Session EF: Heavy Ion Reactions

Important role of medium effect in high-density region on nucleus-nucleus elastic scattering

We investigate the sensitivity of the medium effect in the high density region on the nucleus-nucleus elastic scattering in the framework of the double-folding (DF) model with the complex $G$-matrix interaction. First, the evaluating position of the local density, which is an ambiguity of the DF model, is investigated. However, the effect has a minor role to the $^{16}$O~+~$^{16}$O system. Next, the medium effect including three-body-force (TBF) effect is investigated with two methods. In the both methods, the important role of the medium effect is clearly seen on the potential and the elastic cross section, but not on the total reaction cross section. Finally, we make clear the crucial role of the TBF effect up to $k_F =$ 1.6 fm$^{-1}$ in the nucleus-nucleus elastic scattering.   

A new mechanism to explore isospin transport and constrain the symmetry energy

Heavy-ion collisions provide a unique probe of the isospin dependence of the nuclear Equation of State. Measurements of the isospin transport between the projectile and target in a heavy-ion collision has been intensively used to constrain the density dependence of the symmetry energy. These experiments require measurements of multiple systems and the isospin transport phenomenon occurs over a relatively short time period ($\sim$100-150 fm/c). Recently, the Indiana University reactions group studied the production and decay of dinuclear systems from mid-peripheral heavy-ion collisions. The IU group experimentally measured the two fragments from the decay of the dinuclear system and the results suggested that isospin transport was occurring over a long time-scale (600-900 fm/c) between the fragments. In this work, the experimental results are confronted with theoretical calculations using the Constrained Molecular Dynamics (CoMD) model to examine their sensitivity to the symmetry energy. The results show that the dynamics of the reaction are well reproduced by CoMD and that measurements of the N/Z of the fragments from the decay of the dinculear system are sensitive to the symmetry energy.   

Production of Spin Polarized Li Isotope Beam through Heavy Ion reactions

Spin polarized radioactive nuclear beams are useful for materials science as beta-NMR probes which exhibit extremely high sensitivity to the NMR detection. So far, short-lived beta-NMR probe nucleus $^{8}$Li ($I=$ 2$^{+}$, $T_{1/2}=$ 838 ms) has been used strongly for the materials science. In the present study, we have performed the production test of polarized $^{8}$Li and another candidate of $^{9}$Li ($I=$ 3/2$^{-}$, $T_{1/2}=$ 178 ms) through the heavy ion reactions at HIMAC in NIRS. Polarized $^{8,9}$Li nuclei were produced by using $^{10,11}$B or $^{12}$C primary beams at an energy of 70 $A $MeV with a Be target. Polarization was observed for $^{8}$Li at high momentum side with other than a $^{10}$B beam. The result of $^{9}$Li suggests the possibility that large polarization is generated with a $^{10}$B beam which picks up a neutron in the reaction process.   

Alpha gas state in 36Ar

The $\alpha$ cluster structures in light nuclei with N=Z are expected to appear abov the threshold energy of breakup into $\alpha$ particles. After the proposal of an $\alpha$ cluster wave function with $\alpha$ particle condensate type, such condensate states are both theoretically and experimentally discussed extensively. Theoretically, the existence of dilute $\alpha$ cluster state in nuclei with mass region of A$>$16, experimentally, is not confirmed for N-$\alpha$ cluster states in nuclei heavier than A=16. Recently, we measured $\alpha$ inelastic scattering of $^{36}$Ar followed by $\alpha$ decay in an inverse kinematics setup. A 50 MeV/u $^{36}$Ar beam from RCNP ring cyclotron was used to bombard a $^{4}$He gas target. $\alpha$ particles were detected in the magnetic spectrometer LAS which was set at 0 degrees. $\alpha$   

Nuclear proximity potentials studied by deflection effect in peripheral reactions at $E = $ 290 MeV/u

The analysis of angular distributions observed in elastic scattering reaction has provided attractive nuclear potentials between projectile and target nuclei at $E=$100 MeV/u or lower. Recent theoretical studies have predicted that the real part of the heavy-ion potentials changes from attractive to repulsive character around the incident energy $E=$200 - 300 MeV/u, however few experimental studies have been performed to prove the transition. To investigate behaviors of heavy-ion potentials at this energy region, angular distributions of nuclei, produced through exchange or 1-nucleon transfer reactions of 12C, 40Ar and 84Kr beam with C, Al, Nb, Tb, and Au targets at 290 MeV/u, were observed. An orbital deflection effect was observed in the angular distributions and has shown remarkable target dependence. The orbital deflection will be analyzed based on the combined contribution of Coulomb and nuclear proximity potentials.   

Experimental study of capture barrier distributions for $^{58}$Ni+$^{60}$Ni

Current coupled channels (CC) models treat fusion as a coherent quantum-mechanical process, in which coupling between the collective states of the colliding nuclei influences fusion probability in near-barrier reactions. While CC models have been used to successfully describe many experimental barrier distribution (BD) measurements, the CC approach has failed in several key cases. The reason for these failures is poorly understood; however, it has been postulated that dissipative processes may play a role. Traditional fusion barrier distribution experiments can only probe the physics of fusion for collisions at the top of the Coulomb barrier ($L=0 \hbar$). In this work, we will present a novel method of probing dissipative processes within the Coulomb barrier by exploiting the predicted sharp onset of fission at $L\sim 60\hbar$ for reactions forming compound nuclei with $A<160$. Using the ANU's 14UD tandem accelerator and CUBE spectrometer, fission outcomes have been measured for the $^{58}$Ni+$^{60}$Ni reaction at a wide range of energies, in order to measure a fission BD. First results will be presented in the context of complementary fusion BD measurements.   

