Effect of nuclear shell structure on fusion reaction

The dependence of the fusion reaction on the nuclear shell structure was investigated for the two reaction systems ⁸²Se + ¹³⁸Ba and ⁸²Se + ¹³⁴Ba, where the nucleus ¹³⁸Ba has a closed neutron shell N=82, while the nucleus ¹³⁴Ba has a neutron number 78. Evaporation residues for these fusion reactions were measured near the Coulomb barrier region. The measured evaporation residue cross sections for the reaction system ⁸²Se + ¹³⁸Ba were two orders of magnitude larger than those for the reaction system ⁸²Se + ¹³⁴Ba in the excitation energy region of 20–30 MeV. The evaporation residue cross sections were compared with those of the other reaction systems that produce the same compound nucleus as the present systems. It was found that the fusion reaction ⁸²Se + ¹³⁸Ba occurs without hindrance, while that of ⁸²Se + ¹³⁴Ba is considerably hindered, as commonly observed in the massive reaction system with the charge product Z _p Z _t >1800 of projectile and target. This suggests the importance of the shell closure N=82 in the heavy-ion fusion reaction.     

Direct inner shell ionization accompanying nuclear fusion reactions with heavy ions

Probabilities for K-shell ionization prior to fusion (half-trajectory collisions) are determined for ⁵¹V, ⁵⁹Co, ⁶²Ni target atoms and 4.5 MeV/u ⁴⁰Ar projectiles. Also measured are energy shifts of the K_α and K_β X-ray lines of residue atoms resulting from multiple inner shell ionization.     

Effect of shell structure on energy dissipation in heavy-ion collisions

The effect of shell structure on the distribution of the excitation energy between fragments of the deep inelastic collisions is analysed in the microscopic approach. It is shown that the density of the single-particle levels of the proton and neutron subsystems near the Fermi surface determines the ratio between the excitation energies of fragments at the initial stage of the collision. It is shown also that the shell structure strongly influences the correlations between the width of the charge distributions and the total kinetic energy losses. Calculations are performed for the ^40,48Ca+²⁴⁸Cm reactions. The results obtained suggest a possible interpretation for the observed concentration of the excitation energy in the light fragment in deep inelastic collisions for a wide range of the total kinetic energy losses. Received: 27 July 1999 / Accepted: 29 March 2000     

Incomplete fusion reactions

A review of incomplete fusion reactions is given. The localization of the entrance chanel angular momentum window is discussed for incident energies lower than 10 MeV per nucleon and is found to depend on the target deformation. With deformed target “peripheral” collisions are observed with ? values in the vicinity of ?_cr for complete fusion, while for spherical targets, the ? window is centered around values lower than 0.5 ?_cr. These properties are discussed on the grounds of available theoretical models. All other properties of the reactions such as cross sections, angular distributions, shape of the energy spectra of light ejectiles are discussed to bring additional informations about the reaction mechanism.     

Effect of positive Q-value neutron transfers on sub-barrier fusion reactions

The role of positive Q-value neutron transfers in sub-barrier fusion reactions has been studied with a modified quantum coupled channels model with all order couplings(CCFULL model). Neutron rearrangement related only to the dynamical matching condition with no free parameters is implemented in the model, which provides a way to understand especially the Q-value dependence of sub-barrier fusion reactions. The fusion cross sections of the collision systems ~(40)Ca+~(94,96)Zr have been calculated and analyzed. The general trend of experimental data can be reproduced well with additional channels for neutron rearrangement. We find that enhancement of sub-barrier fusion cross sections is closely related to the Q-value of the transferred neutrons, in particular for channels with sequential even number transferred neutrons.     

Liquid-drop effects in sub-barrier fusion reactions

We introduce an operational measure for the enhancement of the fusion cross section at sub-barrier energies in terms of an asymptotic energy shift ΔE. It is shown that ΔE has a continuously growing trend with the size of the system. This trend is explained in terms of neck formation using the liquid-drop model. Deviations from this trend are attributed to strong coupling to specific channels.     

Channel-coupling effects in heavy-ion fusion reactions

A coupled-channel framework for fusion reactions is considered where an ingoing-wave boundary condition allows the effect of strong coupling in the barrier region to be studied. It is shown analytically within the sudden limit and, more generally, with model calculations that the couplings to reaction channels act to enhance the transmission through the barrier at low energies. This appears to be a natural mechanism for explaining the relatively large sub-barrier heavy-ion fusion cross sections which have recently been observed.     

Dinuclear systems in complete fusion reactions

Formation and evolution of dinuclear systems in reactions of complete fusion are considered. Based on the dinuclear system concept, the process of compound nucleus formation is studied. Arguments confirming the validity of this concept are given. The main problems of describing the complete fusion in adiabatic approximation are listed. Calculations of evaporation residue cross sections in complete fusion reactions leading to formation of superheavy nuclei are shown. Isotopic trends of the cross sections of heavy nuclei formation in complete fusion reactions are considered.     

