arXiv subjects

K. Hagino

Publications and source records attributed to K. Hagino.

At least 127 records · Page 7Linked to original sources

arXiv2013

Non-collective excitations in low-energy heavy-ion reactions: applicability of the random-matrix model

We investigate the applicability of a random-matrix model to the description of non-collective excitations in heavy-ion reactions around the Coulomb barrier. To this end, we study fusion in the reaction $^{16}$O + $^{208}$Pb, taking account of the known non-collective excitations in the $^{208}$Pb nucleus. We show that the random-matrix model for the corresponding couplings reproduces reasonably well the exact calculations, obtained using empirical deformation parameters. This implies that the model may provide a powerful method for systems in which the non-collective couplings are not so well known.

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arXiv2013

Correlated two-neutron emission in the decay of unbound nucleus 26O

The particle unbound $^{26}$O nucleus is located outside the neutron drip line, and spontaneously decays by emitting two neutrons with a relatively long life time due to the centrifugal barrier. We study the decay of this nucleus with a three-body model assuming an inert $^{24}$O core and two valence neutrons. We first point out the importance of the neutron-neutron final state interaction in the observed decay energy spectrum. We also show that the energy and and angular distributions for the two emitted neutrons manifest a clear evidence for the strong neutron-neutron correlation in the three-body resonance state. In particular, we find an enhancement of two-neutron emission in back-to-back directions. This is interpreted as a consequence of {\it dineutron correlation}, with which the two neutrons are spatially localized before the emission.

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arXiv2013

Sum rule approach to a soft dipole mode in $Λ$ hypernuclei

Applying the sum rule approach, we investigate the energy of a soft dipole motion in $Λ$ hypernuclei, which results from a dipole oscillation of a $Λ$ hyperon against the core nucleus. To this end, we systematically study single-$Λ$ hypernuclei, from $^{16}_{\;\,Λ}$O to $^{208}_{\;\;\;Λ}$Pb, for which the ground state wave function is obtained in the framework of Hartree-Fock method with several Skyrme-type $ΛN$ interactions. Our results indicate that the excitation energy of the soft dipole $Λ$ mode, $E_{sdΛ}$, decreases as the mass number increases. We find that the excitation energy is well parametrized as $E_{sdΛ}=26.6A^{-1/3}+11.2A^{-2/3}$ MeV as a function of mass number $A$.

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arXiv2013

Application of the inverse Hamiltonian method to Hartree-Fock-Bogoliubov calculations

We solve the Hartree-Fock-Bogoliubov (HFB) equations for a spherical mean field and a pairing potential with the inverse Hamiltonian method, which we have developed for the solution of the Dirac equation. This method is based on the variational principle for the inverse Hamiltonian, and is applicable to Hamiltonians that are bound neither from above nor below. We demonstrate that the method works well not only for the Dirac but also for the HFB equations.

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arXiv2012

Competition between $T$=1 and $T$=0 pairing in $pf$-shell nuclei with $N=Z$

The pairing correlation energy for two-nucleon configurations with the spin-parity and isospin of $J^π=0^+$, $T$=1 and $J^π=1^+$, $T$=0 are calculated with $T$=1 and $T$=0 pairing interactions, respectively. To this end, we consider the $(1f2p)$ shell model space, including single-particle angular momenta of $l=3$ and $l=1$. It is pointed out that a two-body matrix element of the spin-triplet $T$=0 pairing is weakened substantially for the $1f$ orbits, even though the pairing strength is much larger than that for the spin-singlet $T$=1 pairing interaction. In contrast, the spin-triplet pairing correlations overcome the spin-singlet pairing correlations for the $2p$ configuration, for which the spin-orbit splitting is smaller than that for the $1f$ configurations, if the strength for the T=0 pairing is larger than that for the T=1 pairing by 50% or more. Using the Hartree-Fock wave functions, it is also pointed out that the mismatch of proton and neutron radial wave functions is at most a few % level, even if the Fermi energies are largely different in the proton and neutron mean-field potentials. These results imply that the configuration with $J^π=0^+$, $T$=1 is likely in the ground state of odd-odd $pf$ shell nuclei even under the influence of the strong spin-triplet $T$=0 pairing, except at the middle of the $pf$ shell, in which the odd proton and neutron may occupy the $2p$ orbits. These results are consistent with the observed spin-parity $J^π=0^+$ for all odd-odd $pf$ shell nuclei except for $^{58}_{29}$Cu, which has $J^π=1^+$.

