nCN) fission component competes with the compound nucleus (CN) fission processes. Consequently, the possibility of nCN contribution is also explored in the decay of the $^{260}\mathrm{No}^{*}$ compound nucleus. With an aim to have a comprehensive analysis of CN and nCN fission mechanisms, the role of the center-of-mass energy (${E}_{\mathrm{c}.\mathrm{m}.}$) and angular momentum ($\ensuremath{\ell}$) is explored in terms of various parameters of DCM such as fragmentation potential, preformation probability, barrier modification, etc.

Fragmentation analysis of Z=112 –116 nuclei using a Skyrme energy density formalism within the collective clusterization approach

The Skyrme energy density formalism is applied to address the capture cross sections of $_{112}^{286}\mathrm{Cn}^{*}$, $_{114}^{292}\mathrm{Fl}^{*}$, and $_{116}^{296}\mathrm{Lv}^{*}$ superheavy nuclei formed in $^{48}\mathrm{Ca}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}^{238}\mathrm{U},^{244}\mathrm{Pu},^{248}\mathrm{Cm}$ reactions. The dynamics of $^{48}\mathrm{Ca}$-induced reactions is investigated by including GSkI and SSk forces for the hot-optimum configurations of decay fragments. The neutron evaporation $(3n \mathrm{and} 4n)$, compound nucleus fission (CN fission) and quasifission (QF) cross sections are calculated with the application of both forces, but the GSkI force seems to be appropriate only for the neutron evaporation channels. However, by including SSk force various decay processes can be handled by including the only parameter of the model, called the neck length parameter $(\mathrm{\ensuremath{\Delta}}R)$. The fragment mass distribution of $Z=112$--116 nuclei shows considerable modifications in the fission, symmetric quasifission $({\mathrm{QF}}_{\mathrm{sym}.})$ and asymmetric quasifission $({\mathrm{QF}}_{\mathrm{asym}.})$ regions with the inclusion of the SSk force as compared to analysis based on the density independent potentials. In addition to this, the role of ${\ensuremath{\beta}}_{2}$ deformations and magic shell effects (around the Pb isotope) is scrutinized through the fragmentation behavior and preformation probability of $_{112}^{286}\mathrm{Cn}^{*}$, $_{114}^{292}\mathrm{Fl}^{*}$, and $_{116}^{296}\mathrm{Lv}^{*}$ nuclei. Finally, the distribution of the average total kinetic energy of fragments is calculated with the density dependent SSk force and compared with experimental data and earlier work based on the proximity potential.

Experimental study of fast fission and quasifission in the Ca40+Pb208 reaction leading to the formation of the transfermium nucleus No248

Background: The stability of the transfermium nucleus against fission is mainly determined by the shell correction depending on its angular momentum and excitation energy.Purpose: The study of the fast fission process of the transfermium nucleus $^{248}\mathrm{No}$ and its dependence on the interaction energy and introduced angular momentum.Methods: Mass-energy distributions of the $^{248}\mathrm{No}$ fission fragments formed in the $^{40}\mathrm{Ca}$ + $^{208}\mathrm{Pb}$ reaction at energies above the Coulomb barrier have been measured using the double-arm time-of-flight spectrometer CORSET at the $^{40}\mathrm{Ca}$-beam energies of 223, 250, and 284 MeV.Results: The contribution of the fast fission process is determined from the calculations of the driving potential, taking into account shell effects and rotational energy and amounts to 39% and 61% at 250 and 284 MeV, respectively. The mass-energy distributions of the quasifission and fast fission fragments have been extracted by subtracting the mass-energy matrices associated with compound nucleus fission from those of all measured fissionlike events. The asymmetric fragments with masses 97 and 151 u were found to be the most probable in the fast fission of $^{248}\mathrm{No}$. With increasing $^{40}\mathrm{Ca}$ energy from 250 to 284 MeV the mass distributions of the fast fission fragments change slightly.Conclusions: Contrary to quasifission in which the fragments are focused mainly around the closed neutron or proton shells, the influence of known proton or neutron shells on the asymmetric mass distribution in the fast fission process was not observed.

