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Impacts of the Cutoff of Coupled-Channels Effects on Heavy-Ion Fusion Reactions at Extreme Sub-Barrier Energies

Lin, C. J.
Fonte: Oxford University Press Publicador: Oxford University Press
Tipo: Artigo de Revista Científica Formato: text/html
EN
Relevância na Pesquisa
46.21%
Based on the one dimensional barrier penetration model with semiclassical WKB method, the dynamic polarization potentials in heavy-ion reactions are extracted from the fusion cross sections. By choosing the proper parameters, the real polarization potential can be calculated from the imaginary part according to the dispersion relation in the framework of the linear-schematic model. The imaginary potential comes from the coupled-channels effects and shows a rapid cuttoff at energies far below the Coulomb barrier. Due to the absence of the coupled-channels effects, the fusion excitation function fall off more quickly than the usual expectation. Moreover, the pure potential in fusion reactions is proposed in hopes to reproduce the fusion cross sections at all energies in the full coupled-channels calculation.

Laboratory Condition for Fusion Reactions in Nuclear Astrophysics

Itahashi, Takahisa; Kudomi, Nobuyuki; Donets, E. D.
Fonte: Oxford University Press Publicador: Oxford University Press
Tipo: Artigo de Revista Científica Formato: text/html
EN
Relevância na Pesquisa
46.22%
The laboratory condition where charged particle fusion reactions have been studied is not exactly the same as stellar condition. To probe details of solar fusion and to test the prediction of standard solar model, we need more precise data for charged particle fusion reactions in the laboratory. We propose several experimental approaches to reduce the ambiguity of the estimation of screening potential value which is crucial for obtaining the astrophysical S-factor. The laboratory experiments of reactions with bare target and bare beam will be achieved by using proposed Electron Beam Ion Trap Apparatus (NARITA).

Reconstituting Intracellular Vesicle Fusion Reactions: The Essential Role of Macromolecular Crowding

Yu, Haijia; Rathore, Shailendra S.; Shen, Chong; Liu, Yinghui; Ouyang, Yan; Stowell, Michael H.; Shen, Jingshi
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
46.23%
Intracellular vesicle fusion is mediated by SNAREs and Sec1/Munc18 (SM) proteins. Despite intensive efforts, the SNARE-SM mediated vesicle fusion reaction has not been faithfully reconstituted in biochemical assays. Here, we present an unexpected discovery that macromolecular crowding is required for reconstituting the vesicle fusion reaction in vitro. Macromolecular crowding is known to profoundly influence the kinetic and thermodynamic behaviors of macromolecules, but its role in membrane transport processes such as vesicle fusion remains unexplored. We introduced macromolecular crowding agents into reconstituted fusion reactions to mimic the crowded cellular environment. In this crowded assay, SNAREs and SM proteins acted in concert to drive efficient membrane fusion. In uncrowded assays, by contrast, SM proteins failed to associate with the SNAREs and the fusion rate decreased more than 30-fold, close to undetectable levels. The activities of SM proteins were strictly specific to their cognate SNARE isoforms and sensitive to biologically relevant mutations, further supporting that the crowded fusion assay accurately recapitulates the vesicle fusion reaction. Using this crowded fusion assay, we also showed that the SNARE-SM mediated fusion reaction can be modulated by two additional factors: NSF and α-SNAP. These findings suggest that the vesicle fusion machinery likely has been evolutionarily selected to function optimally in the crowded milieu of the cell. Accordingly...

