Effect of neutron transfer channels in fusion reactions with weakly bound nuclei at subbarrier energies

The role of neutron transfer in fusion reactions of weakly-bound nuclei at subbarrier energies is studied within the empirical model of channel coupling. The results from calculating the fusion cross sections for the ⁷Li + ²⁰⁹Bi, ^{9, 11}Li + ^{208, 206}Pb, ^{6, 7, 9, 11}Li + ¹⁵²Sm reactions are presented. Good agreement with the available experimental data is shown. Several combinations of colliding nuclei for which the strong enhancement of subbarrier fusion due to the effect of neutron transfer processes are predicted.     

Fusion and transfer reactions in beams of light loosely bound nuclei at energies in the vicinity of the Coulomb barrier

Experiments devoted to studying cross sections for fusion and transfer reactions induced by the interaction of beams of halo-like (⁶He), cluster (⁶Li and ⁷Li), and loosely bound (³He) nuclei with nuclei of light and heavy elements are described. The cross sections obtained experimentally for such reactions are analyzed. Special features in the behavior of the cross sections for the formation of evaporation residues and products of transfer reactions at energies in the vicinity of the Coulomb barrier are revealed. In particular, an increase in the cross sections for fusion and transfer reactions involving halo-like nuclei and proceeding at energies in the subbarrier region is observed. The cross sections for neutron-transfer and light-cluster-transfer reactions reachmaximum values at an energy in the vicinity of the Coulomb barrier for the reaction being considered.     

Evidence of complete fusion in the subbarrier 16O+238U reaction

Evaporation residue cross sections in the ¹⁶O+²³⁸U reaction were measured for the energy range from above-to extreme subbarrier. We used a He-gas-jet system to transport the fusion products, and the α decay of the evaporation residues was measured by using a rotating wheel system. The measured cross sections for ^248,249,250Fm are reproduced by a statistical model calculation, for which partial cross sections are calculated by a coupled-channel model taking into account the prolate deformation of ²³⁸U. We conclude that complete fusion is the main process in the subbarrier energy region, and quasifission is not an important channel. The text was submitted by the authors in English. On leave from China Institute of Atomic Energy, Beijing, China.     

放射性核引起的反应

评述了放射性核自身的特征和它引起的反应机制研究．论述了晕核、中子皮核、电磁断裂和软偶极共振以及放射性核所引起的弹性散射、转移反应和亚位垒融合等. The characteristic of the nuclei far from stability and the mechanism of the reactions induced by these nuclei are reviewed. The disscusion includes the structure beyond the dripline and examples of elastic and inelastic scattering,inverse kinematic and nuclear break up, and of subbarrier fusion using radioactive beams.     

Quantum calculations of Coulomb reorientation for sub-barrier fusion

Classical mechanics and time dependent Hartree-Fock (TDHF) calculations of heavy ions collisions are performed to study the rotation of a deformed nucleus in the Coulomb field of its partner. This reorientation is shown to be independent of the charges and relative energy of the partners. It only depends upon the deformations and inertias. TDHF calculations predict an increase by 30% of the induced rotation due to quantum effects while the nuclear contribution seems negligible. This reorientation modifies strongly the fusion cross section around the barrier for light deformed nuclei on heavy collision partners. For such nuclei a hindrance of the sub-barrier fusion is predicted.     

On the interpretation of heavy-ion sub-barrier fusion data

We study the well-known deviation of measured fusion cross-sections below the barrier from predictions of one-dimensional tunnelling models. A simple parametrisation of measured excitation functions is discussed which allows to identify a smooth general trend depending only on gross nuclear properties. The parametrisation is substantiated by quantum mechanically exact calculations in a two-dimensional tunnelling model. Various possibilities for the geometrical interpretation of the second degree of freedom are discussed. Neck formation is identified as the most probable degree of freedom effecting the overall behavior of sub-barrier fusion.     

