Search

Nuclear friction causes energy dissipation in heavy-ion collisions. Its understanding and inclusion in quantum mechanical reaction models are crucial for advancing the physics of heavy-ion reactions forming heavy elements. The effects of nuclear friction on heavy-ion fusion reactions are investigated using the coupled-channels density-matrix method. In this open-quantum-system description, a phenomenological nuclear friction form factor is introduced along with coherent coupled-channels effects. The key nucleus was the 92Zr target, due to its high density of low-lying non-collective excited states, which was recently theorised to be cause of nuclear friction. The calculations using the 16O + 92Zr collision showed that the inclusion of nuclear friction effects increased the fusion probability significantly, and that the agreement between the theoretical and experimental fusion barrier distributions was improved when nuclear friction effects were included.

Earlier, it was shown in the literature that accounting for the zero-point oscillations (ZPO) of the colliding nuclei shapes results in significant increase of the calculated sub-barrier capture (fusion) cross sections. Yet, such calculations had been performed in a simplified approach with a schematic nucleus–nucleus potential. The present paper aims to evaluate accurately the sub-barrier capture cross sections in the collision of oxigen-16 with spherical nuclei using a realistic potential and to compare the calculated cross sections with the experimental data. The nucleus–nucleus potential is evaluated within the framework of the double-folding model with M3Y–Paris nucleon–nucleon forces. The nucleon densities are borrowed from the IAEA data base, they were computed using the Hartree–Fock–Bogolubov methos and approximated by the two-parameter Fermi formula. The transmission coefficients are evaluated using the WKB approximation below the barrier and the parabolic barrier approximation above the barrier. The proposed novel method to account for ZPO does not use any adjustable parameters. It is demonstrated that ignoring ZPO results in underestimating experimental data whereas accounting for ZPO allows to reach a better agreement. [ABSTRACT FROM AUTHOR]

We perform a comprehensive analysis of complete fusion cross section data with the aim to derive, in a completely data-driven way, a model suitable to predict the integrated cross section of the fusion between light-to-medium mass nuclei at above barrier energies. To this end, we adopted a novel artificial intelligence approach, based on a hybridization of genetic programming and artificial neural networks, capable to derive an analytical model for the description of experimental data. The approach enables to perform a global search for computationally simple models over several variables and a considerable body of nuclear data. The derived phenomenological formula can serve to reproduce the trend of fusion cross section for a large variety of light to intermediate mass collision systems in an energy domain ranging approximately from the Coulomb barrier to the onset of multi-fragmentation phenomena. [ABSTRACT FROM AUTHOR]

We investigate the occurrence of nuclear structure effects in the cross section of heavy-ion fusion reactions at energies above the Coulomb barrier. To this end, we initially develop a universal phenomenological model capable to reproduce, with an unprecedented accuracy, all previously published experimental fusion excitation functions with a few parameters. The new model, which foresees exclusively charge, mass, and energy of the colliding systems, shows a clear saturation of the critical angular momentum and avoids analytical non-regularities. The predictions of the newly developed model are then inspected to pin down residual discrepancies with the data, which could be ascribed to the structure of the colliding systems. In this framework, we obtain the following findings: (1) for the first time, we suggest an anomaly in the optimum value of the fusion cross section for systems having nearly-zero fusion Q-values; (2) we point out the occurrence of shell closure effects in the fusion of light systems; (3) we suggest that shell effects are washed-out at relative velocities vrel≳0.07c; (4) in the higher energy part of the fusion excitation function, the cross section for colliding systems involving fluorine or neon isotopes impinging on 2p3/2-1f5/2 nuclei is suppressed, possibly due to the occurrence of α-clustering effects enhancing α-transfer reactions.