Evidence for competing bi-faceted compound nucleus fission modes in 232Th(α,f) reaction

Relative isotopic yield distributions have been extracted for nine correlated fission fragment pairs following a detailed analysis of prompt γ−γ coincidence events from 232Th(α,f) reaction at Elab = 30 MeV. Simultaneously, the charge and mass yield distributions of the even-even fission fragments have been obtained from the measured relative isotopic yields. The onset of a triple-hump structure is seen in the extracted charge as well as mass yield distributions of the fragments. The results are consistent with the two-mode fission hypothesis of compound nucleus and in good agreement with the theoretical predictions, based on relevant fission models. The detailed results provide experimental evidence for the presence of two distinct compound nucleus fission modes, where the asymmetric and symmetric fission components are 83% and 17%, respectively. An average neutron multiplicity value of 4.60 ± 0.09 has been obtained, and the extracted neutron multiplicity distribution pattern corroborates the observed features of the multifaceted fission modes. Comparing the measured yield distributions following the 232Th(α,f) surrogate reaction, which produces 236U⁎ at an excitation energy of 21.5 MeV, with that of the direct reaction of 14 MeV neutron induced fission of 235U, an increase of about 11% in the yield of the symmetric component has been observed.

Investigations on Fission of Compound Nuclei with Z=120

Investigations on Fission of Compound Nuclei with Z=120

Energy sharing based on microscopic level densities

The transformation of a moderately excited heavy nucleus into two excited fission fragments is modeled as a strongly damped evolution of the nuclear shape. The resulting Brownian motion in the multi-dimensional deformation space is guided by the shape-dependent level density which has been calculated microscopically for each of nearly ten million shapes (given in the three-quadratic-surfaces parametrization) by using a previously developed combinatorial method that employs the same single-particle levels as those used for the calculation of the pairing and shell contributions to the five-dimensional macroscopic-microscopic potential-energy surface. The stochastic shape evolution is followed until a small critical neck radius is reached, at which point the mass, charge, and shape of the two proto-fragments are extracted. The available excitation energy is divided statistically on the basis of the microscopic level densities associated with the two distorted fragments. Specific fragment structure features may cause the distribution of the energy disvision to deviate significantly from expectations based on a Fermi-gas level density. After their formation at scission, the initially distorted fragments are being accelerated by their mutual Coulomb repulsion as their shapes relax to their equilibrium forms. The associated distortion energy is converted to additional excitation energy in the fully accelerated fragments. These subsequently undergo sequential neutron evaporation which is calculated using again the appropriate microscopic level densities. The resulting dependence of the mean neutron multiplicity on the fragment mass, as well as the dependence of on the initial excitation energy of the fissioning compound nucleus, exhibit features that are similar to the experimentally observed behavior, suggesting that the microscopic energy sharing mechanism plays an important role in low-energy fission.

Effect of nuclear structure and fissility on quasifission

In order to investigate the interplay between nuclear structure (shell closure) and dynamics, fission fragment mass distributions have been measured for reactions $^{35,37}\mathrm{Cl}+^{206,208}\mathrm{Pb}$. These reactions have similar entrance channel parameters except for the number of closed shell configuration in the entrance channel. Fission fragment mass distributions have been measured for these reactions in the energy region close to the Coulomb barrier. No mass-angle correlation was observed in any of the four reactions, indicating the absence of fast quasifission (QF), but mass distributions were wider than that of compound nucleus (CN) fission, which indicates the presence of slow QF. With comparable fissility and deformation values, extra stability against QF was observed for reactions which have more closed shell configurations in the entrance channel. Mass-angle distributions of these reactions do not follow the systematics with respect to mean fissility. A new systematics has been obtained by considering quadrupole deformation of the heavy reaction partner along with the fissility, which explains the absence of mass-angle correlation in the presently measured reactions.

