Heavy Compound Nuclei Research Articles

Study of binary fragmentation dynamics of 260Rf compound nucleus at an excitation energy of 85.7 MeV

The fission dynamics has been studied for a near super heavy compound nucleus 260Rf populated through 28Si + 232Th reaction at an excitation energy of 85.7 MeV. Full momentum transfer binary events were separated from the transfer induced fission events. The contribution from transfer induced fission has been found to be 7±2%. Mas ratio distribution, mass-total kinetic energy (TKE), and mass angle correlation have been extracted for the full momentum transfer events using two body kinematics. The experimentally extracted width of mass distribution is higher than the mass width calculated theoretically using the saddle-point model, which indicates the presence of non-compound nuclear fission in the reaction under study. The mass-TKE distribution obtained for 260Rf nucleus matches with the theoretical predictions from the Viola systematics and GEneral description of Fission observables (GEF) model. The mass-angle distribution for the reaction under study indicates no significant correlation between the mass and emission angles of the fission fragments.

Effects of isospin and shell on probing nuclear dissipation with survival probabilities of heavy systems

Using the Langevin model, we calculate the survival probability (Peva) of 228U, 234U and 240U as a function of the presaddle dissipation strength (β). It is shown that Peva displays a greater sensitivity to β with increasing the isospin of the U system and with decreasing excitation energy and angular momentum, and that shell effects can significantly enhance the sensitivity. Furthermore, we compare calculated and measured evaporation residue cross sections of heavy compound nuclei 224Th produced in the 16O + 208Pb reaction and find that the best-fit value of β required to satisfactorily describe the measured data has an apparent drop as shell effects are taken into account in the calculation compared to that neglecting the effects, clearly indicating a strong influence of shell effects on the precise determination of β. These results demonstrate that to more stringently constrain the strength of presaddle dissipation with survival probabilities of heavy nuclei, on the experimental side, it is best to produce high-isospin compound systems with low energy and low spin; theoretically, it is important to account for the shell effects in the calculations.

Influence of the asymmetry parameter and dissipation coefficient of the K coordinate on different aspects of fission of excited compound nuclei

The dynamics of fission of the excited compound nuclei $^{256}\mathrm{Fm}, ^{215}\mathrm{Fr}, ^{187}\mathrm{Ir}, ^{172}\mathrm{Yb}, ^{162}\mathrm{Yb}$, and $^{142}\mathrm{Ce}$ produced in fusion reactions with 158.8 MeV $^{18}\mathrm{O}$ has been studied by solving three- and four-dimensional Langevin equations with dissipation generated through the chaos weighted wall and window friction formula. The constant dissipation coefficients of $K, {\ensuremath{\gamma}}_{K}=0.077\phantom{\rule{0.28em}{0ex}}{(\mathrm{MeV}\phantom{\rule{0.16em}{0ex}}\mathrm{zs})}^{\ensuremath{-}1/2}, {\ensuremath{\gamma}}_{K}=0.2\phantom{\rule{0.16em}{0ex}}{(\mathrm{MeV}\phantom{\rule{0.16em}{0ex}}\mathrm{zs})}^{\ensuremath{-}1/2}$ and a nonconstant dissipation coefficient of $K$ have been used to reproduce the experimental data for both symmetric and asymmetric splitting of the fissioning systems. The average kinetic energies of fission fragments, the pre-scission neutron multiplicities, the fission time, and the variances of the mass and kinetic energy of fission fragments are calculated for the excited compound nuclei $^{256}\mathrm{Fm}, ^{215}\mathrm{Fr}, ^{187}\mathrm{Ir}, ^{172}\mathrm{Yb}, ^{162}\mathrm{Yb}, ^{142}\mathrm{Ce}$, and results of the calculations are compared with each other and with the experimental data. Comparison of the theoretical results with the experimental data calculated by using different values of ${\ensuremath{\gamma}}_{K}$ shows that the difference is small between the results of calculations for symmetric and asymmetric simulations of the fission process of excited intermediate nuclei, whereas for heavy compound nuclei the difference is slightly high. In other words, the effect of the asymmetry parameter on the fission process of intermediate nuclei is smaller than the effect on heavy nuclei. Furthermore, we show that the pre-scission neutron multiplicity decreases rapidly with increasing fragment asymmetry.

