Fission Barrier Research Articles

Neutron multiplicity measurement and investigation of nuclear dissipation and shell effects in Si30+W182,184,186 reactions

Background: Enhanced prescission neutron multiplicity (${\ensuremath{\nu}}_{\mathrm{pre}}$) over statistical model calculations assuming Bohr-Wheeler fission width is explained using Kramers' fission width incorporating dissipative effects. The dissipation strength obtained from such studies reported significant effect of the neutron shell closure of $N=126$ of the fissioning system as well as nuclear temperature. The dependence of dissipation strength on shell effect and temperature is also attributed to the choice of input parameters in the statistical model calculations.Purpose: We investigate the role of $N/Z$, shell effect, collective enhancement of level density (CELD), and excitation energy on ${\ensuremath{\nu}}_{\mathrm{pre}}$ in reactions forming isotopes of the Ra nucleus.Methods: The neutron multiplicity excitation function is measured for the $^{30}\mathrm{Si}+^{182,184,186}\mathrm{W}$ reactions populating $^{212,214,216}\mathrm{Ra}$ nuclei using the National Array of Neutron Detectors (NAND) at the Inter-University Accelerator Centre, New Delhi. Among these compound nuclei, $^{214}\mathrm{Ra}$ has a major neutron shell closure of $N=126$. Measured ${\ensuremath{\nu}}_{\mathrm{pre}}$ are analyzed within the framework of a statistical model incorporating dynamical hindrance in nuclear fission due to dissipation, shell corrections in the fission barrier and level density, and CELD.Results: Experimental ${\ensuremath{\nu}}_{\mathrm{pre}}$ show a marginal isotopic dependence at all excitation energies. ${\ensuremath{\nu}}_{\mathrm{pre}}$ values do not show any noticeable effect of neutron magic number $N=126$. Dissipation strength of $\ensuremath{\beta}=8\phantom{\rule{4pt}{0ex}}{\mathrm{zs}}^{\ensuremath{-}1}$ ($5.27\phantom{\rule{4pt}{0ex}}\mathrm{MeV}/\ensuremath{\hbar}$) reasonably reproduces the experimental ${\ensuremath{\nu}}_{\mathrm{pre}}$ excitation functions for all three nuclei in the measured energy range. Appreciable variations in presaddle neutron emissions are observed when shell effect or CELD is excluded in the calculations. Even though both the shell and CELD are found to impact primarily in the presaddle sector, they alter the excitation energy and multiplicity in the saddle-to-scission sector as well in a complementary manner.Conclusions: A temperature independent dissipation coefficient is observed to reproduce the experimental results in this study throughout the excitation energy range measured. Shell corrections in fission barrier and level density parameters and CELD in fission and particle evaporation widths also influence the neutron multiplicities. The observed effect of CELD in the presaddle phase is attributed to the large enhancement of level density at the saddle due to its large deformation and consequent enhancement of fission width.

Least action description of dynamic pairing correlations in the fission of curium and californium isotopes based on the Gogny energy density functional

The impact of dynamic pairing correlations and their interplay with Coulomb antipairing effects on the systematic of the spontaneous fission half-lives for the nuclei $^{240-250}$Cm and $^{240-250}$Cf is analyzed, using a hierarchy of approximations based on the parametrization D1M of the Gogny energy density functional (EDF). First, the constrained Hartree-Fock-Bogoliubov (HFB) approximation is used to compute deformed mean-field configurations, zero-point quantum corrections and collective inertias either by using the Slater approximation to Coulomb exchange and neglecting Coulomb antipairing or by fully considering the exchange and pairing channels of the Coulomb interaction. Next, the properties of the {\it{least action}} and {\it{least energy}} fission paths are compared. In the {\it{least action}} case, pairing is identified as the relevant degree of freedom in order to minimize the action entering the Wentzel-Kramers-Brillouin (WKB) approximation to the tunneling probability through the fission barrier. Irrespective of the treatment of Coulomb exchange and antipairing, it is shown that the {\it{least action}} path obtained taking into account the pairing degree of freedom leads to stronger pairing correlations that significantly reduce the spontaneous fission half-lives $t_{SF}$ improving thereby the comparison with the experiment by several orders of magnitude. It is also shown that the Coulomb antipairing effect is, to a large extent, washed out by the {\it{least action}} procedure and therefore the $t_{SF}$ values obtained by the two different treatments of the Coulomb exchange and pairing are of similar quality.