Isospin conservation in compound nuclear fusion–fission reactions – empirical evidence

Isospin symmetry has gained importance in nuclear phenomena involving heavy nuclei in recent years. We present empirical evidence of isospin dependence in the compound nuclear fission cross-sections and fission widths, which suggests the idea that the compound nucleus (CN) possibly retains the memory of the isospin when it is formed and affects the CN fission. This idea was first proposed by E.L. Yadrovsky [E.L. Yadrovsky, Phys. Lett. 55B, 157 (1975)]. An observable difference between the fission branching ratios in two different isospin states suggests that the CN seems to remember its isospin at the point of formation. Further, the data also suggest a possible new signature of weakening of the CN process and gradual transition to non-compound processes as the energy rises. This is a very interesting and useful observation which needs to be confirmed by further and more precise experiments.

Potential energy models of excited compound nucleus

Potential energy is important ingredient in static and dynamical investigations of a nuclear fission process. The calculation of surface, nuclear, Coulomb, rotational, curvature, congruence, and Wigner energy functionals is presented for large variety of nuclear shapes generated by a {c,h,α} parametrization. Using these functionals one can calculate the potential energy in several macroscopic models of the liquid-drop type. A numerical code which calculates all these quantities is made available. Program summaryProgram Title: FRM_epotCPC Library link to program files:http://dx.doi.org/10.17632/h7r43t7dnh.1Licensing provisions: CC BY NC 3.0Programming language: C++Nature of problem: The shape dependence of the surface, nuclear, Coulomb, rotational, curvature, congruence, and Wigner energy functionals together with the moments of inertia for rotation, center-of-mass distance between nascent fragments are calculated for deformed shapes of the fissioning compound nucleus. These functionals allow to calculate the potential energy in several macroscopic models of the liquid-drop type for a large variety of nuclear shapes.Solution method: The shape of the fissioning nucleus is described by the {c,h,α} parametrization. This parametrization determines the axially symmetric shapes in cylindrical coordinates. The potential energy functionals are calculated using numerical integration procedure as a function of the three shape parameters. Using these functionals one can generate potential energy of the nucleus in four different versions of the liquid-drop model. All subroutines, which define the nuclear shape are separated from the code, which calculates the potential energy functionals. Thus, the program could be easily adopted for any other shape parametrization in case if it is given in cylindrical coordinates.

Formation and Decay of the Composite System Z = 120 in Reactions with Heavy Ions at Energies Near the Coulomb Barrier

The mass, energy and angular distributions of binary fragments formed in the reactions 64Ni + 238U, 58Fe + 244Pu, 52Cr + 248Cm, 54Cr + 248Cm at energies near the Coulomb barrier have been measured. The analysis of energy distributions of the symmetric fragments with mass numbers $${{{{A}_{{{\text{CN}}}}}} \mathord{\left/ {\vphantom {{{{A}_{{{\text{CN}}}}}} 2}} \right. \kern-0em} 2} \pm 20$$ formed in these reactions have been applied to separate compound nucleus fission and quasi-fission. The estimated fusion probability for the reactions Cr, Fe, and Ni ions with actinide targets shows an exponential dependence on the mean fissility parameter of the system and shows also that reaction with Cr ions is more favorable for production of the super heavy element with Z = 120.

Excitation energy partition in fission

The transformation of an atomic nucleus into two excited fission fragments is modeled as a strongly damped evolution of the nuclear shape. As in previous studies, it is assumed that the division of mass and charge is frozen in at a critical neck radius of c0=2.5fm. In order to also determine the energetics, we follow the system further until scission occurs at a smaller neck radius, at which point the shapes of the proto-fragments are extracted. The statistical energy available at scission is then divided on the basis of the respective microscopic level densities. This approach takes account of important (and energy-dependent) finite-size effects. After the fragments have been fully accelerated and their shapes have relaxed to their equilibrium forms, they undergo sequential neutron evaporation. The dependence of the resulting mean neutron multiplicity on the fragment mass, ν¯(A), including the dependence on the initial excitation energy of the fissioning compound nucleus, agrees reasonably well with observations, as demonstrated here for 235U(n,f), and the sawtooth appearance of ν¯(A) can be understood from shell-structure effects in the level densities.