Evaporation residue cross-section measurements for Ti48 -induced reactions

Background: A significant research effort is currently aimed at understanding the synthesis of heavy elements. For this purpose, heavy ion induced fusion reactions are used and various experimental observations have indicated the influence of shell and deformation effects in the compound nucleus (CN) formation. There is a need to understand these two effects.Purpose: To investigate the effect of proton shell closure and deformation through the comparison of evaporation residue (ER) cross sections for the systems involving heavy compound nuclei around the ${Z}_{\mathrm{CN}}=82$ region.Methods: A systematic study of ER cross-section measurements was carried out for the $^{48}\mathrm{Ti}+^{142,150}\mathrm{Nd}, ^{144}\mathrm{Sm}$ systems in the energy range of $140--205\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$. The measurement has been performed using the gas-filled mode of the hybrid recoil mass analyzer present at the Inter University Accelerator Centre (IUAC), New Delhi. Theoretical calculations based on a statistical model were carried out incorporating an adjustable barrier scaling factor to fit the experimental ER cross section. Coupled-channel calculations were also performed using the ccfull code to obtain the spin distribution of the CN, which was used as an input in the calculations.Results: Experimental ER cross sections for $^{48}\mathrm{Ti}+^{142,150}\mathrm{Nd}$ were found to be considerably smaller than the statistical model predictions whereas experimental and statistical model predictions for $^{48}\mathrm{Ti}+^{144}\mathrm{Sm}$ were of comparable magnitudes.Conclusion: Though comparison of experimental ER cross sections with statistical model predictions indicate considerable non-compound-nuclear processes for $^{48}\mathrm{Ti}+^{142,150}\mathrm{Nd}$ reactions, no such evidence is found for the $^{48}\mathrm{Ti}+^{144}\mathrm{Sm}$ system. Further investigations are required to understand the difference in fusion probabilities of $^{48}\mathrm{Ti}+^{142}\mathrm{Nd}$ and $^{48}\mathrm{Ti}+^{144}\mathrm{Sm}$ systems.

Dynamical Dipole mode in heavy-ion fusion reactions

The excitation of the Dynamical Dipole mode along the fusion path was investigated for the first time in the formation of a heavy compound nucleus in the A~190 mass region. The compound nucleus was formed at identical conditions of excitation energy and spin from two entrance channels having different charge asymmetry: the charge asymmetric 40 Ca + 152 Sm and the nearly charge symmetric 48 Ca + 144 Sm at Elab =11 and 10.1 MeV/nucleon, respectively. Both fusion–evaporation and fission events were studied simultaneously for the first time. The Dynamical Dipole mode excitation in the charge asymmetric channel was evidenced, in a model-independent way, by comparing the γ -ray multiplicity spectra and angular distributions of the two entrance channels with each other.