Estimation of fission barrier heights for even–even superheavy nuclei using machine learning approaches

With the fission barrier height information, the survival probabilities of super-heavy nuclei can also be reached. Therefore, it is important to have accurate knowledge of fission barriers, for example, the discovery of super-heavy nuclei in the stability island in the super-heavy nuclei region. In this study, five machine learning techniques, Cubist model, Random Forest, support vector regression, extreme gradient boosting and artificial neural network were used to accurately predict the fission barriers of 330 even–even super-heavy nuclei in the region 140 ≤ N ≤ 216 with proton numbers between 92 and 120. The obtained results were compared both among themselves and with other theoretical model calculation estimates and experimental results. According to the results obtained, it was concluded that the Cubist model, support vector regression and extreme gradient boosting methods generally gave better results and could be a better tool for estimating fission barrier heights.

Cassini-oval description of the multidimensional potential energy surface for U236 : Role of octupole deformation and calculation of the most probable fission path

Multidimensional potential energy surfaces (PES) are calculated using the microscopic-macroscopic approach. The nuclear shapes are described by Cassinian ovals generalized by the inclusion of ${\ensuremath{\alpha}}_{1}$, ${\ensuremath{\alpha}}_{3}$, and ${\ensuremath{\alpha}}_{4}$ shape parameters in addition to the main fission coordinate $\ensuremath{\alpha}$. The influence of the octupole deformation $({\ensuremath{\alpha}}_{3})$ on the PES is studied in the case of the nuclear fission of $^{236}\mathrm{U}$. It is found that ${\ensuremath{\alpha}}_{3}$ plays an important role in the last stage of the fission process; for instance, it lowers the third minimum and the third barrier. Two methods to calculate the static fission path are investigated. They are found to be consistent in the sense that they lead to the same fission barrier. In certain subspaces, the least energy paths from the ground state to scission present discontinuities around one of the saddles. They are caused by sharp changes in the nuclear shapes involved occurring without a change in energy. Such transitions are smoothed out by the principle of stationary action, which transforms a discontinuous path into a continuous one. Finally, various macroscopic models have been employed in order to study their influence on the energies and positions of the saddles and the minima.

Probing the structural evolution along the fission path in the superheavy nucleus $$^{256}$$Sg

The evolution of structure property along the fission path in the superheavy nucleus $^{256}$Sg is predicted through the multi-dimensional potential-energy(or Routhian)-surface calculations,in which the phenomenological deformed Woods-Saxon potential is adopted. Calculated nuclear deformations and fission barriers for $^{256}_{106}$Sg$_{150}$ and its neighbors, e.g., $^{258,260}$Sg, $^{254}$Rf and $^{252}$No are presented and compared with other theoretical results. A series of energy maps and curves are provided and used to evaluate the corresponding shape-instability properties, especially in the directions of triaxial $\gamma$ and different hexadecapole deformations (e.g., $\alpha_{40}$, $\alpha_{42}$ and $\alpha_{44}$). It is found that the triaxial deformation may help the nucleus bypass the first fission-barrier of the axial case. After the first minimum in the nuclear energy surface, the fission pathway of the nucleus can be affected by $\gamma$ and hexadecapole deformation degrees of freedom. In addition, microscopic single-particle structure, pairing and Coriolis effects are briefly investigated and discussed.

Microscopic study of neutron-induced fission process of 239Pu via zero- and finite-temperature density functional theory* *Supported by the National Natural Science Foundation of China (11790325, 12275081, 11790320, 11790321, 11961131010, 11605054, 12105369, 12147219, 12047568) and the Continuous Basic Scientific Research Project (WDJC-2019-09)