Quasi-molecular states of neutrons in dinuclear systems and their effect on fusion of heavy atomic nuclei

The evolution of the wave functions of valence neutrons upon collisions of heavy nuclei having energies near the Coulomb barrier has been investigated by numerical solution of the multidimensional time-dependent Schrödinger equation. It is shown that even before overcoming the barrier by nuclei the wave functions of valence neutrons are extended to their volumes with occupation of quasi-molecular states. This process results in a significant effect of neutron transfers on nuclear fusion and, in particular, in a significant increase in the probability of subbarrier fusion for certain combinations of nuclei.     

Fusability and fissionability in 86Kr-induced reactions near and below the fusion barrier

Evaporation-residue excitation functions for the reactions ⁸⁶Kr + ^70,76Ge, ^92,100 Mo, ^99,102,104 Ru have been measured using activation methods and the velocity filter SHIP. The data span the region from well below the fusion barrier up to, and beyond, the energy where limitation by fission competition takes place. The data are shown to be compatible with the concept of complete fusion followed by the statistical decay of the equilibrated compound nucleus. Information on both the fusion probability at and below the fusion threshold and the fissionability of the compound nuclei formed, is extracted. The model dependence of the extracted fission barriers is discussed in detail. In analogy to studies involving lighter projectiles, strong correlations between the low-energy nuclear-structure properties of the nuclei and the subbarrier fusion probability are found. A relative shift of the fusion barrier to higher energies, that increases with the number of valence neutrons in the target nuclei, is observed.     

Telepistemology - knowledge in the age of the Internet

The crucial role of atomic and molecular collision processes in a number of outstanding issues in current magnetic fusion research is discussed. These issues are related to the achievement and control of thermonuclear burn conditions, thermal power and particle exhaust, and the optimization of fusion reactor performance. The status of the present knowledge on the characteristics of collision processes (cross sections, reaction rate coefficients) involved in these fusion research issues is presented. Significant gaps in this knowledge that prevent a full understanding of the background physics of the above mentioned fusion research problems and may affect the ways of their solution are identified. One of the purposes of the present article is to provide a motivation for undertaking the necessary atomic and molecular physics studies for bridging these gaps.     

The fate of Li and Be in stars and in the laboratory

We connect the observed under-abundances of Li and Be in dwarfs, with recent results on nuclear cross sections at low energies: for collisions of protons with atomic or molecular targets, the measured cross sections seem too high with respect to extrapolations for bare nuclei. Phenomenologically, these anomalous nuclear interactions can be described in terms of an effective screening potentialU _lab in the range of few hundred eV: in the presence of the electron cloud, nuclei become more transparent to each other as if the effective collision energy is aumented byU _lab. This implies that fusion cross sections are enlarged and at the same time elastic cross sections are lowered. If something similar occurs in stellar plasma, the nuclear burning temperatures are lowered, whereas diffusion processes are enhanced. We find that the observed Li and Be abundances in the Hyades and in the Sun can be reproduced for effective screening potentials of the plasma in the range of 600–700 eV, close to that found by experiments in the laboratory.     

Cold fusion in symmetric 90Zr-induced reactions

Excitation functions for evaporation residues were measured for the reactions ⁹⁰Zr + ⁸⁹Y, ⁹⁰Zr, ⁹²Zr, ⁹⁶Zr, and ⁹⁴Mo. Deexcitation by γ-radiation only was found for the compound nuclei ¹⁷⁹Au, ¹⁸⁰Hg, ¹⁸²Hg and ¹⁸⁴Pb.Fusion probabilities as well as fusion-barrier distributions were deduced from the measured cross sections. There are strong nuclear structure effects in subbarrier fusion. For energies far below the fusion barrier the increase of the fusion probabilities with increasing energy is found to be much steeper than predicted by WKB calculations.As a by-product of this work new α-spectroscopic information was obtained for neutron-defi- cient isotopes between Ir and Pb.     