Alignment effects in subbarrier fusion reactions

Subbarrier fusion reactions in which one constitutent is deformed and aligned are considered in the framework of a coupled-channel model with incoming-wave boundary conditions. The effect of alignment on the fusion cross section is estimated to be 40% for aligned ¹⁶⁵Ho. The semiclassical analog to the coupled-channel equations is developed, yielding an expression for the cross section in terms of a weighted average over all orientations of the deformed nucleus.     

Spin distributions in heavy-ion fusion reactions

Spin distributions are calculated for the ¹⁶O + ²³²Th fusion reaction, taking into account both inelastic and transfer channels. The static deformations of the target play an important role. The calculated cross sections for complete fusion are in good agreement with the measured fission yields, whereas the anisotropies are underpredicted. This indicates that there is a serious inconsistency in the interpretation of the data.     

Quantum calculation of the dipole excitation in fusion reactions

The excitation of the giant dipole resonance induced by fusion reaction is studied with N/Z asymmetry in the entrance channel. The time dependent Hartree-Fock solution exhibits a strong dipole vibration which can be associated with a giant vibration along the main axis of the deformed compound nucleus. This dipole motion appears to be nonlinearly coupled to the shape oscillation, leading to a strong modulation of its frequency. These phenomena can be detected in the gamma-ray emission from hot compound nuclei.     

Probing the low angular momentum window in fusion reactions

The angular correlation between two alpha particles evaporated from the compound nucleus is sensitive to the initial spin population, and can thus be used to test the possible existence of angular momentum windows in the fusion cross section. A model calculation performed for the ²⁸Si + ²⁸Si fusion reaction at E_cm = 60 MeV is used as an illustration.     

Nuclear shell structure in the systematics of nuclear properties

Existing experimental data allow us to study the behavior of nuclear shell structure by analyzing characteristics of different nature. In this work, one of the most striking phenomena of nuclear dynamics is examined: nucleon pairing. Nucleon pairing for different nuclei chains as a function of the number of protons or neutrons in a nucleus explains why high numbers of states with positive parity in even-odd nuclei are observed for excitation energies E* < 4 MeV that form the multiplet of a nucleus’s ground state.     

On the influence of shell structure in dissipative collisions

A kinematically complete study of the symmetric systems ¹⁴⁴Sm+¹⁴⁴Sm and ¹⁵⁴Sn+¹⁵⁴Sm has been performed at energies 30% in excess of the interaction barrier. They have been chosen because of their different internal structure: ¹⁴⁴Sm has a closed N = 82 neutron shell and a spherical ground-state configuration; ¹⁵⁴Sm with ten neutrons outside this shell is strongly deformed. The observed gross features of both reactions like energy, angular and total mass or element distributions are very similar; the ratio of the mass variances as function of the total kinetic energy loss indicates the number of exchanged nucleons to be comparable in all stages of the reactions. At small energy losses, however, the element distributions of the ¹⁴⁴Sm + ¹⁴⁴Sm reaction are considerably broader, pointing at an enhanced proton transfer at the cost of the number of exchanged neutrons in this system. This observation is attributed to the influence of the closed shell which seems to block the transfer of neutrons at low excitation energies. These results can be explained quantitatively by the different gradients of the shell-corrected potential energy surfaces of the two systems.     

Effects of the entrance channel mass asymmetry in fusion reactions

The symmetric and asymmetric fusion reaction systems forming the same compound nuclei 26Al,30Si,38Ar and 170Hf are investigated with the frame of improved isospin dependent quantum molecular dynamics model.The entrance channel mass asymmetry dependence of compound nucleus formation is found by analyzing the shell correction energies,the Coulomb barriers and the fusion cross sections.The calculated fusion cross sections agree quantitatively with the experimental data.The results indicate that compound nucleus formation is favorable for the systems with larger mass asymmetry because of the smaller Coulomb contribution to the fusion barrier.     

熔合反应中入射道质量不对成度效应

The symmetric and asymmetric fusion reaction systems forming the same compound nuclei ^26Al, ^30Si, ^38Ar and ^170Hf are investigated with the frame of improved isospin dependent quantum molecular dynamics model. The entrance channel mass asymmetry dependence of compound nucleus formation is found by analyzing the shell correction energies, the Coulomb barriers and the fusion cross sections. The calculated fusion cross sections agree quantitatively with the experimental data. The results indicate that compound nucleus formation is favorable for the systems with larger mass asymmetry because of the smaller Coulomb contribution to the fusion barrier.     

Analysis of the fusion hindrance in mass-symmetric heavy ion reactions

The fusion hindrance,which is also denominated by the term extra-push,is studied on mass-symmetric systems by the use of the liquid drop model with the two-center parameterization.Following the idea that the fusion hindrance exists only if the liquid drop barrier(saddle point) is located at the inner side of the contact point after overcoming the outer Coulomb barrier,the reactions in which two barriers are overlapped with each other are determined.It is shown that there are many systems where the fusion hi...     

Effect of shell structure on transversal heating and cooling of channeled ions

It is shown that an abrupt change in ionization potential at the transition from one shell to the next one can result in a repeated transition from the heating of channeled ion beam to the cooling and vice versa with an increase in ion velocity.