To study the neutron-induced fission of Pu, potential energy surface (PES) calculations were performed using zero and finite-temperature density functional theory (FT-DFT) with the Skyrme force. The energy of the incident neutron was simulated by the temperature of the FT-DFT. The variations of the least-energy fission path, fission barrier, total kinetic energy, scission line, and mass distribution of fission fragments with the incident neutron energy were analyzed. It was learned that an increase in the temperature lowers the barrier height, the isomeric-state energy, and the ridge between symmetric and asymmetric fission valleys. Additionally, the gaps of the single particle levels become smaller with an increase in the temperature. As the temperature increases, the pre-fission region shrinks, and the scission occurs at smaller deformation around the symmetric fission channel. At low temperatures, the pairing correlations in the collective space are similar to those in zero-temperature DFT, and when the temperature is 0.3 MeV, the pairing gaps decrease rapidly. Two different methods were used to calculate the fission yields of the neutron-induced fission Pu (n, f) with different incident neutron energies, in the framework of time-dependent generator coordinate method (TDGCM). One way to calculate the fission yield of Pu (n, f) is to solve the collective equation of the TDGCM by using the PES from the FT-DFT with the corresponding temperature. The other involves using the PES from the zero-temperature DFT and adjusting the initial collective energy of the wave packet in the TDGCM according to the incident neutron energy. For the cases of the lower incident neutron energies, these two methods gave similar results and reproduced the experimental peak and width of fission fragment distribution. However, for the highest incident neutron energy considered in this study, the results from the TDGCM using the PES from zero-temperature DFT deviated explicitly from the experimental data, whereas those obtained by using the PES from FT-DFT remained close to the experimental data. This indicated that, with the increase in the incident neutron energy, the shell structure of the compound nuclei changed explicitly; thus, it may not be effective to use the PES from zero-temperature to perform the fission dynamic calculation.

Decay Times of Compound Nuclei in Heavy Ion Fusion Reactions

The decay time of the compound nuclei has been studied using different statistical models. In the present work, we have improved the back-shifted microscopic level density model by including the pairing effect, an angular momentum-dependent moment of inertia, fission barriers, and a dynamical effect. The decay time of the compound nuclei is evaluated with and without dynamical effect and also compared with various statistical models and available experiments. It is also observed that the decay times of the compound nuclei formed during inverse kinematics is found to be larger when compared to direct kinematics at same excitation energy. Furthermore, the possible fission fragments have been identified using modified generalised liquid drop model. The identified fission fragments are compared with available experiments. The failure of the experiments to synthesize the superheavy element Z = 119 and 120 may be confirmed by the identification of fission fragments in these reactions. The predicted decay times of the compound nuclei and possible fission fragments gives a complete over view of failure of reactions to synthesize the superheavy element Z = 119 and 120.

Double-humped barrier effects in the R-matrix for fitting of fissile isotope

The Reich--Moore approach has been extensively used in the resolved resonance energy range (RRR) for a wide range of isotopes. The approximation was suggested for cross-section representation of fissile isotopes since experimental fission width distribution according to given resonance spin and parity showed that only a few degrees of freedom (DoF) was involved during the fission process. Experimental cross-section data in RRR were successfully reproduced, and the interference in fission channels were well described. The fitting of the fission cross-section data was done according to one or two fission channels for a given resonance spin (J) and parity (π). Using the two-fission channel representation, channel interference effects observed on cross-section data for fissile heavy isotopes were adequately treated but only on a phenomenological basis. Indeed, this approach is physically unsatisfactory since no fission penetrability is involved in reduced fission channel width evaluation, and therefore no actual connection between R-matrix fission channel widths and Aage Bohr transition fission channels can be made neither in terms of individual barrier height or by the shape. This paper intends to address model deficiency by including ‘fluctuating’ fission barrier penetrability as a function of resonance spin and parity.

Comprehensive investigation of fission yields by using spallation- and (p,2p)- induced fission reactions in inverse kinematics

In the last decades, measurements of spallation, fragmentation and Coulex induced fission reactions in inverse kinematics have provided valuable data to accurately investigate the fission dynamics and nuclear structure at large deformations of a large variety of stable and non-stable heavy nuclei. To go a step further, we propose now to induce fission by the use of quasi-free (p,2p) scattering reactions in inverse kinematics, which allows us to reconstruct the excitation energy of the compound fissioning system by using the four-momenta of the two outgoing protons. Therefore, this new approach might permit to correlate the excitation energy with the charge and mass distributions of the fission fragments and with the fission probabilities, given for the first time direct access to the simultaneous measurement of the fission yield dependence on temperature and fission barrier heights of exotic heavy nuclei, respectively. The first experiment based on this methodology was realized recently at the GSI/FAIR facility and a detailed description of the experimental setup is given here.