A recent experiment on 12C + 24Mg sub-barrier fusion was performed. Standard coupled-channels calculations clearly over-estimated the low energy cross sections and there was indication of a maximum of the astrophysical S factor vs energy. In the present work further measurements have been performed on the same system, and we extended the excitation function to lower energies down to σ ≃ 4 ÎĽ b, confirming the existence of the hindrance phenomenon. Additional energy points near the barrier were added as well. In agreement with the result of previous experiment, the hindrance energy threshold is observed to correspond to a remarkably large cross section. The present new data allow to better define the S -factor maximum, that is well fitted using two parametrizations of either the hindrance or the adiabatic model. We also performed coupled-channels calculations using a Woodsâ€"Saxon potential. Far below the barrier we observe that the coupling strengths decrease and tend to vanish, indeed one-dimensional tunnelling calculations well reproduce the excitation function. Further precise cross section measurements at slightly lower energies are needed to discriminate between the adiabatic and hindrance models. This would give a reliable guidance for the extrapolation to the systems of astrophysical interest, like 12C + 12C and 16O + 16O. [ABSTRACT FROM AUTHOR]

In this work, the production of I111n radionuclide has been investigated theoretically via heavy-ion fusion reactions of two stable nuclei: C37l+G74e, M26g+R85b, S30i+B81r, and C46a+C65u reactions. Fusion cross-sections, barrier distributions, and potential energies on mutual orientations in the reactions planes of all reactions have been researched in detail around the barrier region via a coupled channel (CC) model using different codes. First of all, the most suitable codes and calculation parameter sets were determined through the C37l+G74e reaction, whose experimental data were available. The compatibility of the calculations via NRV knowledge base, CCFULL, CCDEF codes, and Wong’s formula with experimental data was analyzed. Barrier distributions and cross-sections for heavy-ion fusion reactions have been investigated with miscellaneous codes and vibrational-rotational nuclei combinations for interacting nuclei. Afterward, calculations were made with the determined parameter values for new reaction suggestions (M26g+R85b, S30i+B81r, and C46a+C65u reactions) and the results were compared. This study aims to suggest the new reaction combinations for the production of 111In radionuclide, to explore the impacts of different calculation codes and nuclear parameter combinations on the heavy-ion fusion cross-sections and barrier distributions, to demonstrate that the results are reliable, and to emphasize the importance of developing these studies in the preparation of new experiments.

We investigate unprecedented experimental information on the fusion reaction induced by the radioactive projectile 8Li on a 4He gas target, at center-of-mass energies between 0.6 and 5 MeV. The main issue is the tendency of the dimensionless fusion cross section σfπƛ2 to form well visible plateaus alternated to steep rises. This finding is likely to be the most genuine consequence of the discrete nature of the intervening angular momenta observed so far in fusion reactions right above the Coulomb barrier. A partial-wave analysis, exclusively based on a pure quantal penetration fusion model and sensitive to the interaction potential, identifies a remarkably low-height barrier.

Abstract The study of sub-barrier fusion is important to understand the dynamics of reactions involving weakly bound exotic nuclei. In most cases cross sections well below ≈ 1 mb need to be determined, but the limited beam intensity and background poses a great experimental challenge to measure it accurately. To overcome these problems we discuss a novel technique based on the use of radiochemical analysis. The advantages and limitations of this method are compared with the direct α -counting technique using the reaction 208Pb( 7 Li, 210Po) at E l a b = 39.3 MeV. The experiments were carried out at the BARC-TIFR Pelletron-Linac accelerator facility in Mumbai, India, and the radiochemical analysis at the University of Huelva, Spain. [ABSTRACT FROM AUTHOR]

Abstract Microscopic methods and tools to describe nuclear dynamics have considerably been improved in the past few years. They are based on the time-dependent Hartree–Fock (TDHF) theory and its extensions to include pairing correlations and quantum fluctuations. The TDHF theory is the lowest level of approximation of a range of methods to solve the quantum many-body problem, showing its universality to describe many-fermion dynamics at the mean-field level. The range of applications of TDHF to describe realistic systems allowing for detailed comparisons with experiment has considerably increased. For instance, TDHF is now commonly used to investigate fusion, multi-nucleon transfer and quasi-fission reactions. Thanks to the inclusion of pairing correlations, it has also recently led to breakthroughs in our description of the saddle to scission evolution, and, in particular, the non-adiabatic effects near scission. Beyond mean-field approaches such as the time-dependent random-phase approximation (TDRPA) and stochastic mean-field methods have reached the point where they can be used for realistic applications. We review recent progresses in both techniques and applications to heavy-ion collision and fission. [ABSTRACT FROM AUTHOR]

