| Professor | Noboru Takigawa |
| Associate Professor | Kouichi Hagino |
| Assistant Professors | Masahiro Maruyama, Akira Ono |
| Secretary | Kaori Yamaji |
| Research Students | Tomomasa Asano, Zakarya Mohamed(from September, 2005) |
| Graduate Students | Takuya Furuta, and Takayuki Takehi Yasuo Kato, Nyein Wink Lwin,and Kouhei Washiyama (D3) Muhammad Zamrun F (D2) Futoshi Minato(D1) Akihiro Suda and Yoshikazu Watanabe (M2) Naoki Takahashi and Myaing Thi Win (M1) |
|
We applied the quantum diffusion approach, which we have been
developing in the past few years,
to 48Ca+244Pu and 70Zn+208Pb
reactions to elucidate the quantum effects in the formation process of
a heavy compound nucleus in the region of super-heavy elements,
and found the followings: 1) quantum fluctuations noticeably affect the formation probability of a compound nucleus, 2) quantum fluctuations affect the mass distribution of quasi-fission fragments. |
| Based on the adiabatic picture for heavy-ion reactions, in which the neck formation in the one-body system is taken into account, we proposed a two-step model for fusion cross sections at deep sub-barrier energies. This model consists of the capture process in the two-body potential pocket, which is followed by the penetration of the adiabatic one-body potential to reach a compound state after the touching configuration. We described the former process with the coupled-channels framework, while the latter with the Wentzel-Kramers-Brillouin (WKB) approximation by taking into account the coordinate dependent inertia mass. The effect of the one-body barrier is important at incident energies below the potential energy at the touching configuration. We showed that this model well accounts for the steep fall-off phenomenon of fusion cross sections at deep sub-barrier energies for the 64Ni+64Ni and 58Ni+58Ni reactions. |
| We systematically evaluated the potential energy at the touching configuration for heavy-ion reactions using various potential models. We pointed out that the energy at the touching point, especially that estimated with the Krappe-Nix-Sierk (KNS) potential, strongly correlates with the threshold incident energy for steep falloff of fusion cross sections observed recently for several systems at extremely low energies. This clearly indicates that the steep fall-off phenomenon can be attributed to the dynamics after the target and projectile touch with each other, e.g., the tunneling process and the nuclear saturation property in the overlap region. |
| We inverted experimental data for heavy-ion fusion reactions at energies well below the Coulomb barrier in order to directly determine the internucleus potential between the colliding nuclei. In contrast to the previous applications of the inversion formula, we explicitly took into account the effect of channel couplings on fusion reactions, by assuming that fusion cross sections at deep sub-barrier energies are governed by the lowest barrier in the barrier distribution. We applied this procedure to the 16O+144Sm and 16O+208Pb reactions, and found that the inverted internucleus potential are much thicker than phenomenological potentials. A relation to the steep fall-off phenomenon of fusion cross sections recently found at deep sub-barrier energies was also discussed. |
| We carried out coupled reaction channels calculations for a precise fusion excitation function for the 12C+208Pb reaction at energies around the barrier. A bare potential previously claimed to uniquely describe a wide range of 12C+208Pb near-barrier reaction channels failed to reproduce the new fusion data. The nuclear potential diffuseness of 0.95 fm which fits the fusion excitation function over a broad energy range failed to reproduce the elastic scattering. A diffuseness of 0.55 fm reproduced the fusion barrier distribution and elastic scattering data, but significantly overpredicted the fusion cross sections at high energies. This may be due to physical processes not included in the calculations. |
| Elastic scattering of identical nuclei results in oscillatory Mott scattering. The separation of the peaks is perturbed by the presence of the nuclear potential. Coupled-channels calculations were performed for near-barrier measurements of Mott scattering of 58Ni from 58Ni to obtain information on the diffuseness of the nuclear potential, giving a diffuseness parameter of 0.62 ± 0.04 fm. |
| We analysed high precision quasi-elastic scattering excitation functions measured at energies well below the Coulomb barrier for the reactions of 32S with 208Pb, 197Au, 186W, and 170Er. Single-channel and coupled-channels calculations have been performed to extract the diffuseness parameter of the nuclear potential. For the reactions involving near-spherical targets, both theoretical analyses gave the same diffuseness parameter. On the other hand, for deformed systems, couplings are important even at deep sub-barrier energies. In general, the effect of couplings is to reduce the diffuseness parameter value extracted from a single-channel potential. Single-channel fits to quasi-elastic scattering data resulted in a=0.72-0.82 fm, whereas coupled-channels calculations gave diffuseness parameters in the range 0.58-0.75 fm. |
| We developped an approximate treatment of exchange kernels in the resonating group method (RGM) which works for heavy systems. That is, we treated the center of mass motion approximately and evaluated the normalization kernel using an idea similar to the so called Gaussian overlap approximation. The normalization kernel yields an explicit energy dependence in an effective inter-nucleus potential. Applying this method to subbarrier fusion of 16O + 208Pb and 32S + 208Pb systems, we showed that the energy dependence of the fusion excitation functions at energies above the Coulomb barrier can be well reproduced. |