Survival probability of compound nuclei in heavy-ion fusion reaction

We have developed an empirical formula to calculate the survival probability of superheavy nuclei by analyzing about 95 fusion experiments, which were classified into four categories based on the deformation parameters of a projectile–target system. Every category is analyzed using ten different entrance channel parameters; one of these displays the smoothest variation. That channel is Z 2/A, where Z and A are the atomic number and mass number, respectively. Further, the predicted empirical relation is improved by the inclusion of the fission barrier, separation energy, level density parameter, center of mass energy, and fusion barrier height for the Z 2/A term. Furthermore, the evaporation residue cross-section is maximum at a certain beam energy, called the optimal energy. Thus, finding the survival probability at the optimal energy is important in the synthesis of a superheavy nucleus. We have developed an empirical formula for the survival probability at the optimal energy to facilitate future superheavy nucleus synthesis.

Skyrme–Hartree–Fock–Bogoliubov mass models on a 3D mesh: II. Time-reversal symmetry breaking

Models based on nuclear energy density functionals can provide access to a multitude of observables for thousands of nuclei in a single framework with microscopic foundations. Such models can rival the accuracy of more phenomenological approaches, but doing so requires adjusting parameters to thousands of nuclear masses. To keep such large-scale fits feasible, several symmetry restrictions are typically imposed on the nuclear configurations. One such example is time-reversal invariance, which is generally enforced via the Equal Filling Approximation (EFA). Here we lift this assumption, enabling us to access the spin and current densities in the ground states of odd-mass and odd-odd nuclei, which contribute to the total energy of such nuclei through so-called ‘time-odd’ terms. We present here the Skyrme-based BSkG2 model whose parameters were adjusted to essentially all known nuclear masses without relying on the EFA, refining our earlier work [G. Scamps et al., EPJA 57, 333 (2021)]. Moving beyond ground state properties, we also incorporated information on the fission barriers of actinide nuclei in the parameter adjustment. The resulting model achieves a root-mean-square (rms) deviation of (i) 0.678 MeV on 2457 known masses, (ii) 0.027 fm on 884 measured charge radii, (iii) 0.44 MeV and 0.47 MeV, respectively, on 45 reference values for primary and secondary fission barriers of actinide nuclei, and (iv) 0.49 MeV on 28 fission isomer excitation energies. We limit ourselves here to a description of the model and the study the impact of lifting the EFA on ground state properties such as binding energies, deformation and pairing, deferring a detailed discussion of fission to a forthcoming paper.

On the time-scale of quasifission and Coulomb fission

The Coulomb fission may take place in a reaction if the maximum Coulomb excitation energy transfer exceeds the fission barrier of either the projectile or the target nucleus. This condition is satisfied in all the reactions used for the earlier blocking measurements of fission time-scale except for the reaction \(^{208}\hbox {Pb}~+\) natural Ge crystal, where the time-scale is below the measurement limit of the blocking technique \(<1\) as. Inclusion of Coulomb fission in the data analysis of the blocking experiments leads us to interpret the measured time-scales longer than a few attoseconds (as) (about 1–2.2 as) due to slow Coulomb fission and those shorter than 1 as, as due to quasifission and fast Coulomb fission. Consequently, this finding resolves the critical discrepancies between the fission time-scales measured using the nuclear and blocking techniques. This, in turn, validates the fact that the quasifission and fast Coulomb fission time-scales are indeed of the order of zeptosecond (zs) in accordance with the nuclear experiments and theories. The present results thus provide an essential input to the understanding of the fusion evaporation reaction during the formation of heavy elements.

Fission Properties of Neutron-Rich Nuclei around the End Point of r-Process

We studied the fission properties of neutron-rich nuclei 278, 286Cf around the end point of r-process by microscopic self-consistent approaches. The fission barriers and potential energy surfaces are obtained by constrained static Skyrme Hartree–Fock-BCS calculations. Fission fragments are studied by dynamical time-dependent Hartree–Fock+BCS calculations. Results show that 286Cf has an octupole deformation at ground state, which can increase the fission barrier height by 1.1 MeV and enhance significantly the spontaneous fission half-life. To search possible fission channels, dynamical calculations with a broad coverage of initial deformations result in two slightly asymmetric peaks around A = 128 and 150 for 278Cf, and A = 133 and 153 for 286Cf. Very asymmetric fission channels as given by semi-empirical models are not found in our results.