Heavy-element research with SHIP at GSI is reviewed including the discovery of the chemical elements bohrium to copernicium, experimental developments, cold fusion of heavy ions, and the discovery of a shell region around hassium. Elements bohrium and heavier are located beyond the limit of liquid-drop stability. They exist by shell stabilization. A universal, sensitive, and fast method: in-flight separation and identification of single atomic nuclei has been developed with the velocity filter SHIP and the detector system to measure decay sequences of individual atoms. Research with single atomic nuclei including detection methods, identification, and physics results will be discussed. Experiments with actinide targets as well as prospects with NUSTAR at FAIR will be addressed. [ABSTRACT FROM AUTHOR]

In this contribution, heavy-element research at RIKEN, Japan is reviewed. Nuclei of the heaviest elements were produced by the reactions 208 Pb( 64 Ni, n ) 271 Ds, 209 Bi( 64 Ni, n ) 272 Rg, 208 Pb( 70 Zn, n ) 277 Cn, and 209 Bi( 70 Zn, n ) 278 113. Alpha decays of those products and their consecutive alpha/spontaneous fission decays were studied using a gas-filled type recoil separator. New nuclei, 278 113, 274 Rg, and 270 Mt, were observed in the 209 Bi + 70 Zn reaction. [ABSTRACT FROM AUTHOR]

The precise estimation of stopping power is crucial to predict the beam energy loss in the target for heavy-ion fusion and heavy-ion-driven high-energy density physics experiments. The electromagnetic shock wave has been proposed to generate a well-defined atomic hydrogen target for the stopping power measurement with dissociation effects. We measured the angular distribution profile of the discharge plasma and the plasma velocity in the electromagnetic shock tube by high-speed framing cameras. To improve the uniformity of the discharge plasma and the velocity, an external magnetic field was applied in the electromagnetic shock tube. The plasma velocity was up to approximately 40 km/s for an initial hydrogen gas pressure of 100 Pa and the velocity decreased with the initial pressure and the propagation length. The framing cameras showed that angular distributions of the discharge plasmas were not uniform and the initial angular distributions were important for the development of plasma profiles. The interaction of the plasma with the external magnetic field was estimated using the ratio of the plasma dynamic pressure to the magnetic pressure. The estimations offer more magnetic fields to improve the discharge uniformity due to the interaction. [ABSTRACT FROM PUBLISHER]

Evolution of the fusion cross-section in the O+O reaction has been analysed. It is shown, both analytically and numerically, that in this excitation function some oscillations can be observed. These oscillations are related to the quantum character of the orbital angular momentum increase as well as to the distinct features of the O+O reaction. In order to perform the numerical calculations, the fluctuation-dissipation model and the single barrier penetration model are used. It turns out that the experimental data available in the literature do not have any definite proof about the presence or absence of the oscillations. We stress, that the question still remains unanswered for more than three decades whereas during this time lapse the experimental errors for other reactions are reduced to 1-2%. [ABSTRACT FROM AUTHOR]

Twenty years ago an approximation for the spin distribution of the dinuclear systems formed in capture reactions with heavy ions was proposed. This approximation is used nowadays. However, since that time the experimental errors of the measured capture cross sections were reduced drastically. We show that the accuracy of the old spin distribution approximation is certainly out of date and propose a new approximation built on the dynamical modeling of the capture process. Results suggest that this new approximation might be useful especially for performing modeling of decay of excited dinuclear systems (compound nuclei) formed during heavy-ion collisions. [ABSTRACT FROM AUTHOR]

Fusion cross sections of 28 Si + 28 Si have been measured in a range above the barrier with a very small energy step ( Δ E lab = 0.5 MeV ). Regular oscillations have been observed, best evidenced in the first derivative of the energy-weighted excitation function. For the first time, quite different behaviors (the appearance of oscillations and the trend of sub-barrier cross sections) have been reproduced within the same theoretical frame, i.e., the coupled-channel model using the shallow M3Y + repulsion potential. The calculations suggest that channel couplings play an important role in the appearance of the oscillations, and that the simple relation between a peak in the derivative of the energy-weighted cross section and the height of a centrifugal barrier is lost, and so is the interpretation of the second derivative of the excitation function as a barrier distribution for this system, at energies above the Coulomb barrier. [ABSTRACT FROM AUTHOR]

