arXiv subjects

K. Hagino

Publications and source records attributed to K. Hagino.

At least 199 records · Page 11Linked to original sources

arXiv2003

Fusion dynamics around the Coulomb barrier

We perform exact coupled-channels calculations, taking into account properly the effects of Coulomb coupling and the finite excitation energy of collective excitations in the colliding nuclei, for three Fm formation reactions, $^{37}$Cl + $^{209}$Bi, $^{45}$Sc + $^{197}$Au, and $^{59}$Co + $^{181}$Ta. For the $^{37}$Cl + $^{209}$Bi and $^{45}$Sc + $^{197}$Au reactions, those calculations well reproduce the experimental total fission cross sections, and a part of the extra-push phenomena can be explained in terms of the Coulomb excitations to multi-phonon states. On the other hand, for the heaviest system, the deep-inelastic collisions become much more significant, and the fission cross sections are strongly overestimated. We also discuss the surprisingly large surface diffuseness parameters required to fit recent high-precision fusion data for medium-heavy systems, in connection with the fusion supression observed in massive systems.

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arXiv2003

Continuum QRPA response for deformed neutron-rich nuclei

We discuss properties of the quadrupole collective excitation of the deformed neutron-rich nucleus $^{38}$Mg within the framework of quasi-particle random phase approximation (QRPA). We first solve the coupled-channels equations to obtain the single-particle levels, and construct the ground state by treating the pairing correlations in the BCS approximation. We then solve the QRPA equation using the response function formalism, by including the continuum spectra with the box dicscretization method. We show that the collectivity of the gamma vibration (the lowest $K^π=2^+$ mode) is significantly enhanced if protons and neutrons have different deformations. We also discuss an attempt towards full continuum QRPA calculations for deformed nuclei.

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arXiv2003

Role of virtual break-up of projectile in astrophysical fusion reactions

We study the effect of virtual Coulomb break-up, commonly known as the dipole polarizability, of the deuteron projectile on the astrophysical fusion reaction 3He(d,p)4He. We use the adiabatic approximation to estimate the potential shift due to the E1 transition to the continuum states in the deuteron, and compute the barrier penetrability in the WKB approximation. We find that the enhancement of the penetrability due to the deuteron break-up is too small to resolve the longstanding puzzle observed in laboratory measurements that the electron screening effect is surprisingly larger than theoretical prediction based on an atomic physics model. The effect of the 3He break-up in the 3He(d,p)4He reaction, as well as the 7Li break-up in the 7Li(p,alpha)4He reaction is also discussed.

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arXiv2003

Structure of positive energy states in a deformed mean-field potential

We investigate the properties of single-particle resonances in a non-spherical potential by solving the coupled-channels equations for the radial wave functions. We first generalize the box discretization method for positive energy states to a deformed system. As in the spherical case, we find that the discretized energy is stabilized against the box size when a resonance condition is met. Using the wave functions thus obtained, we then discuss the energy and the radial dependences of scattering wave functions in the vicinity of an isolated resonance. In the eigenchannel basis, where the $S$-matrix is diagonal, we propose a generalized expression for the factorization formula for the multi-channel wave function. We find that the factorized wave function agrees well with the exact solution inside the centrifugal barrier when the energy distance from the resonance is less than the resonance width.

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arXiv2003

Variational RPA for the Mie resonance in jellium

The surface plasmon in simple metal clusters is red-shifted from the Mie frequency, the energy shift being significantly larger than the usual spill-out correction. Here we develop a variational approach to the RPA collective excitations. Using a simple trial form, we obtain analytic expressions for the energy shift beyond the spill-out contribution. We find that the additional red shift is proportional to the spill-out correction and can have the same order of magnitude.

physics.atm-clus↗

arXiv2003

Generator Coordinate Truncations

We investigate the accuracy of several schemes to calculate ground-state correlation energies using the generator coordinate technique. Our test-bed for the study is the $sd$ interacting boson model, equivalent to a 6-level Lipkin-type model. We find that the simplified projection of a triaxial generator coordinate state using the $S_3$ subgroup of the rotation group is not very accurate in the parameter space of the Hamiltonian of interest. On the other hand, a full rotational projection of an axial generator coordinate state gives remarkable accuracy. We also discuss the validity of the simplified treatment using the extended Gaussian overlap approximation (top-GOA), and show that it works reasonably well when the number of boson is four or larger.

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arXiv2003

Fusion cross sections at deep subbarrier energies

A recent publication reports that heavy-ion fusion cross sections at extreme subbarrier energies show a continuous change of their logarithmic slope with decreasing energy, resulting in a much steeper excitation function compared with theoretical predictions. We show that the energy dependence of this slope is partly due to the asymmetric shape of the Coulomb barrier, that is its deviation from a harmonic shape. We also point out that the large low-energy slope is consistent with the surprisingly large surface diffusenesses required to fit recent high-precision fusion data.