Effects of isospin and dissipation coefficient of the K coordinate in simulation of fission dynamics of excited compound nuclei

A stochastic approach based on four-dimensional (4D) dynamical model has been used to simulate the fission process of the excited compound nuclei [Formula: see text]Fr, [Formula: see text]Fr and [Formula: see text]Fr produced in fusion reactions. Effects of isospin and dissipation coefficient of the [Formula: see text] coordinate, [Formula: see text], on estimation of the evaporation residue (ER) cross-section, the prescission neutron multiplicity, the variance of the mass and energy distributions of fission fragments and the anisotropy of fission fragments angular distribution have been investigated for the excited compound nuclei [Formula: see text]Fr, [Formula: see text]Fr and [Formula: see text]Fr. Three collective shape coordinates [Formula: see text] plus the projection of total spin of the compound nucleus to the symmetry axis, [Formula: see text], were considered in the 4D dynamical model. In the 4D dynamical model, the magnitude of the dissipation coefficient of [Formula: see text], [Formula: see text], was considered as a free parameter and its magnitude inferred by fitting measured data on the ER cross-section. Results of the extracted dissipation coefficients of [Formula: see text] for different isotopes of Fr were shown that the magnitude of the dissipation coefficient of [Formula: see text] increases with decreasing isospin of fissioning compound nucleus. It was also shown that the prescission neutron multiplicity and the anisotropy of fission fragments angular distribution increase with increasing isospin whereas the variance of the mass and energy distributions of fission fragments decrease with increasing isospin of fissioning compound nucleus. Furthermore, it was shown that the calculated values of prescission neutron multiplicity and the variance of the mass distribution of fission fragments for the excited compound nuclei [Formula: see text]Fr, [Formula: see text]Fr and [Formula: see text]Fr decrease with the dissipation strength of [Formula: see text], whereas the variance of the energy distribution of fission fragments and the anisotropy of fission fragments angular distribution increase with the dissipation strength of [Formula: see text].

Fission of compound nuclei and the developments of microscopic fission theory

Nuclear fission process is a complex large-amplitude dynamic process of quantum many-body systems, and the development of predictive fission descriptions is a well-known challenge in nuclear physics. To describe the fission observables, there are some phenomenological and semi-microscopic models, which rely on adjustments of parameters. The microscopic fission theory based on effective nuclear interactions in principle can avoid free parameters, but they are not ready for accurate applications. This work discussed about the fission of compound nuclei, which is particularly useful for synthesizing superheavy elements. In addition, we discussed the adiabatic and non-adiabatic microscopic fission models. The adiabatic fission model performs better in describing fission observables, while non-adiabatic fission models are valuable to explore fission mechanisms. We discussed the issues and challenges of current fission theories, and realized the fission process is extremely complicated, and some prospects of fission theory have also been discussed.

Using excitation-energy dependent fission yields to identify key fissioning nuclei in r-process nucleosynthesis

The possibility that nucleosynthesis in neutron star mergers may reach fissioning nuclei introduces significant uncertainties in predicting the relative abundances of r-process material from such events. We evaluate the impact of using sets of fission yields given by the 2016 GEF code for spontaneous (sf), neutron-induced ((n, f)), and β-delayed (βdf) fission processes which take into account the approximate initial excitation energy of the fissioning compound nucleus. We further explore energy-dependent fission dynamics in the r process by considering the sensitivity of our results to the treatment of the energy sharing and de-excitation of the fission fragments using the FREYA code. We show that the asymmetric-to-symmetric yield trends predicted by GEF 2016 can reproduce the high-mass edge of the second r-process peak seen in solar data and examine the sensitivity of this result to the mass model and astrophysical conditions applied. We consider the effect of fission yields and barrier heights on the nuclear heating rates used to predict kilonova light curves. We find that fission barriers influence the contribution of 254Cf spontaneous fission to the heating at ∼100 d, such that a light curve observation consistent with such late-time heating would both confirm that actinides were produced in the event and imply the fission barriers are relatively high along the 254Cf β-feeding path. We lastly determine the key nuclei responsible for setting the r-process abundance pattern by averaging over thirty trajectories from a 1.2–1.4 M⊙ neutron star merger simulation. We show it is largely the odd-N nuclei undergoing (Z, N)(n, f) and (Z, N)βdf that control the relative abundances near the second peak. We find the ‘hot spots’ for β-delayed and neutron-induced fission given all mass models considered and show most of these nuclei lie between the predicted N = 184 shell closure and the location of currently available experimental decay data.