To realize a heavy-ion inertial fusion (HIF) driver, we have studied a possibility of laser ion source (LIS). A LIS can provide high-current high-brightness heavy-ion beams; however, it was difficult to manipulate the beam parameters. To overcome the issue, we employed a pulsed solenoid in the plasma drift section and investigated the effect of the solenoid field on singly charged iron beams. The rapid ramping magnetic field could enhance limited time slice of the current and simultaneously the beam emittance changed accordingly. This approach may also be useful to realize an ion source for HIF power plant. [ABSTRACT FROM PUBLISHER]

Abstract: The sub-barrier fusion excitation function of 40Ca + 96Zr has been measured down to cross sections ≃2.4 μb, i.e. two orders of magnitude smaller than obtained in a previous experiment, where the sub-barrier fusion of this system was found to be greatly enhanced with respect to 40Ca + 90Zr, and the need of coupling to transfer channels was suggested relying on coupled-channels calculations. The purpose of this work has been to investigate the behavior of 40Ca + 96Zr fusion far below the barrier, thereby disentangling the elusive interplay of effects due to inelastic couplings, transfer couplings and, possibly, the appearance of the fusion hindrance. The smooth trend of the excitation function has been found to continue, and the logarithmic slope increases very slowly. No indication of hindrance shows up, and a comparison with 48Ca + 96Zr is illuminating in this respect. A new CC analysis of the complete excitation function has been performed, including explicitly one- and two-nucleon transfer channels. Such transfer couplings bring significant cross section enhancements, even at the level of a few μb. Locating the hindrance threshold, if any, in 40Ca + 96Zr would require challenging measurements of cross sections in the sub-μb range. [Copyright &y& Elsevier]

Abstract: Induction accelerators are appealing for heavy-ion driven inertial fusion energy (HIF) because of their high efficiency and their demonstrated capability to accelerate high beam current (≥10kA in some applications). For the HIF application, accomplishments and challenges are summarized. HIF research and development has demonstrated the production of single ion beams with the required emittance, current, and energy suitable for injection into an induction linear accelerator. Driver scale beams have been transported in quadrupole channels of the order of 10% of the number of quadrupoles of a driver. We review the design and operation of induction accelerators and the relevant aspects of their use as drivers for HIF. We describe intermediate research steps that would provide the basis for a heavy-ion research facility capable of heating matter to fusion relevant temperatures and densities, and also to test and demonstrate an accelerator architecture that scales well to a fusion power plant. [Copyright &y& Elsevier]

Abstract: A long-standing challenge in the design of a Heavy Ion Fusion power plant is that the ion beams entering the target chamber, which number of order a hundred, all need to be routed from one or two multi-beam accelerators through a set of transport lines. The beams are divided into groups, each of which has a unique arrival time and may have a unique kinetic energy. It is also necessary to arrange for each beam to enter the target chamber from a prescribed location on the periphery of that chamber. Furthermore, it has generally been assumed that additional constraints must be obeyed: that the path lengths of the beams in a group must be equal, and that any delay of “main-pulse” beams relative to “foot-pulse” beams must be provided by the insertion of large delay-arcs in the main beam transport lines. Here we introduce the notion of applying “differential acceleration” to individual beams or sets of beams at strategic stages of the transport lines. That is, by accelerating some beams “sooner” and others “later,” it is possible to simplify the beam line configuration in a number of cases. For example, the time delay between the foot and main pulses can be generated without resorting to large arcs in the main-pulse beam lines. It is also possible to use differential acceleration to effect the simultaneous arrival on target of a set of beams (e.g., for the foot-pulse) without requiring that their path lengths be precisely equal. We illustrate the technique for two model configurations, one corresponding to a typical indirect-drive scenario requiring distinct foot and main energies, and the other to an ion-driven fast-ignition scenario wherein the foot and main beams share a common energy. [Copyright &y& Elsevier]