Applications of Skyrme energy-density functional to fusion reactions spanning the fusion barriers

Liu, Min; Wang, Ning; Li, Zhuxia; Wu, Xizhen; Zhao, Enguang
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.27%
The Skyrme energy density functional has been applied to the study of heavy-ion fusion reactions. The barriers for fusion reactions are calculated by the Skyrme energy density functional with proton and neutron density distributions determined by using restricted density variational (RDV) method within the same energy density functional together with semi-classical approach known as the extended semi-classical Thomas-Fermi method. Based on the fusion barrier obtained, we propose a parametrization of the empirical barrier distribution to take into account the multi-dimensional character of real barrier and then apply it to calculate the fusion excitation functions in terms of barrier penetration concept. A large number of measured fusion excitation functions spanning the fusion barriers can be reproduced well. The competition between suppression and enhancement effects on sub-barrier fusion caused by neutron-shell-closure and excess neutron effects is studied.; Comment: 28 pages, 13 figures and 2 tables. accepted by Nucl. Phys. A

Dynamic study on fusion reactions for $^{40,48}$Ca+$^{90,96}$Zr around Coulomb barrier

Wang, Ning; Wu, Xizhen; Li, Zhuxia
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 09/01/2003
Relevância na Pesquisa
46.32%
By using the updated improved Quantum Molecular Dynamics model in which a surface-symmetry potential term has been introduced for the first time, the excitation functions for fusion reactions of $^{40,48}$Ca+$^{90,96}$Zr at energies around the Coulomb barrier have been studied. The experimental data of the fusion cross sections for $^{40}$Ca+$^{90,96}$Zr have been reproduced remarkably well without introducing any new parameters. The fusion cross sections for the neutron-rich fusion reactions of $^{48}$Ca+$^{90,96}$Zr around the Coulomb barrier are predicted to be enhanced compared with a non-neutron-rich fusion reaction. In order to clarify the mechanism of the enhancement of the fusion cross sections for neutron-rich nuclear fusions, we pay a great attention to study the dynamic lowering of the Coulomb barrier during a neck formation. The isospin effect on the barrier lowering is investigated. It is interesting that the effect of the projectile and target nuclear structure on fusion dynamics can be revealed to a certain extent in our approach. The time evolution of the N/Z ratio at the neck region has been firstly illustrated. A large enhancement of the N/Z ratio at neck region for neutron-rich nuclear fusion reactions is found.; Comment: 21 pages...

Screened Coulomb potentials for astrophysical nuclear fusion reactions

Liolios, Theodore E.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.28%
The electron-screening acceleration of laboratory fusion reactions at astrophysical energies is an unsolved problem of great importance to astrophysics. That effect is modeled here by considering the fusion of hydrogen-like atoms whose electron probability density is used in Poisson's equation in order to derive the corresponding screened Coulomb potential energy. That way atomic excitations and deformations of the fusing atoms can be taken into account. Those potentials are then treated semiclassically in order to obtain the screening (accelerating) factor of the reaction. By means of the proposed model the effect of a superstrong magnetic field on laboratory Hydrogen fusion reactions is investigated here for the first time showing that, despite the considerable increase in the cross section of the $% dd$ reaction, the $pp$ reaction is still too slow to justify experimentation. The proposed model is finally applied on the $H^{2}(d,p) H^{3}$ fusion reaction describing satisfactorily the experimental data although some ambiguity remains regarding the molecular nature of the deuteron target. Notably, the present method gives a sufficiently high screening energy for Hydrogen fusion reactions so that the take-away energy of the spectator nucleus can also be taken into account.; Comment: 11 (RevTex) pages + 3 ps figures. Accepted for publication in Eur.Phys.J. A

Production of exotic isotopes in complete fusion reactions with radioactive beams

Sargsyan, V. V.; Zubov, A. S.; Adamian, G. G.; Antonenko, N. V.; Heinz, S.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 18/11/2013
Relevância na Pesquisa
46.22%
The isotopic dependence of the complete fusion (capture) cross section is analyzed in the reactions $^{130,132,134,136,138,140,142,144,146,148,150}$Xe+$^{48}$Ca with stable and radioactive beams. It is shown for the first time that the very neutron-rich nuclei $^{186-191}$W can be reached with relatively large cross sections by complete fusion reactions with radioactive ion beams at incident energies near the Coulomb barrier. A comparison between the complete fusion and fragmentation reactions for the production of neutron-rich W and neutron-deficient Rn isotopes is performed.; Comment: 13 pages, 6 figures, accepted in PRC