Fission fragment angular distributions in pre-actinide nuclei

Background: Complete fusion of two nuclei leading to formation of a heavy compound nucleus (CN) is known to be hindered by various fission-like processes, in which the composite system reseparates after capture of the target and the projectile inside the potential barrier. As a consequence of these non-CN fission (NCNF) processes, fusion probability $({P}_{\mathrm{CN}})$ starts deviating from unity. Despite substantial progress in understanding, the onset and the experimental signatures of NCNF and the degree of its influence on fusion have not yet been unambiguously identified.Purpose: This work aims to investigate the presence of NCNF, if any, in pre-actinide nuclei by systematic study of fission angular anisotropies and fission cross sections $({\ensuremath{\sigma}}_{\mathrm{fis}})$ in a number of nuclear reactions carried out at and above the Coulomb barrier $({V}_{\mathrm{B}})$.Method: Fission fragment angular distributions were measured for six $^{28}\mathrm{Si}\text{-induced}$ reactions involving isotopically enriched targets of $^{169}\mathrm{Tm},\phantom{\rule{0.16em}{0ex}}^{176}\mathrm{Yb},\phantom{\rule{0.16em}{0ex}}^{175}\mathrm{Lu},\phantom{\rule{0.16em}{0ex}}^{180}\mathrm{Hf},\phantom{\rule{0.16em}{0ex}}^{181}\mathrm{Ta}$, and $^{182}\mathrm{W}$ leading to probable formation of CN in the pre-actinide region, at a laboratory energy $({E}_{\mathrm{lab}})$ range of 129--146 MeV. Measurements were performed with large angular coverage $({\ensuremath{\theta}}_{\mathrm{lab}}={41}^{\ensuremath{\circ}}$--${170}^{\ensuremath{\circ}})$ in which fission fragments (FFs) were detected by nine hybrid telescope $(E\text{\ensuremath{-}}\mathrm{\ensuremath{\Delta}}E)$ detectors. Extracted fission angular anisotropies and ${\ensuremath{\sigma}}_{\mathrm{fis}}$ were compared with statistical model (SM) predictions.Results: Barring two reactions involving targets with large non-zero ground state spin $(\mathcal{J})$, viz., $^{175}\mathrm{Lu}\left({\frac{7}{2}}^{+}\right)$ and $^{181}\mathrm{Ta}\phantom{\rule{4pt}{0ex}}\left({\frac{7}{2}}^{+}\right)$, experimental fission angular anisotropies were found to be higher in comparison with predictions of the statistical saddle point model (SSPM), at ${E}_{\mathrm{c}.\mathrm{m}.}$ near ${V}_{\mathrm{B}}$. Comparison of present results with those from neighboring systems revealed that experimental anisotropies increasingly deviated from SSPM predictions as one moved from pre-actinide to actinide nuclei. For reactions involving targets with large nonzero $\mathcal{J}$, this deviation was subdued. Comparison between measured ${\ensuremath{\sigma}}_{\mathrm{fis}}$ and predictions of SM indicated the presence of NCNF in at least four systems, when shell effects, both in the level density and the fission barrier, were included in the calculation.Conclusions: Systematic SM analysis of measured fission angular anisotropies and ${\ensuremath{\sigma}}_{\mathrm{fis}}$ confirmed the onset of NCNF in pre-actinide nuclei. Discrepancies between results about the degree of its influence on complete fusion, as deduced from various experimental probes, remain challenges to be solved. Complete measurement of all signatures of NCNF for many systems and preferably a dynamical description of the collisions between projectile and target nuclei are warranted for a deeper understanding.

Evidence of dynamical dipole excitation in the fusion-evaporation of theCa40+Sm152heavy system

The excitation of the dynamical dipole mode along the fusion path was investigated for the first time in the formation of a heavy compound nucleus in the A∼190 mass region. The compound nucleus was formed at identical conditions of excitation energy and spin from two entrance channels: the charge-asymmetric Ca40+Sm152 and the nearly charge-symmetric Ca48+Sm144 at Elab=11 and 10.1 MeV/nucleon, respectively. High-energy γ rays and light charged particles were measured in coincidence with evaporation residues by means of the MEDEA multidetector array (Laboratori Nazionali del Sud, Italy) coupled to four parallel plate avalanche counters. The charged particle multiplicity spectra and angular distributions were used to pin down the average excitation energy, the average mass, and the average charge of the compound nucleus. The γ-ray multiplicity spectrum and angular distribution related to the nearly charge-symmetric channel were employed to obtain new data on the giant dipole resonance in the compound nucleus. The dynamical dipole mode excitation in the charge-asymmetric channel was evidenced, in a model-independent way, by comparing the γ-ray multiplicity spectra and angular distributions of the two entrance channels with each other. Calculations of the dynamical dipole mode in the Ca40+Sm152 channel, based on a collective bremsstrahlung analysis of the reaction dynamics, are presented. Possible interesting implications in the superheavy-element quest are discussed.2 MoreReceived 19 January 2016DOI:https://doi.org/10.1103/PhysRevC.93.044619©2016 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective levelsLow & intermediate energy heavy-ion reactionsTechniquesNuclear reactionsNuclear Physics

