N. Takigawa

Quantum tunneling in nuclear fusion

Recent theoretical advances in the study of heavy-ion fusion reactions below the Coulomb barrier are reviewed. Particular emphasis is given to new ways of analyzing data (such as studying barrier distributions), new approaches to channel coupling (such as the path-integral and Greens function formalisms), and alternative methods to describe nuclear structure effects (such as those using the interacting boson model). The roles of nucleon transfer, asymmetry effects, higher-order couplings, and shape phase transitions are elucidated. The current status of the fusion of unstable nuclei and very massive systems are briefly discussed.

Subbarrier Fusion Reactions and Many-Particle Quantum Tunneling

Low-energy heavy-ion fusion reactions are governed by quantum tunneling through the Coulomb barrier formed by the 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 the general problem of quantum tunneling in the presence of couplings, which has been a popular subject in recent decades in many branches of physics and chemistry. Here, we review theoretical aspects of heavy-ion subbarrier fusion reactions from the viewpoint 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 multichannel penetrability. We also discuss an application of the barrier distribution method to elucidate the mechanism of the dissociative adsorption of H2 molecules in surface science. Subject Index: 062, 211, 223, 226, 330

Structure of 12C

Abstract The low-lying states of 12 C are investigated with the Heitler-London approximation of the clustering wave functions. The distances between α-clusters are determined to make the total energy minimum; they are relatively small in the ground state band but large in the excited band. The l · s force is taken into account in the ground-state band by hybridizing the shell model and the cluster model and is found important to raise the excitation energies of the 2 + and 4 + states. Even though the clusterization is small in the ground-state band, it has an important effect on the enhanced transition probabilities and the form factors for electron scattering. The clusterization is essential to explain the observed properties of the excited band such as the low excitation energy and the large α-decay width.

Interaction potential and fusion of a halo nucleus

Abstract The interaction potential for the scattering of a neutron-rich nucleus is studied using the double folding procedure of the M3Y force. It is shown that the height of the potential barrier considerably decreases when the separation energy of the valence neutrons becomes small enough to develop a neutron halo similarly to the 11Li nucleus. A simple model calculation of the fusion cross section, which takes into account the realistic interaction potential and the coupling of the translational motion to a soft dipole mode of excitation of the halo nucleus, is then performed to show that the fusion cross section of a halo nucleus is drastically enhanced at low energies because of the lowering of the fusion barrier and the coupling of the translational motion to soft modes of excitation associated with the neutron halo.

Adiabatic quantum tunneling in heavy-ion sub-barrier fusion

The measured fusion barrier distributions for {sup 40}Ca+{sup 192}Os, {sup 194}Pt show significant features due to projectile excitation, while none are seen for {sup 16}O+{sup 144}Sm. This conflict is reconciled using realistic coupled-channel calculations, which show that the higher excitation energy of the 3{sup {minus}} state in {sup 16}O produces an adiabatic potential renormalization, without affecting the structure in the barrier distribution. This result indicates that adiabatic effects restrict, in a natural way, the states which influence the {ital shape} of a fusion barrier distribution. {copyright} {ital 1997} {ital The American Physical Society}

Clustering in Light Nuclei

The nucleus is a very interesting and unique object because of its identical constituents, i.e., nucleons, its finite size, and its abundance of different modes of motion. Among many modes of nuclear motion, we can find the shell model mode and the collective mode. The nucleus is the best subject for studying the quantal collective motion—a form of motion in which many nucleons participate. On the other hand a few nucleons in the nucleus will sometimes correlate strongly with each other to add further variety to modes of nuclear motion. Among such correlations the following are important: (i) the pairing correlation (ii) the neutron-proton pair correlation (iii) the four-body correlation

Elastic scattering of a halo nucleus at medium energies

Abstract The angular distribution of elastic scattering of a halo nucleus is compared to that between stable nuclei. We show that the optical limit approximation of the Glauber theory, which is conventionally used to describe collisions of stable nuclei, overestimates the reaction cross section when the projectile has an extended neutron halo. This suggests the important role played by the breakup reactions in the elastic scattering of halo nuclei, and also that the corresponding dynamic polarization potential has a sizable positive real part. The overestimate is shown to occur for a wide range of the incident energy, though it decreases with increasing bombarding energy.

Effects of beta(6) deformation and low lying vibrational bands on heavy ion fusion reactions at subbarrier energies

We study fusion reactions of

Life time of soft multipole excitations

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Validity of the linear coupling approximation in heavy-ion fusion reactions at sub-barrier energies

O with