Formation of superheavy nuclei in cold fusion reactions

Feng, Zhao-Qing; Jin, Gen-Ming; Li, Jun-Qing; Scheid, Werner
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.22%
Within the concept of the dinuclear system (DNS), a dynamical model is proposed for describing the formation of superheavy nuclei in complete fusion reactions by incorporating the coupling of the relative motion to the nucleon transfer process. The capture of two heavy colliding nuclei, the formation of the compound nucleus and the de-excitation process are calculated by using an empirical coupled channel model, solving a master equation numerically and applying statistical theory, respectively. Evaporation residue excitation functions in cold fusion reactions are investigated systematically and compared with available experimental data. Maximal production cross sections of superheavy nuclei in cold fusion reactions with stable neutron-rich projectiles are obtained. Isotopic trends in the production of the superheavy elements Z=110, 112, 114, 116, 118 and 120 are analyzed systematically. Optimal combinations and the corresponding excitation energies are proposed.; Comment: 18 pages, 8 figures

Microscopic dynamics simulations of heavy-ion fusion reactions induced by neutron-rich nuclei

Wang, Ning; Ou, Li; Zhang, Yingxun; Li, Zhuxia
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.32%
The heavy-ion fusion reactions induced by neutron-rich nuclei are investigated with the improved quantum molecular dynamics (ImQMD) model. With a subtle consideration of the neutron skin thickness of nuclei and the symmetry potential, the stability of nuclei and the fusion excitation functions of heavy-ion fusion reactions $^{16}$O+$^{76}$Ge, $^{16}$O+$^{154}$Sm, $^{40}$Ca+$^{96}$Zr and $^{132}$Sn+$^{40}$Ca are systematically studied. The fusion cross sections of these reactions at energies around the Coulomb barrier can be well reproduced by using the ImQMD model. The corresponding slope parameter of the symmetry energy adopted in the calculations is $L \approx 78$ MeV and the surface energy coefficient is $g_{\rm sur}=18\pm 1.5$ MeVfm$^2$. In addition, it is found that the surface-symmetry term significantly influences the fusion cross sections of neutron-rich fusion systems. For sub-barrier fusion, the dynamical fluctuations in the densities of the reaction partners and the enhanced surface diffuseness at neck side result in the lowering of the fusion barrier.; Comment: 10 figures, the version published in PRC

Fusion reactions initiated by laser-accelerated particle beams in a laser-produced plasma

Labaune, C.; Baccou, C.; Depierreux, S.; Goyon, C.; Loisel, G.; Yahia, V.; Rafelski, J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 08/10/2013
Relevância na Pesquisa
46.25%
The advent of high-intensity pulsed laser technology enables the generation of extreme states of matter under conditions that are far from thermal equilibrium. This in turn could enable different approaches to generating energy from nuclear fusion. Relaxing the equilibrium requirement could widen the range of isotopes used in fusion fuels permitting cleaner and less hazardous reactions that do not produce high energy neutrons. Here we propose and implement a means to drive fusion reactions between protons and boron-11 nuclei, by colliding a laser-accelerated proton beam with a laser-generated boron plasma. We report proton-boron reaction rates that are orders of magnitude higher than those reported previously. Beyond fusion, our approach demonstrates a new means for exploring low-energy nuclear reactions such as those that occur in astrophysical plasmas and related environments.; Comment: 7 pages, 5 figures