Abstract: The fusion excitation function of 48Ca+ 48Ca has been measured above and well below the Coulomb barrier, thereby largely extending the energy range of a previous experiment down to very low cross sections. This system has a negative Q-value for compound nucleus formation. The fusion cross section decreases steadily below the barrier with no conspicuous change of slope below . Coupled-channels calculations using a Woods–Saxon potential indicate that a large diffuseness parameter is needed to reproduce the sub-barrier cross sections. A close analogy with the case of 36S+ 48Ca, with , is pointed out. The sign of the Q-value does not influence fusion cross sections down to the 300–600 nb level. [Copyright &y& Elsevier]

Abstract: Complete fusion excitation functions for have been measured at energies around the respective Coulomb barriers, and the existing complete fusion measurements for have been extended to higher energies. The measurements show significant reduction of complete fusion cross sections at above-barrier energies for both the reactions, and , when compared to those for . The comparison shows that the extent of suppression of complete fusion cross sections is correlated with the α-separation energies of the projectiles. Also, the two reactions, and were found to produce incomplete fusion products at energies near the respective Coulomb barriers, with the α-particle emitting channel being the favoured incomplete fusion process in both the cases. [Copyright &y& Elsevier]

Abstract: It is well known that electron heat conductivity coefficient and range of α-particles decrease in magnetic field; therefore, thermonuclear mixture ignition is feasible at lower densities, resulting in a reduction of target sensitivity to temperature field asymmetry at the capsule surface. This paper estimates the 2D perturbation level that does not impede ignition with magnetic field. [Copyright &y& Elsevier]

Abstract: An end-to-end simulation of a Heavy-Ion Fusion driver is very challenging due to the wide range of scales involved. We have merged the Particle-In-Cell and the Adaptive Mesh Refinement (AMR) methods and have applied it to the simulation of HIF beam sources. We describe our past progress and present our recent implementation of automatic AMR in WARP. [Copyright &y& Elsevier]

Abstract: The need to reach high temperatures in an inertial fusion energy (IFE) target (or a target for the study of High Energy Density Physics, HEDP) requires the ability to focus ion beams down to a small spot. System models indicate that within the accelerator, the beam radius will be of the order of centimeters, whereas at the final focal spot on the target, a beam radius of the order of millimeters is required, so radial compression factors of order ten are required. The IFE target gain (and hence the overall cost of electricity) and the HEDP target temperature are sensitive functions of the final spot radius on target. Because of this sensitivity, careful attention needs to be paid to the spot radius calculation. We review our current understanding of the elements that enter into a systems model (such as emittance growth from chromatic, geometric, and non-linear space charge forces) for the final focus based on a quadrupolar magnet system. [Copyright &y& Elsevier]

Abstract: Negative halogen ions have recently been proposed as a possible alternative to positive ions for heavy-ion fusion drivers because electron accumulation would not be a problem in the accelerator, and if desired, the beams could be photodetached to neutrals (Nucl. Instr. and Meth. A 464 (2001) 315; Fusion Sci. Technol. 43 (2003) 191; Laser Part. Beams 21 (2003) 545). To test the ability to make suitable quality beams, an experiment was conducted at Lawrence Berkeley National Laboratory using chlorine in an RF-driven ion source. Without introducing any cesium (which is required to enhance negative ion production in hydrogen ion sources) a negative chlorine current density of 45mA/cm2 was obtained under the same conditions that gave 57mA/cm2 of positive chlorine, suggesting the presence of nearly as many negative ions as positive ions in the plasma near the extraction plane. The negative-ion spectrum was 99.5% atomic chlorine ions, with only 0.5% molecular chlorine, and essentially no impurities. Although this experiment did not incorporate the type of electron suppression technology that is used in negative hydrogen beam extraction, the ratio of co-extracted electrons to Cl− was as low as 7 to 1, many times lower than the ratio of their mobilities, suggesting that few electrons are present in the near-extractor plasma. This, along with the near-equivalence of the positive- and negative-ion currents, suggests that the plasma in this region was mostly an ion–ion plasma. The negative chlorine current density was relatively insensitive to pressure, and scaled linearly with RF power. If this linear scaling continues to hold at higher RF powers, it should permit current densities of 100mA/cm2, sufficient for present heavy-ion fusion injector concepts. The effective ion temperatures of the positive and negative ions appeared to be similar and relatively low for a plasma source. [Copyright &y& Elsevier]