Two-dimensional Langevin modeling of fission dynamics of the excited compound nuclei 188Pt, 227Pa and 251Es

A stochastic approach based on one- and two-dimensional Langevin equations is applied to calculate the pre-scission neutron multiplicity, fission probability, anisotropy of fission fragment angular distribution, fission cross section and the evaporation cross section for the compound nuclei 188Pt, 227Pa and 251Es in an intermediate range of excitation energies. The chaos weighted wall and window friction formula are used in the Langevin equations. The elongation parameter, c, is used as the first dimension and projection of the total spin of the compound nucleus onto the symmetry axis, K, considered as the second dimension in Langevin dynamical calculations. A constant dissipation coefficient of K, γK = 0.077(MeV zs)−1/2, is used in two-dimensional calculations to reproduce the above mentioned experimental data. Comparison of the theoretical results of the pre-scission neutron multiplicity, fission probability, fission cross section and the evaporation cross section with the experimental data shows that the results of two-dimensional calculations are in better agreement with the experimental data. Furthermore, it is shown that the two-dimensional Langevin equations together with a dissipation coefficient of K, γK = 0.077(MeV zs)−1/2, can satisfactorily reproduce the anisotropy of fission fragment angular distribution for the heavy compound nucleus 251Es. However, a larger value of γK = 0.250(MeV zs)−1/2 is needed to reproduce the anisotropy of fission fragment angular distribution for the lighter compound nucleus 227Pa.

Dynamical Dipole mode in the40,48Ca +152,144Sm fusion reactions at 11 MeV/nucleon

The excitation of the dynamical dipole mode along the fusion path was investigated in the formation of a heavy compound nucleus in the A=190 mass region. To form the compound nucleus, the 40 Ca + 152 Sm and 48 Ca + 144 Sm reactions were employed at Elab =11 and 10.1 MeV/nucleon, respectively. Both fusion–evaporation and fission events were studied simultaneously for the first time. Our results for evaporation and fission events (preliminary) show that the dynamical dipole mode survives in reactions involving heavier nuclei than those studied previously.

Heavy ion collision dynamics of10,11B+10,11B reactions

The dynamical cluster-decay model (DCM) of Gupta and collaborators has been applied successfully to the decay of very-light (A ∼ 30), light (A ∼ 40−80), medium, heavy and super-heavy mass compound nuclei for their decay to light particles (evaporation residues, ER), fusion-fission (ff), and quasi-fission (qf) depending on the reaction conditions. We intend to extend here the application of DCM to study the extreme case of decay of very-light nuclear systems 20,21,22Ne∗ formed in 10,11B+10,11B reactions, for which experimental data is available for their binary symmetric decay (BSD) cross sections, i.e., σBSD. For the systems under study, the calculations are presented for the σBSD in terms of their preformation and barrier penetration probabilities P0 and P. Interesting results are that in the decay of such lighter systems there is a competing reaction mechanism (specifically, the deep inelastic orbiting of non-compound nucleus (nCN) origin) together with ff. We have emipirically estimated the contribution of σnCN. Moreover, the important role of nuclear structure characteristics via P0 as well as angular momentum ℓ in the reaction dynamics are explored in the study.