Transfer/Breakup Channel Couplings in Sub-barrier Fusion Reactions

Beck, C.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.32%
With the recent availability of state-of-the-art radioactive ion beams, there has been a renew interest in the investigation of nuclear reactions with heavy ions near the Coulomb barrier. The role of inelastic and transfer channel couplings in fusion reactions induced by stable heavy ions can be revisited. Detailed Analysis of recent experimental fusion cross sections by using standard coupled-channel calculations is first discussed. Multi-neutron transfer effects are introduced in the fusion process below the Coulomb barrier by analyzing 32S+90,96Zr as benchmark reactions. The enhancement of fusion cross sections for 32S+96Zr is well reproduced at sub-barrier energies by NTFus code calculations including the coupling of the neutron-transfer channels following the Zagrebaev semi-classical model. Similar effects for 40Ca+90Zr and 40Ca+96Zr fusion excitation functions are found. The breakup coupling in both the elastic scattering and in the fusion process induced by weakly bound stable projectiles is also shown to be crucial. In the second part of this work, full coupled-channel calculations of the fusion excitation functions are performed by using the breakup coupling for the more neutron-rich reaction and for the more weakly bound projectiles. we clearly demonstrate that Continuum-Discretized Coupled-Channel calculations are capable to reproduce the fusion enhancement from the breakup coupling in 6Li+59Co.; Comment: Invited Talk (Reaction and Structure parallel Session) given by C. Beck at the 11th International Conference on Nucleus-Nucleus Collisions (NN2012)...

Systematic investigations of deep sub-barrier fusion reactions using an adiabatic approach

Ichikawa, Takatoshi
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 03/10/2015
Relevância na Pesquisa
46.25%
To describe fusion hindrance observed in fusion reactions at extremely low incident energies, I propose a novel extension of the standard CC model by introducing a damping factor that describes a smooth transition from sudden to adiabatic processes. I demonstrate the performance of this model by systematically investigating various deep sub-barrier fusion reactions. I extend the standard CC model by introducing a damping factor into the coupling matrix elements in the standard CC model. I adopt the Yukawa-plus-exponential (YPE) model as a basic heavy ion-ion potential, which is advantageous for a unified description of the one- and two-body potentials. For the purpose of these systematic investigations, I approximate the one-body potential with a third-order polynomial function based on the YPE model. Calculated fusion cross sections for the medium-heavy mass systems of $^{64}$Ni + $^{64}$Ni, $^{58}$Ni + $^{58}$Ni, and $^{58}$Ni + $^{54}$Fe, the medium-light mass systems of $^{40}$Ca + $^{40}$Ca, $^{48}$Ca + $^{48}$Ca, and $^{24}$Mg + $^{30}$Si, and the mass-asymmetric systems of $^{48}$Ca + $^{96}$Zr and $^{16}$O + $^{208}$Pb are consistent with the experimental data. The astrophysical S factor and logarithmic derivative representations of these are also in good agreement with the experimental data. Since the results calculated with the damping factor are in excellent agreement with the experimental data in all systems...

Effects of $\beta_{6}$ deformation and low-lying vibrational bands on heavy-ion fusion reactions at sub-barrier energies

Rumin, Tamanna; Hagino, Kouichi; Takigawa, Noboru
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/08/1999
Relevância na Pesquisa
46.3%
We study fusion reactions of $^{16}$O with $^{154}$Sm, $^{186}$W and $^{238}$U at sub-barrier energies by a coupled-channels framework. We focus especially on the effects of $\beta_{6}$ deformation and low-lying vibrational excitations of the target nucleus. It is shown that the inclusion of $\beta_{6}$ deformation leads to a considerable improvement of the fit to the experimental data for all of these reactions. For the $^{154}$Sm and $^{238}$U targets, the octupole vibration significantly affects the fusion barrier distribution. The effect of $\beta$ band is negligible in all the three reactions, while the $\gamma$ band causes a non-negligible effect on the barrier distribution at energies above the main fusion barrier. We compare the optimum values of the deformation parameters obtained by fitting the fusion data with those obtained from inelastic scatterings and the ground state mass calculations. We show that the channel coupling of high multipolarity beyond the quadrupole coupling is dominated by the nuclear coupling and hence higher order Coulomb coupling does not much influence the optimum values of $\beta_4$ and $\beta_6$ parameters. We also discuss the effect of two neutron transfer reactions on the fusion of $^{16}$O with $^{238}$U.; Comment: 11 pages...