| We performed coupled-channels analyses for the quasi-elastic barrier distribution for the 48Ti,54Cr,56Fe,64Ni, 70Zn + 208Pb reactions. We showed that the coupled-channels calculations which include the multi-phonon excitations in the colliding nuclei reproduce reasonably well the experimental excitation functions for quasi-elastic scattering at backward angles and the barrier distribution for these reactions. |
| We studied the low energy d+d reactions in matter from two aspects, i.e. (1) the effect of screening by conduction electrons and (2) the effect of quantum motion of the target deuteron. As for the former, we solved the Kohn-Sham equation to analyze the non-linear response of electrons to the reacting nuclei. Comparing the results with those of the linear response theory, we found the importance of non-linear effects in the electron screening in matter. However, the resultant screening effect explains neither the huge screening potentials reported experimentally for some host materials nor the strong host material dependence. As for the latter, we developed a coupled-channels framework in an Eikonal approximation to calculate the barrier penetrability by taking the quantum motion of the target deuteron embedded in a confinement potential at an interstitial site into account. We found that the effect of quantum motion is too small to explain the experimentally reported huge enhancement of the reaction rate in matter as long as the parameters of the confinement potential are assessed from the data of neutron scattering for studying the diffusion of hydrogens in matter. In order to see the circumstances under which the huge enhancement of the reaction rate could be attributed to the quantum motion of the target deuteron, we also examined the dependence of the effect of quantum motion on the properties of the confinement potential by varying the parameters of the potential from those provided by the neutron scattering. |
| We applied a three-body model consisting of two valence neutrons and the core nucleus 14C in order to investigate the ground state properties and electric quadrupole transition of the 16C nucleus. The discretized continuum spectrum within a large box is taken into account by using a single-particle basis obtained from a Woods-Saxon potential. The calculated B(E2) value from the first 2+ state to the ground state shows good agreement with the observed data with the core polarization charge which reproduces the experimental B(E2) value for 15C. We also show that the present calculation accounts well for the longitudinal momentum distribution of the 15C fragment from the breakup of the 16C nucleus. We point out that the dominant (d5/2)2 configuration in the ground state of 16C plays a crucial role in these agreements. |
| We investigated the spatial structure of the two-neutron wave function in the Borromean nucleus 11Li, using a three-body model of 9Li+n+n, which includes many-body correlations stemming from the Pauli principle. The behavior of the neutron pair at different densities was simulated by calculating the two-neutron wave function at several distances between the core nucleus 9Li and the center of mass of the two neutrons. With this representation, a strong concentration of the neutron pair on the nuclear surface was for the first time quantitatively established for neutron-rich nuclei. That is, the neutron pair wave function in 11Li has an oscillatory behavior at normal density, while it becomes a well-localized single peak in the dilute density region around the nuclear surface. We pointed out that these features qualitatively correspond to the BCS- and BEC-like structures of the pair wave function found in infinite nuclear matter. |
| We investigated the consistency of the measured charge radius and dipole response of 11Li within a three-body model. We showed how these observables are related to the mean-square distance between the 9Li core and the center of mass of the two valence neutrons. In this representation we found by considering the effect of smaller corrections that the discrepancy between the results of the two measurements is of the order of 1.5. We also investigated the sensitivity to the three-body structure of 11Li and found that the charge radius measurement favors a model with a 50% s-wave component in the ground state of the two-neutron halo, whereas the dipole response is consistent with a smaller s-wave component of about 25% value. |
| We performed constrained Hartree-Fock calculations introducing a double constraint both on the proton and the neutron quadrupole moments. We have systematically applied the double constraint method to neutron-rich carbon isotopes. We employed both the relativistic mean field (RMF) model and the non-relativistic Skyrme Hartree Fock (SHF) method. For neutron-rich isotopes, 16,18,20C, our calculation showed that the valley in the two-dimensional energy surface runs through a line which deviates from the diagonal β p=βn line. For the 16C nucleus, we investigated two cofigurations. One is the configuration in which the neutron and proton symmetry axes are parallel to each other, and the other perpendicular to each other. The latter configuration had been claimed to be favoured in the antisymmetrized molecular dynamics (AMD) calculations. In contrast, we found that the former configuration is energetically favored as compared to the latter configuration, at least in the level of mean-field approximation. This conclusion was found to be independent of the paring correlation, the nucleon-nucleon effective force used in the calculations, and the strength of spin-orbit interaction. |