Measurement of fusion-evaporation cross sections for reactions of Ca44 with Gd154,156,157,160 targets

Background: Previously reported fusion-evaporation cross sections of residues in $^{45}\mathrm{Sc}$-induced reactions with lanthanide targets are much smaller than $^{48}\mathrm{Ca}$-induced reactions on the same targets. $^{44}\mathrm{Ca}$ is one proton removed from $^{45}\mathrm{Sc}$ and could be used to produce nuclei with a relative neutron content between those produced in the $^{45}\text{Sc-}$ and $^{48}\mathrm{Ca}$-induced reactions.Purpose: Several experiments worldwide have attempted to discover elements beyond the currently heaviest known element, oganesson ($Z=118$). Due to a lack of appropriate targets, these efforts focused on projectiles other than $^{48}\mathrm{Ca}$, which has been widely used for a number of successful element discovery experiments. The present study continues our previous work to understand the influence of various projectiles on the compound nucleus in fusion-evaporation reactions, and addresses the influence of target neutron number on fusion-evaporation cross sections.Methods: In experiments performed at the Cyclotron Institute at Texas A University, a beam of $^{44}\mathrm{Ca}^{6+}$ with an energy of $\ensuremath{\approx}5\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}/u$ was delivered by the K500 superconducting cyclotron to the Momentum Achromat Recoil Spectrometer (MARS). The $^{44}\mathrm{Ca}$ projectiles bombarded various isotopically enriched Gd targets in the MARS target chamber to create evaporation residues, which were spatially separated from unreacted projectiles by MARS and identified via their characteristic $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{decay}$ energies. Excitation functions for the reactions of $^{44}\mathrm{Ca}$ with $^{154,156,157,160}\mathrm{Gd}$ were measured at several projectile energies each.Results: The maximum $4n$ cross sections in the $^{44}\mathrm{Ca}{+}^{154,156,157,160}\mathrm{Gd}$ reactions were $0.038\ifmmode\pm\else\textpm\fi{}0.008, 0.83\ifmmode\pm\else\textpm\fi{}0.08, 3.8\ifmmode\pm\else\textpm\fi{}0.2$, and $3.0\ifmmode\pm\else\textpm\fi{}0.5$ mb, respectively. Production cross sections for the more neutron-rich targets were surprisingly constant even given the substantial changes in the difference in neutron binding energy and fission barrier of the compound nuclei.Conclusions: Collective enhancements to level density caused a reduction in compound nucleus survivability for all targets. While this effect was required to obtain good agreement between theoretical calculations and experimental data, it was not sufficient to explain the cross sections for the reaction with the most neutron-deficient target studied $(^{154}\mathrm{Gd})$. Instead, it appears that the difference in the fission barrier and neutron binding energy is the dominant factor affecting the survival of the compound nucleus in this case.

Rules of thumb for synthesizing superheavy elements

Synthesizing any elements in the eighth period using either cold or hot fusion reactions remains a big challenge to date. The quasifission mechanism restricts complete fusion to an indeterminably low evaporation residue (ER) cross-section for superheavy nuclei. The entrance channel parameters of the heavy-ion reaction, the fission barrier of the compound nucleus, the deformation parameters of the projectile and target nuclei, and the kinetic energy of the projectile are mainly responsible for governing the scale of the quasifission. The role of these factors has been examined explicitly by the experimental ER cross-sections. Thorough comparisons lead us to infer that the entrance channel criteria contribute to a much lower extent than the deformation parameters do. The effect of deformation can be categorized into four rules as validated by all the reactions used except one Uue. The null result from this reaction is explained by the improper choice of the projectile energy, as shown theoretically by means of a statistical model approach, which is valid for a system having a large nucleon number so as to intrinsically have a high density of excited states. The optimal selection of the beam energy sets another rule. Therefore, these five rules can be treated as the rules of thumb for synthesizing the superheavy elements. Application of the first four rules can enable us primarily to spot a suitable reaction, and finally, exploitation of the fifth rule chooses the most appropriate reaction at a preferable excited energy to achieve the highest ER cross-section for a superheavy element.