Systematics of fusion probability in "hot" fusion reactions

Wang, Ning; Tian, Junlong; Scheid, Werner
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/12/2011
Relevância na Pesquisa
46.29%
The fusion probability in "hot" fusion reactions leading to the synthesis of super-heavy nuclei is investigated systematically. The quasi-fission barrier influences the formation of the super-heavy nucleus around the "island of stability" in addition to the shell correction. Based on the quasi-fission barrier height obtained with the Skyrme energy-density functional, we propose an analytical expression for the description of the fusion probability, with which the measured evaporation residual cross sections can be reproduced acceptably well. Simultaneously, some special fusion reactions for synthesizing new elements 119 and 120 are studied. The predicted evaporation residual cross sections for 50Ti+249Bk are about 10-150fb at energies around the entrance-channel Coulomb barrier. For the fusion reactions synthesizing element 120 with projectiles 54Cr and 58Fe, the cross sections fall to a few femtobarns which seems beyond the limit of the available facilities.; Comment: 5 figures, 1 table

Dynamical analysis on heavy-ion fusion reactions near Coulomb barrier

Feng, Zhao-Qing; Jin, Gen-Ming; Zhang, Feng-Shou
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 23/11/2007
Relevância na Pesquisa
46.32%
The shell correction is proposed in the improved isospin dependent quantum molecular dynamics (ImIQMD) model, which plays an important role in heavy-ion fusion reactions near Coulomb barrier. By using the ImIQMD model, the static and dynamical fusion barriers, dynamical barrier distribution in the fusion reactions are analyzed systematically. The fusion and capture excitation functions for a series of reaction systems are calculated and compared with experimental data. It is found that the fusion cross sections for neutron-rich systems increase obviously, and the strong shell effects of two colliding nuclei result in a decrease of the fusion cross sections at the sub-barrier energies. The lowering of the dynamical fusion barriers favors the enhancement of the sub-barrier fusion cross sections, which is related to the nucleon transfer and the neck formation in the fusion reactions.; Comment: 20 pages, 12 figures

Subbarrier fusion reactions and many-particle quantum tunneling

Hagino, K.; Takigawa, N.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.33%
Low energy heavy-ion fusion reactions are governed by quantum tunneling through the Coulomb barrier formed by a strong cancellation of the repulsive Coulomb force with the attractive nuclear interaction between the colliding nuclei. Extensive experimental as well as theoretical studies have revealed that fusion reactions are strongly influenced by couplings of the relative motion of the colliding nuclei to several nuclear intrinsic motions. Heavy-ion subbarrier fusion reactions thus provide a good opportunity to address a general problem on quantum tunneling in the presence of couplings, which has been a popular subject in the past decades in many branches of physics and chemistry. Here we review theoretical aspects of heavy-ion subbarrier fusion reactions from the view point of quantum tunneling in systems with many degrees of freedom. Particular emphases are put on the coupled-channels approach to fusion reactions, and the barrier distribution representation for multi-channel penetrability. We also discuss an application of the barrier distribution method to elucidation of the mechanism of dissociative adsorption of H$_2$ melecules in surface science.; Comment: 45 pages, 16 eps figures. An invited paper to be published in Prog. Theo. Phys

Applications of Skyrme energy-density functional to fusion reactions for synthesis of superheavy nuclei

Wang, Ning; Wu, Xizhen; Li, Zhuxia; Liu, Min; Scheid, Werner
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 18/09/2006
Relevância na Pesquisa
46.42%
The Skyrme energy-density functional approach has been extended to study the massive heavy-ion fusion reactions. Based on the potential barrier obtained and the parameterized barrier distribution the fusion (capture) excitation functions of a lot of heavy-ion fusion reactions are studied systematically. The average deviations of fusion cross sections at energies near and above the barriers from experimental data are less than 0.05 for 92% of 76 fusion reactions with $Z_1Z_2<1200$. For the massive fusion reactions, for example, the $^{238}$U-induced reactions and $^{48}$Ca+$^{208}$Pb the capture excitation functions have been reproduced remarkable well. The influence of structure effects in the reaction partners on the capture cross sections are studied with our parameterized barrier distribution. Through comparing the reactions induced by double-magic nucleus $^{48}$Ca and by $^{32}$S and $^{35}$Cl, the 'threshold-like' behavior in the capture excitation function for $^{48}$Ca induced reactions is explored and an optimal balance between the capture cross section and the excitation energy of the compound nucleus is studied. Finally, the fusion reactions with $^{36}$S, $^{37}$Cl, $^{48}$Ca and $^{50}$Ti bombarding on $^{248}$Cm, $^{247...