Study of fission dynamics of the excited nuclei produced in fusion reactions in the framework of the four-dimensional Langevin equations

The dynamics of fission of excited nuclei has been studied by solving four-dimensional Langevin equations with dissipation generated through the chaos-weighted wall and window friction formula. The projection of the total spin of the compound nucleus to the symmetry axis, K, was considered as the fourth dimension in Langevin dynamical calculations. The average pre-scission neutron multiplicities, mean kinetic energy of fission fragments and the variances of the mass and kinetic energy have been calculated in a wide range of fissile parameter for compound nuclei 162Yb, 172Yb, 215Fr, 224Th, 248Cf, 260Rf and results compared with the experimental data. Calculations were performed with a constant dissipation coefficient of K, γK (MeV zs)-1/2, and with a non-constant dissipation coefficient. Comparison of the theoretical results for the average pre-scission neutron multiplicities, mean kinetic energy of fission fragments and the variances of the mass and kinetic energy with the experimental data showed that the results of four-dimensional Langevin equations with a non-constant dissipation coefficient are in better agreement with the experimental data. Furthermore, the difference between the results of two models for compound nuclei with low fissile parameter is low whereas, for heavy compound nuclei, is high.

Simulation of recoil trajectories in gas-filled magnetic separators

A computer code has been developed to simulate the production of heavy element compound nucleus recoils and their trajectories through gas-filled magnetic separators. The simulation is carried out in three steps: positions and trajectories of heavy element recoils in the target layer, propagation through remaining target material, and trajectories through the gas-filled separator. Separators with quite different magnetic configurations are modeled: the Berkeley gas-filled separator (BGS) and two magnetic configurations for the TransActinide separator and chemistry apparatus (TASCA). While computing trajectories through the gas-filled separator, special attention is paid to the charge exchange/equilibration and scattering in the gas. New features of these simulations include mixed He/H2/N2 gas operation and a gas density (pressure) effect. Numerical procedures used in the simulations are explained in detail. Results of the simulations are presented, showing the gas mixtures/pressures that result in the highest efficiency for collecting compound nucleus recoils at the focal plane of the gas-filled separator. Comparison between simulation and experimental results are presented for average recoil ion charge in various gases, focal plane image size, and magnetic rigidity dispersion.

Comment on “Systematic description of evaporation spectra for light and heavy compound nuclei”

Contrary to what is claimed in the article by R. J. Charity [Phys. Rev. C 82, 014610 (2010)], the papers by Gontchar and Aktaev [Phys. Rev. C 80, 044601 (2009)] and Lestone and McCalla [Phys. Rev. C 79, 044611 (2009)] do not contradict but rather complement each other with respect to the time delay of the fission process.

Systematic description of evaporation spectra for light and heavy compound nuclei

To systematically describe evaporation spectra for light and heavy compound nuclei over a large range of excitation energies, it was necessary to consider three ingredients in the statistical model. First, transmission coefficients or barrier penetration factors for charged-particle emission are typically taken from global fits to elastic-scattering data. However, such transmission coefficients do not reproduce the barrier region of evaporation spectra and reproduction of the data requires a distributions of Coulomb barriers. This is possibly associated with large fluctuations in the compound-nucleus shape or density profile. Second, for heavy nuclei, an excitation-energy dependent level-density parameter is required to describe the slope of the exponential tails of these spectra. The level-density parameter was reduced at larger temperatures, consistent with the expected fadeout of long-range correlation, but the strong $A$ dependence of this effect is unexpected. Last, to describe the angular-momentum dependence of the level density in light nuclei at large spins, the macroscopic rotational energy of the nucleus has to be reduced from the values predicted with the finite-range liquid-drop model.

Project of the experimental setup dedicated for gamma and electron spectroscopy of heavy nuclei at FLNR JINR

Important information on the structure of super heavy elements (SHE) can come from the study of lighter deformed transfermium (Z∼100–106) elements. The cross-section for the formation of these nuclei is many orders of magnitude higher than for Z⩾110 so that detailed spectroscopy becomes possible. The advantage of Dubna is the unique possibility to use radioactive actinide targets, which yield higher production cross-sections for heavy elements. The use of actinide targets also makes Dubna a complementary site for heavy element spectroscopy because, until the availability of intense radioactive beams, it is the only way to produce less neutron deficient heavy isotopes. The opportunity to have high intensity (⩾1pμA) accelerated beams with A⩽40 together with the use of exotic targets provide the possibility to study many aspects of heavy ion induced reactions exploiting new generation of high efficiency, high resolution experimental setups. The scientific fields of the application of the new setup could be concentrated on the study of asymmetric combinations leading to excited heavy compound nuclei.