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arXiv2012

Collective excitations of $Λ$ hypernuclei

We discuss low-lying collective excitations of $Λ$ hypernuclei using the self-consistent mean-field approaches. We first discuss the deformation properties of $Λ$ hypernuclei in the $sd$-shell region. Based on the relativistic mean-field (RMF) approach, we show that the oblate deformation for $^{28}$Si nucleus may disappear when a $Λ$ particle is added to this nucleus. We then discuss the rotational excitations of $^{25}_Λ$Mg nucleus using the three-dimensional potential energy surface in the deformation plane obtained with the Skyrme-Hartree-Fock method. The deformation of $^{25}_Λ$Mg nucleus is predicted to be slightly reduced due to an addition of $Λ$ particle. We demonstrate that this leads to a reduction of electromagnetic transition probability, $B(E2)$, in the ground state rotational band. We also present an application of random phase approximation (RPA) to hypernuclei, and show that a new dipole mode, which we call a soft dipole $Λ$ mode, appears in hypernuclei, which can be interpreted as an oscillation of $Λ$ particle against the core nucleus.

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arXiv2012

Role of T=0 pairing in Gamow-Teller states in N=Z nuclei

Gamow-Teller (GT) states in N=Z nuclei with the mass number A from 48 to 64 are studied by using Hartree-Fock-Bogoliubov + quasi-particle random phase approximation (HFB+QRPA) with Skyrme interactions. The isoscalar spin-triplet (T=0,S=1) pairing interaction is taken into account in QRPA calculations. It is found in the context of SU(4) symmetry in the spin-isospin space that the GT strength of lower energy excitations is largely enhanced by the T=0 pairing interaction which works cooperatively with the T=1 pairing interaction in the ground state. A two-peaked structure observed recently in $(p,n)$ reaction on $^{56}$Ni can be considered as a manifestation of the role of T=0 pairing in the GT collective states.

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arXiv2012

Pairing correlations in exotic nuclei

The BCS and HFB theories which can accommodate the pairing correlations in the ground states of atomic nuclei are presented. As an application of the pairing theories, we investigate the spatial extension of weakly bound Ne and C isotopes by taking into account the pairing correlation with the Hartree-Fock-Bogoliubov (HFB) method and a 3-body model, respectively. We show that the odd-even staggering in the reaction cross sections of $^{30,31,32}$Ne and $^{14,15,16}$C are successfully reproduced, and thus the staggering can be attributed to the unique role of pairing correlations in nuclei far from the stability line. A correlation between a one-neutron separation energy and the anti-halo effect is demonstrated for $s$- and p-waves using the HFB wave functions. We also propose effective density-dependent pairing interactions which reproduce both the neutron-neutron ($nn$) scattering length at zero density and the neutron pairing gap in uniform matter. Then, we apply these interactions to study pairing gaps in semi-magic finite nuclei, such as Ca, Ni, Sn and Pb isotopic chains.

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arXiv2012

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 a 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 a general problem on quantum tunneling in the presence of couplings, which has been a popular subject in the past decades in many branches of physics and chemistry. Here we review theoretical aspects of heavy-ion subbarrier fusion reactions from the view point 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 multi-channel penetrability. We also discuss an application of the barrier distribution method to elucidation of the mechanism of dissociative adsorption of H$_2$ melecules in surface science.