Anharmonic phonon excitations in subbarrier fusion reactions

Hagino, K.; Takigawa, N.; Kuyucak, S.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 29/09/1997
Relevância na Pesquisa
46.22%
Recently measured high precision data of fusion excitation function have enabled a detailed study on the effects of nuclear collective excitations on fusion reactions. Using such highly accurate data of the $^{16}$O + $^{144,148}$Sm reactions, we discuss the anharmonic properties of collective phonon excitations in $^{144,148}$Sm nuclei. It is shown that subbarrier fusion reactions are strongly affected by the anharmonic effects and thus offer an alternative method to extract the static quadrupole moments of phonon states in a spherical nucleus.; Comment: 10 pages, To be published in the Proceedings of the Tours Symposium on Nuclear Physics III, Tours, France, September 1997 (American Institute of Physics)

Excitation of nuclear anharmonic vibrations in heavy-ion fusion reactions

Hagino, K.; Kuyucak, S.; Takigawa, N.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 05/11/1997
Relevância na Pesquisa
46.25%
We discuss the effects of multi-phonon excitations on heavy-ion fusion reactions at energies near and below the Coulomb barrier, focusing especially on the role of anharmonicities. We carry out a systematic study of those effects on the excitation function of the fusion cross section and on the fusion barrier distribution, by using the vibrational limit of the interacting boson model. We also analyze the recently measured high-precision data of the $^{16}$O + $^{148}$Sm fusion reaction with this model and discuss the anharmonic properties of the quadrupole as well as the octupole vibrations in $^{148}$Sm. Negative and positive static quadrupole moments are deduced for the first 2$^+$ and 3$^-$ states in $^{148}$Sm, respectively. It is shown that the fusion barrier distribution strongly depends on the sign of the quadrupole moments, suggesting that subbarrier fusion reactions offer an alternative method to extract the static quadrupole moments of phonon states in spherical nuclei.; Comment: 21 pages, 9 figures, Submitted to Physical Review C

Unexpected inhibition of fusion in nucleus-nucleus collisions

Berriman, Annette; Hinde, David; Dasgupta, Mahananda; Morton, Clyde; Butt, Rachel D; Newton, John
Fonte: Macmillan Publishers Ltd Publicador: Macmillan Publishers Ltd
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.25%
Unstable heavy atomic nuclei not found in nature can be created by fusing two stable nuclei, in a process analogous to colliding charged droplets of liquid. Recently, the formation of a handful of super-heavy nuclei with atomic numbers 114 (ref. 1) and 116 (reft 2) has been achieved by fusion of heavy nuclei. The electrostatic energy of such systems is very large (which is the reason super-heavy nuclei are unstable), so although the two nuclei may initially be captured by the nuclear potential, rather than fusing, they almost always separate after transfer of mass to the lighter nucleus. This process, called quasi-fission3,4, can inhibit fusion by many orders of magnitude. Understanding this inhibition may hold the key to forming more super-heavy elements. Theoretically, inhibition is predicted (ref. 5 and references therein) when the product Z1Z2 of the charges of the projectile and target nuclei is larger than about 1,600. Here we report measurements of three fusion reactions with Z1Z2 around half this value, each forming 88216 Ra. We find convincing model-independent evidence both of inhibition of fusion, and of the presence of quasi-fission. These results defy interpretation within the standard picture of nuclear fusion and fission.