Measurement of the 230Th(n,f) cross-section at the CERN n_TOF facility

The study of neutron induced reactions on actinides is of great importance both for the design of advanced nuclear systems and for fundamental research. The design of Generation IV reactors, Accelerator Driven Systems (ADS), as well as the study of alternative fuel cycles, such as the thorium fuel cycle, highlight the need for accurate cross-section data. In addition, fission cross-section data can assist in the study of the fission process and the fission barrier.Specifically, 230Th is produced from the alpha decay of 234U in the thorium fuel cycle, thus its fission cross-section is required with high accuracy. In addition, in the fission cross-section of thorium isotopes narrow resonances and fine structures are present, which cannot be explained efficiently by the double-humped fission barrier. For 230Th, previous measurements have shown a resonance at ~720 keV with additional fine structures. However, few discrepant experimental data exist in literature covering the energy region from the fission threshold up to 25 MeV.For these reasons, the fission cross-section of 230Th was measured at the CERN n_TOF facility, with the time-of-flight technique in order to cover the energy range of the previous measurements and extend the measurements to higher energies. The measurements were performed with Micromegas detectors, while the high purity 230Th were produced at JRC-Geel in Belgium. The analysis procedure for the cross-section results are on the final stages. In this work, a study on the angular distributions of 230Th, performed for the first time with this kind of Micromegas detectors is presented.

Fission barriers of superheavy nuclei for emitted fragment isotopes near proton magic numbers

Fission barriers for supposed doubly magic $^{298}\mathrm{Fl}$ are calculated using a specialized binary macroscopic-microscopic method. The microscopic effects are obtained from the developed binary Strutinsky shell correction method. The main input data are the proton and neutron energy levels calculated with the deformed two-center shell model. One is able in this way to provide the microscopic transition via the level schemes, from the parent to two overlapped and finally separated level schemes. The macroscopic part is acquired by the deformed charged liquid drop model with the double Yukawa-plus-exponential finite range potential. The method is applied to the reactions accompanied by isotopes of Sn and Pb within the fission channels, as being favored by strongly negative shell corrections.

Predictions for the synthesis of the Z=120 superheavy element

The possible stable and radioactive beam-induced hot fusion reactions for the production of new superheavy element (SHE) $Z=120$ are investigated within the dinuclear system model. Our investigation shows that the reaction $^{45}\mathrm{Sc}+^{252}\mathrm{Es}$ has a relatively large evaporation residue cross-section (ERCS) for the synthesis of SHE $Z=120$ due to its slightly larger mass asymmetry. In addition, we analyzed the effects of fission barrier and neutron separation energy of superheavy nuclei on ERCS. To synthesize the double-magic $Z=120$ and $N=184$ nucleus predicted by the relativistic mean-field model, some radioactive beam-induced fusion reactions, which can be performed in available experimental equipment, for producing superheavy nucleus $^{304}120$ in neutron evaporation channels are investigated systematically. We find that the hot fusion reaction $^{50}\mathrm{Ca}+^{257}\mathrm{Fm}$ in the $3n$ evaporation channel is optimal for synthesizing the superheavy nucleus $^{304}120$. We hope these predictions will shed new light timely for the recent experiments on the synthesis of $Z=120$ superheavy element and the search for superheavy stability islands.

Energy competition and pairing effect for the fission path with a microscopic model

We studied the fission barrier of 236U with a microscopic mean-field model employing Skyrme-type effective interaction. It has been known that the microscopic mean-field calculation had a trend of overestimating the fission barriers derived from the fission cross section, and our results were found to be in accord with it. To reveal a major factor of the discrepancy, we investigated various components of the Skyrme energy-density functional building of the fission barrier height by a static mean-field model, including nuclear pairing correlation. We found that the spin-orbit and pairing terms affected the fine structure of the fission barrier as a function of elongation of the nucleus. Therefore, we investigated the sensitivity of the fission barrier height on the pairing strength, considering the change of level density along the calculated fission path.

Attempts to produce new americium isotopes near N=126

Searches for new americium isotopes with neutron number around 126 are made in bombardments of $^{191,193}\mathrm{Ir}$ targets with $^{40}\mathrm{Ar}$ ions by employing the gas-filled recoil separator SHANS and the recoil-$\ensuremath{\alpha}$ correlation technique. No evidence for nuclides of interest is obtained, and the upper limits of the cross sections producing the evaporation residues are estimated to be 31.4 pb and 5.4 pb for the reactions of $^{40}\mathrm{Ar}+^{193}\mathrm{Ir}$ and $^{40}\mathrm{Ar}+^{191}\mathrm{Ir}$, respectively. We have performed microscopic finite-temperature Skyrme Hartree-Fock$+$ (Bardeen- Cooper- Schripffer theory) BCS calculations, and found that the fission barriers of compound nuclei in this region decrease rapidly with increasing proton number. The non-observation of new americium isotopes would be attributed to small survival probabilities of the $^{231}\mathrm{Am}$ and $^{233}\mathrm{Am}$ compound nuclei due to their much reduced fission barriers at high excitations.