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arXiv2012

Reaction cross sections of the deformed halo nucleus 31Ne

Using the Glauber theory, we calculate reaction cross sections for the deformed halo nucleus $^{31}$Ne. To this end, we assume that the $^{31}$Ne nucleus takes the $^{30}$Ne + $n$ structure. In order to take into account the rotational excitation of the core nucleus $^{30}$Ne, we employ the particle-rotor model (PRM). We compare the results to those in the adiabatic limit of PRM, that is, the Nilsson model, and show that the Nilsson model works reasonably well for the reaction cross sections of $^{31}$Ne. We also investigate the dependence of the reaction cross sections on the ground state properties of $^{31}$Ne, such as the deformation parameter and the p-wave component in the ground state wave function.

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arXiv2012

Impurity effect of Lambda particle on the structure of 18F and 19F_Lambda

We perform three-body model calculations for a $sd$-shell hypernucleus $^{19}_Λ$F ($^{17}_Λ{\rm O}+p+n$) and its core nucleus $^{18}$F ($^{16}{\rm O}+p+n$), employing a density-dependent contact interaction between the valence proton and neutron. We find that the $B(E2)$ value from the first excited state (with spin and parity of $I^π=3^+$) to the ground state ($I^π=1^+$) is slightly decreased by the addition of a $Λ$ particle, which exhibits the so called shrinkage effect of $Λ$ particle. We also show that the excitation energy of the $3^+$ state is reduced in $^{19}_Λ$F compared to $^{18}$F, as is observed in a $p$-shell nucleus $^{6}$Li. We discuss the mechanism of this reduction of the excitation energy, pointing out that it is caused by a different mechanism from that in $^{7}_Λ$Li.

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arXiv2012

Exotic nuclei far from the stability line

The recent availability of radioactive beams has opened up a new era in nuclear physics. The interactions and structure of exotic nuclei close to the drip lines have been studied extensively world wide, and it has been revealed that unstable nuclei, having weakly bound nucleons, exhibit characteristic features such as a halo structure and a soft dipole excitation. We here review the developments of the physics of unstable nuclei in the past few decades. The topics discussed in this Chapter include the halo and skin structures, the Coulomb breakup, the dineutron correlation, the pair transfer reactions, the two-nucleon radioactivity, the appearance of new magic numbers, and the pygmy dipole resonances.

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arXiv2012

Transition from subbarrier to deep subbarrier regimes in heavy-ion fusion reactions

We analyze the recent experimental data of heavy-ion fusion cross sections available up to deep subbarrier energies in order to discuss the threshold incident energy for a deep subbarrier fusion hindrance phenomenon. To this end, we employ a one-dimensional potential model with a Woods-Saxon internuclear potential. Fitting the experimental data in two different energy regions with different Woods-Saxon potentials, we define the threshold energy as an intersect of the two fusion excitation functions. We show that the threshold energies so extracted are in good agreement with the empirical systematics as well as with the values of the Krappe-Nix-Sierk (KNS) potential at the touching point. We also discuss the asymptotic energy shift of fusion cross sections with respect to the potential model calculations, and show that it decreases with decreasing energies in the deep subbarrier region although it takes a constant value at subbarrier energies.

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arXiv2012

Role of non-collective excitations in heavy-ion fusion reactions and quasi-elastic scattering around the Coulomb barrier

Despite the supposed simplicity of double-closed shell nuclei, conventional coupled-channels calculations, that include all of the known collective states of the target and projectile, give a poor fit to the fusion cross section for the $^{16}$O + $^{208}$Pb system. The discrepancies are highlighted through the experimental barrier distribution and logarithmic derivative, that are both well defined by the precise experimental fusion data available. In order to broaden our search for possible causes for this anomaly, we revisit this system and include in our calculations a large number of non-collective states of the target, whose spin, parity, excitation energy and deformation paramter are known from high-precision proton inelastic-scattering measurements. Although the new coupled-channels calculations modify the barrier distribution, the disagreemnt with experiment remains both for fusion and for quasi-elastic (QE) scattering. We find that the Q-value distributions for large-angle QE scattering become rapidly more important as the incident energy increases, reflecting the trend of the experimental data. The mass-number dependence of the non-collective excitations is discussed.