Disentangling Effects of Nuclear Structure in Heavy Element Formation

Forming the same heavy compound nucleus with different isotopes of the projectile and target elements allows nuclear structure effects in the entrance channel (resulting in static deformation) and in the dinuclear system to be disentangled. Using three isotopes of Ti and W, forming 232Cm, with measurement spanning the capture barrier energies, alignment of the heavy prolate deformed nucleus is shown to be the main reason for the broadening of the mass distribution of the quasifission fragments as the beam energy is reduced. The complex, consistently evolving mass-angle correlations that are observed carry more information than the integrated mass or angular distributions, and should severely test models of quasifission.

Formation of heavy compound nuclei, their survival, and correlation with longtime-scale fission

Fusion of two massive nuclei with formation of a superheavy compound nucleus is driven by the potential energy gradient, as follows from the analysis of nuclear reaction cross sections. The conservative energy of the system is deduced in a simple approximation using regularized nuclear mass and interaction barrier values. Different reactions for the synthesis of Z = 110−118 nuclei are compared and favorable conditions are found for fusion of the stable W-Pt isotopes with radioactive fission fragment projectiles, like 94Kr or 100Sr. Thus, the cold-fusion method can be extended for a synthesis of elements with Z > 113. Survival of the evaporation residue is defined by the neutron-to-fission probability ratio and by the successful emission of gammas at the final step of the reaction. Numerical estimates are presented. Fixation of evaporation residue products must correlate with longtime-scale fission, and available experimental results are discussed.

Shell stabilization and the survival of heavy compound nuclei

Knowledge of the influence of nuclear shell structure on the survival probability of heavy compound nuclei against fission is important for a quantitative understanding of the production rates of spherical super-heavy elements (SHE). Fission probabilities of N = 126 isotones beyond astatine can be used as test cases for the production of spherical super-heavy elements, as those isotones possess a strong shell correction energy and are highly fissile. Here, we report on two new experimental approaches which probe the effect of the closed neutron shell at N = 126 on the competition between fission and particle evaporation using projectile fragmentation and electromagnetic-induced fission of radioactive beams. We conclude that these nuclei lose at least a great part of their stability against fission at low excitation energies and angular momenta - mostly due to the influence of collective contributions in the level density. Implications on the production of spherical SHE will be discussed.

Fusion-fission dynamics of super-heavy element formation and decay

Formation dynamics of very heavy compound nuclei taking place in strong competition with the process of quasi-fission is discussed. For the first time a common driving potential is defined in the whole configuration space and used for simultaneous description of the whole evolution process starting from approaching of two heavy nuclei and ending in compound nucleus configuration of the system and/or in fission channels (normal and fast) with formation of fission fragments. Theoretical analysis of available experimental data on the “cold” and “hot” fusion-fission reactions was performed and the corresponding cross sections of super-heavy element formation were calculated up toZCN = 120.

Fusion-fission dynamics of super-heavy element formation and decay

Formation dynamics of very heavy compound nuclei taking place in strong competition with the process of quasi-fission is discussed. For the first time a common driving potential is defined in the whole configuration space and used for simultaneous description of the whole evolution process starting from approaching of two heavy nuclei and ending in compound nucleus configuration of the system and/or in fission channels (normal and fast) with formation of fission fragments. Theoretical analysis of available experimental data on the “cold” and “hot” fusion-fission reactions was performed and the corresponding cross sections of super-heavy element formation were calculated up to Z CN = 120.