Observation of the competing fission modes in 178Pt

Fragment mass distributions from fission of the excited compound nucleus 178Pt have been deduced from the measured fragment velocities. The 178Pt nucleus was created at the JAEA tandem facility in a complete fusion reaction 36Ar + 142Nd, at beam energies of 155, 170 and 180 MeV. The data are indicative of a mixture of the mass-asymmetric and mass-symmetric fission modes associated with higher and lower total kinetic energies of the fragments, respectively. The measured fragment yields are dominated by asymmetric mass splits, with the symmetric mode contributing at the level of ≈1/3. This constitutes the first observation of a multimodal fission in the sub-lead region. Most probable experimental fragment-mass split of the asymmetric mode, AL/AH≈79/99, is well reproduced by nuclear density functional theory using the UNEDF1-HFB and D1S potentials. The symmetric mode is associated by theory with very elongated fission fragments, which is consistent with the observed total kinetic energy/fragment mass correlation.

Fission and quasifission of the composite system Z=114 formed in heavy-ion reactions at energies near the Coulomb barrier

Background: Study of competition between compound nucleus fission and quasifission in heavy-ion-induced reactions and its dependence on the reaction entrance channel are important for picking up the right target-projectile combination for the synthesis of superheavy elements.Purpose: We investigate the decrease of fusion probability in the $^{52}\mathrm{Cr}+^{232}\mathrm{Th}$ reaction in comparison with that in the reactions $^{48}\mathrm{Ca}+^{244}\mathrm{Pu}$ and $^{48}\mathrm{Ti}+^{238}\mathrm{U}$. All reactions lead to the formation of composite systems with $Z=114$.Methods: Mass-energy distributions of binary fragments formed in the reaction $^{52}\mathrm{Cr}+^{232}\mathrm{Th}$ leading to the $^{284}\mathrm{Fl}$ composite system at energies of 265, 288, 302, and 320 MeV have been measured using the double-arm time-of-flight spectrometer CORSET. To study the properties and entrance channel dependence of quasifission in more detail, the mass and energy distributions of fragments formed in the $^{86}\mathrm{Kr}+^{198}\mathrm{Pt}$ reaction, leading to the same composite system $^{284}\mathrm{Fl}$, have also been measured.Results: The contribution of symmetric fragments in all fissionlike events is nearly the same for both the reactions $^{48}\mathrm{Ti}+^{238}\mathrm{U}$ and $^{52}\mathrm{Cr}+^{232}\mathrm{Th}$ at energies above the barrier, while it is greater for $^{48}\mathrm{Ca}+^{244}\mathrm{Pu}$. The analysis of total kinetic energy distribution of symmetric fragments formed in the reaction $^{52}\mathrm{Cr}+^{232}\mathrm{Th}$ shows that at an energy 15% over the Bass barrier the contribution of compound nucleus fission in the capture cross section is less than 0.4%.Conclusions: The fusion probability drops down by about a factor of 4 at the transition from the $^{48}\mathrm{Ca}+^{244}\mathrm{Pu}$ reaction to $^{48}\mathrm{Ti}+^{238}\mathrm{U}$ and about a factor of 25 at the transition to $^{52}\mathrm{Cr}+^{232}\mathrm{Th}$ at energies above the barrier. It agrees with the trend of fusion probability to decrease exponentially with increasing the mean fissility parameter of the composite system.