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arXiv2012

Dynamical approach to heavy-ion induced fission using actinide target nuclei at energies around the Coulomb barrier

In order to describe heavy-ion fusion reactions around the Coulomb barrier with an actinide target nucleus, we propose a model which combines the coupled-channels approach and a fluctuation-dissipation model for dynamical calculations. This model takes into account couplings to the collective states of the interacting nuclei in the penetration of the Coulomb barrier and the subsequent dynamical evolution of a nuclear shape from the contact configuration. In the fluctuation-dissipation model with a Langevin equation, the effect of nuclear orientation at the initial impact on the prolately deformed target nucleus is considered. Fusion-fission, quasi-fission and deep quasi-fission are separated as different Langevin trajectories on the potential energy surface. Using this model, we analyze the experimental data for the mass distribution of fission fragments (MDFF) in the reactions of $^{34,36}$S+$^{238}$U and $^{30}$Si+$^{238}$U at several incident energies around the Coulomb barrier. We find that the time scale in the quasi-fission as well as the deformation of fission fragments at the scission point are different between the $^{30}$Si+$^{238}$U and $^{36}$S+$^{238}$U systems, causing different mass asymmetries of the quasi-fission.

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arXiv2012

Odd-even staggering of reaction cross sections for 22,23,24O isotopes

The interaction cross sections of $^{22,23}$O nuclei at 900 MeV/nucleon have been measured recently by Kanungo {\it et al.}. We show that the odd-even staggering parameter of interaction cross sections deduced from these new data agrees well with the theoretical systematics expected for the neutron separation energy of $S_n$=2.74$\pm$0.120 MeV for $^{23}$O. We also discuss briefly the energy dependence of the staggering parameter.

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arXiv2011

Application of random phase approximation to vibrational excitations of double-$Λ$ hypernuclei

Using the Hartree-Fock plus random-phase-approximation (HF+RPA), we study the impurity effect of $Λ$ hyperon on the collective vibrational excitations of double-$Λ$ hypernuclei. To this end, we employ a Skyrme-type $ΛN$ and $ΛΛ$ interactions for the HF calculations, and the residual interactions for RPA derived with the same interactions. We find that inclusion of two $Λ$ hyperons in $^{16}$O shifts the energy of the collective states towards higher energies. In particular, the energy of the giant monopole resonance of $^{\,\,18}_{ΛΛ}$O, as well as that of $^{210}_{ΛΛ}$Pb, becomes larger. This implies that the effective incompressibility modulus increases due to the impurity effect of $Λ$ particle, if the $β$-stability condition is not imposed.

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arXiv2011

Pairing correlations and odd-even staggering in reaction cross sections of weakly bound nuclei

We investigate the odd-even staggering (OES) in reaction cross sections of weakly bound nuclei with a Glauber theory, taking into account the pairing correlation with the Hartree-Fock-Bogoliubov (HFB) method. We first discuss the pairing gap in extremely weakly bound nuclei and show that the pairing gap persists in the limit of zero separation energy limit even for single-particle orbits with the orbital angular momenta $l=0$ and $l=1$. We then introduce the OES parameter defined as the second derivative of reaction cross sections with respect to the mass number, and clarify the relation between the magnitude of OES and the neutron separation energy. We find that the OES parameter increases considerably in the zero separation energy limit for $l=0$ and $l=1$ single-particle states, while no increase is found for higher angular momentum orbits with {\it e.g.,} $l=3$. We point out that the increase of OES parameter is also seen in the experimental reaction cross sections for Ne isotopes, which is well accounted for by our calculations.

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