The influence of the entrance channel mass asymmetry on the reaction mechanism

We have tried to investigate the influence of the entrance channel mass asymmetry on the reaction mechanisms associated with heavy ion collisions. Two systems, one very much asymmetric (O+Mo) and the other one almost symmetric (Cr+Fe), were studied in detail by measuring evaporation residues, deep inelastic collision products and fission fragments. An important fraction of the fragments observed in the Cr+Fe system exhibits all the characteristics of fission fragments. The analysis of these data seems to indicate that these fission like products are most likely emitted by a long lived composite system having not reached full statistical equilibrium for all the degrees of freedom. As a consequence, the fusion cross section for this symmetric system is too low as compared to predictions based on a critical distance approach for fusion, whereas the asymmetric system (O+Mo) is well understood in term of the same model.     

Sub-barrier fusion excitation function data and energy dependent Woods–Saxon potential

This paper analyzed the role of intrinsic degrees of freedom of colliding nuclei in the enhancement of sub-barrier fusion cross-section data of various heavy ion fusion reactions. The influences of inelastic surface vibrations of colliding pairs are found to be dominant and their couplings result in the significantly larger fusion enhancement over the predictions of the one dimensional barrier penetration model at sub-barrier energies. The theoretical calculations are performed by using energy dependent Woods–Saxon potential model (EDWSP model) in conjunction with the one dimensional Wong formula. The effects of dominant intrinsic channels are entertained within framework of the coupled channel calculations obtained by using the code CCFULL. It is quite interesting to note that the energy dependence in Woods–Saxon potential simulates the effects of inelastic surface vibrational states of reactants wherein significantly larger value of diffuseness parameter ranging from a = 0.85fm to a = 0.95fm is required to address the observed fusion excitation function data of the various heavy ion fusion reactions.     

Cross section for <Superscript>246</Superscript>Cm subbarrier fission

The cross section for ²⁴⁶Cm fission induced by neutrons of energy in the range 0.1 eV-20 keV was measured by the neutron lead slowing-down spectrometer (LSDS-100) of the Institute for Nuclear Research (INR, Russian Academy of Sciences, Moscow). The parameters of the resonance area and of the fission width were evaluated for several low-lying s-wave neutron resonances. The parameters of the intermediate structure in the cross section for the subbarrier fusion of ²⁴⁶Cm nuclei were found. The results obtained in this way were compared with available experimental data and with recommended evaluated data.     

Isotopic dependence of fusion cross-sections – linear relationships

A systematic study of isotopic dependence of fusion cross-section is carried out by adding neutrons gradually to N=Z colliding nuclei. We find that fusion barrier position increases and height decreases, both linearly with the increase of N/Z ratio of the compound system. The increase in barrier position is larger compared to decrease in barrier height. In terms of these linear relationships, a parameterized form of fusion cross-sections is given for the neutron-rich colliding nuclei. The fusion cross-sections are also enhanced linearly with the N/Z ratio, and this enhancement is larger for lower incident centre-of-mass energies and independent of the choice of reaction partners. Experimental data and other theoretical studies are called for to verify these results. Received: 27 February 1998 / Revised version: 22 May 1998     

Using molecules to measure nuclear spin-dependent parity violation

Nuclear spin-dependent parity violation arises from weak interactions between electrons and nucleons and from nuclear anapole moments. We outline a method to measure such effects, using a Stark-interference technique to determine the mixing between opposite-parity rotational/hyperfine levels of ground-state molecules. The technique is applicable to nuclei over a wide range of atomic number, in diatomic species that are theoretically tractable for interpretation. This should provide data on anapole moments of many nuclei and on previously unmeasured neutral weak couplings.     

Fusion near the threshold: A comparative study of the systems 40Ar + 112, 116, 122sn and 40Ar + 144, 148, 154Sm

Fusion excitation functions for the systems ⁴⁰Ar + ^{112, 116, 122}Sn and ⁴⁰Ar + ^{144, 148, 154}Sm have been determined, covering cross sections ranging from several hundred mb down to the μb level. The data show a pronounced correlation of the subbarrier behaviour with low-energy collective properties of the nuclei involved and are well reproduced by simplified coupled-channel calculations coupling fusion to inelastic channels. The possibilities of parameterizing the data in terms of a simple dynamic barrier-fluctuation phenomenon are discussed and result in the prediction of remarkably diffuse partial-wave distributions above the barrier. This is shown to be important for the analysis of deexcitation phenomena following fusion reactions.