A large number of precision fusion excitation functions, at energies above the average fusion barriers, have been fitted using the Woods–Saxon form for the nuclear potential. Values of the empirical diffuseness parameter greatly exceed those which generally reproduce elastic scattering data and tend to increase strongly with the reaction charge product Z1Z2. Possible reasons for this may lie in the Woods–Saxon form being inappropriate, or in the reduction of fusion cross sections by processes such as deep-inelastic collisions. These results point to a need for renewed efforts in dynamical calculations of fusion. [Copyright &y& Elsevier]

Successful development of heavy-ion fusion (HIF) will require scientific and technology advances in areas of targets, drivers and chambers. Design work on heavy-ion targets indicates that high gain (60–130) may be possible with a ∼3–6 MJ driver depending on the ability to focus the beams to small spot sizes. Significant improvements have been made on key components of heavy-ion drivers, including sources, injectors, insulators and ferromagnetic materials for long-pulse induction accelerator cells, solid-state pulsers, and superconducting quadrupole magnets. The leading chamber concept for HIF is the thick-liquid-wall HYLIFE-II design, which uses an array of flibe jets to protect chamber structures from X-ray, debris, and neutron damage. Significant progress has been made in demonstrating the ability to create and control the types of flow needed to form the protective liquid blanket. Progress has also been made on neutron shielding for the final focus magnet arrays with predicted lifetimes now exceeding the life of the power plant. Safety analyses have been completed for the HYLIFE-II design using state-of-the-art codes. Work also continues on target fabrication and injection for HIF. A target injector experiment capable of >5 Hz operation has been designed and construction will start in 2002. Methods for mass-production of hohlraum targets are being evaluated with small-scale experiments and analyses. Progress in these areas will be reviewed. [Copyright &y& Elsevier]

In this work, the production of I 111 n radionuclide has been investigated theoretically via heavy-ion fusion reactions of two stable nuclei: C 37 l + G 74 e , M 26 g + R 85 b , S 30 i + B 81 r , and C 46 a + C 65 u reactions. Fusion cross-sections, barrier distributions, and potential energies on mutual orientations in the reactions planes of all reactions have been researched in detail around the barrier region via a coupled channel (CC) model using different codes. First of all, the most suitable codes and calculation parameter sets were determined through the C 37 l + G 74 e reaction, whose experimental data were available. The compatibility of the calculations via NRV knowledge base, CCFULL, CCDEF codes, and Wong's formula with experimental data was analyzed. Barrier distributions and cross-sections for heavy-ion fusion reactions have been investigated with miscellaneous codes and vibrational-rotational nuclei combinations for interacting nuclei. Afterward, calculations were made with the determined parameter values for new reaction suggestions ( M 26 g + R 85 b , S 30 i + B 81 r , and C 46 a + C 65 u reactions) and the results were compared. This study aims to suggest the new reaction combinations for the production of 111In radionuclide, to explore the impacts of different calculation codes and nuclear parameter combinations on the heavy-ion fusion cross-sections and barrier distributions, to demonstrate that the results are reliable, and to emphasize the importance of developing these studies in the preparation of new experiments. [ABSTRACT FROM AUTHOR]

In the present work, the influence of the nuclear matter density on the DF potential and on the Coulomb barrier parameters is studied systematically for collisions of spherical nuclei. The value of the parameter B Z = Z P Z T / (A P 1 / 3 + A T 1 / 3) (estimating the Coulomb barrier height) varies in these calculations from 10 MeV up to 150 MeV. We have introduced self-consistent relativistic mean field (RMF) density in the present analysis. For the nucleon-nucleon effective interaction, the M3Y forces with the finite range exchange term and density dependence are employed. The above barrier fusion cross sections are calculated within the framework of the trajectory model with surface friction. Results are compared with the previous study in which the nuclear density came from the Skyrme Hartree-Fock (HF) calculations and with the high precision experimental data. This comparison demonstrates that i) agreement between the theoretical and experimental cross sections obtained with RMF and HF densities is of the same quality and ii) the values of the only adjustable parameter (friction strength) obtained with RMF and HF densities strongly correlate with each other. [ABSTRACT FROM AUTHOR]