| We carried out calculations for neutrino-nucleus reactions under the supernova environment based on the finite temperature random phase approximation. At finite tempratures, the Fermi surface is distorted. This makes some transitions allowed, which are forbidden at zero temperature. Also, the system undergoes the phase transition from super fluid to normal fluid phases as the temperature increases. We found that, in the absence of pairing correlation, the temperature effect is significant especially for N=Z nuclei. For instance, the cross sections for the 16O is enhanced by a factor of two at T=0.5 MeV, and a factor of 100 at T=1 MeV. On the other hand, in the presence of pairing, we found that cross sections are hindered at temperatures above the critical temperature for the phase transition. In addition to the tempareture effects, we also investigated the effect of tensor interaction, to which single particle energies are sensitive. We found that the tensor interaction slightly quenches the temperature effect on neutrino-nucleus reactions. |
| We investigated the effect of deformation on direct radiative capture reactions of neutron-rich nuclei. To this end, we developped a schematic model using a deformed square well potential. We solved the coupled-channels equations to obtain scattering as well as bound state wave functions of a neutron. We applied this method to the neutron capture of 142Ru and 230Hg nuclei, which lie close to the r-process path. We showed that the deformation effect changes the position and the width of multi-channel resonance states, leading to a large modification in capture cross sections. |
| An important question whether equilibrium is reached in dynamical multifragmentation reactions was studied by utilizing the antisymmetrized molecular dynamics (AMD) model which can describe both dynamical reactions and systems in thermal equilibrium. The reaction ensemble at a given time t is constructed by collecting states from many events of the 40Ca+40Ca central collisions at 35 MeV/nucleon simulated by AMD, while the microcanonical equilibrium ensemble for a given volume and energy was obtained by the long-time AMD simulation of nucleons in a container. By comparing the reaction and equilibrium ensembles for fragment observables (i.e., the charges and excitation energies of fragments), it was found that there exists an equilibrium ensemble which well reproduces the reaction ensemble at each time from t=80 fm/c to the investigated maximum time (300 fm/c). This result suggests that equilibrium is already reached in the late stage of collisions for the investigated fragment observables, supporting the idea to extract equilibrium properties (such as liquid-gas phase transition) from multifragmentation reactions. |
| Cluster correlations may be important in excited systems which undergo multifragmentation. To study this possibility, we explicitly introduced cluster correlations into the time evolution of the antisymmetrized molecular dynamics by considering the cluster formation in the final state of each two-nucleon collision. A kind of impulse approximation was employed to estimate the cluster formation probabilities. The formed clusters are propagated and can be broken later by the collision with other nucleons or clusters. The calculated results show that the cluster formation process has a large effect on the relative yield of light particles (A=2 - 4) to isolated nucleons. Furthermore, the IMF production is also affected by the cluster correlations. Consequently, the issue of the consistency of the nucleon multiplicity and the transverse kinetic energies of IMFs is affected by the cluster correlations. |
| Multifragmentation scenarios, as predicted by antisymmetrized molecular dynamics (AMD) or momentum-dependent stochastic mean-field (BGBD) calculations were compared. Whereas in the BGBD case fragment emission is clearly linked to the spinodal decomposition mechanism (i.e., to mean-field instabilities), in AMD many-body correlations have a stronger impact on the fragmentation dynamics. In fact, the density and momentum fluctuations develop earlier in AMD, suggesting that fragments are formed on shorter time scales in AMD, on about equal footing as light-particle pre-equilibrium emission. |
| The AMD simulations were performed to study the production mechanism of rare isotopes from the projectile fragmentation of Ca and Ni isotopes at 140 MeV/nucleon. By taking account of the statistical decays of excited primary fragments, the experimental data of the final yields were reproduced reasonably well. However, the experimental overall cross section is overestimated by AMD, which may be because the two-nucleon collision cross sections should be reduced in nuclear medium. The shift of the projectilelike fragment volocity from the beam velocity was found to be too large in the AMD calculation. |
| Experimental analyses of moderate-temperature nuclear gases produced in the violent collisions of 35 MeV/nucleon 64Zn projectiles with 92Mo and 197Au target nuclei reveal a large degree of α particle clustering at low densities. For these gases, temperature- and density-dependent symmetry energy coefficients were derived from isoscaling analyses of the yields of nuclei with A≤4. At densities of 0.01 to 0.05 times the ground-state density of symmetric nuclear matter, the temperature- and density-dependent symmetry energies range from 9.03 to 13.6 MeV. This is much larger than those obtained in mean-field calculations and reflects the clusterization of low-density nuclear matter. The results are in quite reasonable agreement with calculated values obtained with a recently proposed virial equation of state calculation. |
| Noncentral collisions of 114Cd projectiles with 92Mo target nuclei at E/A=50 MeV were explored with an antisymmetrized molecular dynamics model. These collisions were found to be essentially binary in character with formation of an excited projectile-like fragment (PLF*) and targetlike fragment (TLF*). The average excitation energy deduced for the PLF* and TLF* saturates for midcentral collisions, 3.5≤ b≤ 6 fm, with its magnitude depending on the cluster recognition time. For short cluster recognition times (t=150 fm/c), an average excitation energy as high as ≈ 6 MeV was determined, indicating a short statistical lifetime for the fragments produced. Evidence for such a rapid deexcitation is observed in the present calculations. |