Strong orientation dependence of multinucleon transfer processes in $^{238}$U$+^{124}$Sn reaction

We investigate multinucleon transfer (MNT) processes induced by a prolately deformed $^{238}$U nuclei. Experimentally, substantial cross sections of MNT processes accompanying more than ten protons have been measured in $^{238}$U$+^{124}$Sn reaction at energies around the Coulomb barrier. We have investigated this reaction employing a microscopic framework of the time-dependent Hartree-Fock (TDHF) theory. From the calculation, we have found that the amount of transferred nucleons depends much on the relative orientation between the deformed axis of $^{238}$U and the relative vector connecting centers of $^{238}$U and $^{124}$Sn nuclei. When the $^{238}$U collides from its tip with $^{124}$Sn, a formation of thick neck is observed. However, when $^{238}$U collides from its side, the neck formation is substantially suppressed. We have found that a large amount of protons is transferred in the tip collision. This is caused by the breaking and absorption of the neck which is composed of both protons and neutrons. In the side collision, we have not observed large probability of MNT due to the suppressed neck formation. We thus consider that the measured MNT processes involving about a tens of protons originate from the neck breaking dynamics following the tip collision of deformed $^{238}$U nucleus.   

Fusion cross sections measurements with MUSIC

The interaction between exotic nuclei plays an important role for understanding the reaction mechanism of the fusion processes as well as for the energy production in stars. With the advent of radioactive beams new frontiers for fusion reaction studies have become accessible. We have performed the first measurements of the total fusion cross sections in the systems $^{10,14,15}$C+$^{12}$C using a newly developed active target-detector system (MUSIC). Comparison of the obtained cross sections with theoretical predictions show a good agreement in the energy region accessible with existing radioactive beams. This type of comparison allows us to calibrate the calculations for cases that cannot be studied in the laboratory with the current experimental capabilities. The high efficiency of this active detector system will allow future measurements with even more neutron-rich isotopes.   

Induced fission studies of Z\textless 82 at the JAEA tandem facility

Recent beta-delayed fission studies of proton rich $^{180}$Hg showed the fission to be dominated by asymmetric fragment masses, in contrast to initial predictions. Models of the potential-energy surface for the path toward scission showed a lower fission barrier for an asymmetrically deformed $^{180}$Hg volume. As a probe of this theoretical interpretation, fission of $^{180}$Hg was induced through the symmetric reaction $^{90}$Z$+^{90}$Z. Mass and energy distributions of fragments for a series of bombarding energies were recorded using 2 MWPCs. As a continuation of Z $<82$ fission studies, fission induced in the $^{7}$Li$+^{182,184,186}$W reactions were measured. Near A$\sim$190 a new region of asymmetric fission is predicted to emerge as one moves towards neutron rich nuclei. In preliminary investigations mass asymmetry was observed in fragments of the $^{7}$Li$+^{186}$W reaction. However, fragment kinematics were inconsistent with fission of $^{193}$Ir formed by fusion. A follow up experiment was conducted in an attempt to confirm the first observation of fission induced by the $^{7}$Li breakup-fusion reaction $^{186}$W$(^{7}$Li,$^{4}$He$)^{189}$Re. Experimental details and initial results will be presented.   

Fission of actinide nuclei using multi-nucleon transfer reactions

We are promoting a campaign to measure fission-fragment mass distributions for neutron-rich actinide nuclei populated by transfer reactions from their ground state up to an excitation energy of several tens MeV. We thus obtain the excitation energy dependence of the mass distribution. The experiment was carried out at the 20 MV JAEA tandem facility at Tokai. We report on the data obtained in the direct reaction $^{18}O+^{232}Th$. Transfer-channels and excitation energies of the fissioning nuclei were identified using silicon dE-E detectors located at forward angle. Two fission fragments were detected in coincidence using multi-wire proportional counters. Fission fragment masses were determined by kinematic consideration. We obtained the fission fragment mass distributions for 13 nuclei from actinium to uranium and some fission barrier heights.   

Exotic radioactive beams production using fusion-fission reactions

Fusion-Fission reaction products produced by a 238U beam at 24 MeV/u on Be and C targets were measured in inverse kinematics by use of the LISE fragment separator. The identification of fragments was done using the dE-TKE-Brho-ToF method. Germanium gamma-detectors were placed in the focal plane near the Si stopping telescope to provide an independent verification of the isotope identification via isomer tagging. The experiment demonstrated excellent resolution, in Z, A, and q. The results demonstrate that a fragment separator can be used to produce radioactive beams using fusion-fission reactions in inverse kinematics, and further that in-flight fusion-fission can become a useful production method to identify new neutron-rich isotopes, investigate their properties and study production mechanisms. Mass, atomic number and charge-state distributions are reported for the two reactions. The comparison of the experimental atomic-number and mass distributions combined with the analysis of the isotopic-distributions properties show that between the 9Be and the 12C target, the reaction mechanism changes substantially, evolving from a complete fusion-fission reaction to incomplete fusion or fast fission.   

Directional Correlation of Nuclear-Collision Probability for Aligned Beams of Deformed Nuclei

In the long history of nuclear physics, a lot of observables have been discussed in connection with the nuclear deformation. In this work, an investigation have been carried out on the directional correlation of the nuclear-collision probability between the direction of beam of deformed nuclei and the deformation axis, to sense the nuclear deformation directly. Few experimental studies of this kind have been done because of the difficulties in producing aligned beams. We utilized the projectile fragmentation to produce the aligned nuclear beams. We tested these measurements for $^{9}$Be, $^{10}$B, and $^{26}$Al at the HIMAC facility. By selecting the parallel momentum using the separator, spin aligned beams were produced, with which the interaction cross sections were measured. The cross sections were precisely measured as a function of longitudinal momentum. We will report on the details of measurements and discussions on this intriguing result.