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arXiv2002

Radiation correction to astrophysical fusion reactions and the electron screening problem

We discuss the effect of electromagnetic environment on laboratory measurements of the nuclear fusion reactions of astrophysical interest. The radiation field is eliminated using the path integral formalism in order to obtain the influence functional, which we evaluate in the semi-classical approximation. We show that enhancement of the tunneling probability due to the radiation correction is extremely small and does not resolve the longstanding problem that the observed electron screening effect is significantly larger than theoretical predictions.

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arXiv2002

Low-energy photodisintegration of $^9$Be with the molecular orbit model

We use a microscopic three-cluster model with the $α+α+n$ configuration to analyze with astrophysical interests the photodisintegration cross section of $^9$Be. The valence neutron in $^9$Be is treated in the molecular orbit model (MO), including continuum states by the box discretization method. It is shown that good agreement is achieved with the recently measured B(E1) transition probability from the ground state to the first 1/2$^+$ state when the box size is enlarged, resulting in reasonable reproduction of the experimental data for the photodisintegration cross section. The Coulomb dissociation of $^9$Be is also discussed.

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arXiv2002

Projection and ground state correlations made simple

We develop and test efficient approximations to estimate ground state correlations associated with low- and zero-energy modes. The scheme is an extension of the generator-coordinate-method (GCM) within Gaussian overlap approximation (GOA). We show that GOA fails in non-Cartesian topologies and present a topologically correct generalization of GOA (topGOA). An RPA-like correction is derived as the small amplitude limit of topGOA, called topRPA. Using exactly solvable models, the topGOA and topRPA schemes are compared with conventional approaches (GCM-GOA, RPA, Lipkin-Nogami projection) for rotational-vibrational motion and for particle number projection. The results shows that the new schemes perform very well in all regimes of coupling.

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arXiv2002

Dynamical particle-phonon couplings in proton emission from spherical nuclei

We present results of coupled-channels calculations which are performed to understand the role of vibrational excitations of the daughter nucleus in proton emission decay of spherical nuclei. It is shown that the experimental decay rates from the 2$d_{3/2}$, 3$s_{1/2}$ and 1$h_{11/2}$ states in proton emitting nuclei $^{160,161}$Re are reproduced simultaneously when the quadrupole phonon excitation of the core nuclei is taken into account. We apply the formalism also to the $^{145}$Tm nucleus and show that good agreement is achieved with the recently measured branching ratio for the fine structure.

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arXiv2001

Surface diffuseness anomaly in heavy-ion fusion potentials

Recent high precision experimental data for heavy-ion fusion cross sections at energies in the vicinity of the Coulomb barrier systematically show that a strikingly large surface diffuseness parameter for a Woods-Saxon potential is required in order to fit the data. We discuss possible origins for this anomaly, including the effects of dissipation and the sensitivity of fusion cross sections to the choice of inter-nuclear potential. Our study suggests that the frozen density approximation, which is often used in analyses of heavy-ion reactions, may have to be re-examined for heavy-ion fusion.

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arXiv2001

Influence of higher-order deformations in the ^{34}S + ^{168}Er fusion reaction

The shape of the measured barrier distribution for the ^{34}S + ^{168}Er reaction is analysed using the coupled-channels description. The ^{168}Er nucleus is a good candidate to test current fusion models description of deformation since it has a large quadrupole deformation with an insignificant hexadecapole deformation. Coupling to weaker channels, the 2^+_1 state in ^{34}S, the 3^-_1 state in ^{168}Er, and the pair neutron transfer channel, all were found to have little influence on the barrier distribution. A successful description of the barrier distribution was only obtained after the hexacontatetrapole deformation term in ^{168}Er (beta_6) was included in the coupling scheme. However, a positive value for beta_6 was needed where the macroscopic-microscopic model predicts a negative one.

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arXiv2001

Role of dynamical particle-vibration coupling in reconciliation of the $d_{3/2}$ puzzle for spherical proton emitters

It has been observed that decay rate for proton emission from $d_{3/2}$ single particle state is systematically quenched compared with the prediction of a one dimensional potential model although the same model successfully accounts for measured decay rates from $s_{1/2}$ and $h_{11/2}$ states. We reconcile this discrepancy by solving coupled-channels equations, taking into account couplings between the proton motion and vibrational excitations of a daughter nucleus. We apply the formalism to proton emitting nuclei $^{160,161}$Re to show that there is a certain range of parameter set of the excitation energy and the dynamical deformation parameter for the quadrupole phonon excitation which reproduces simultaneously the experimental decay rates from the 2$d_{3/2}$, 3$s_{1/2}$ and 1$h_{11/2}$ states in these nuclei.

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arXiv2001

Continuum QRPA in the coordinate space representation

We formulate a quasi-particle random phase approximation (QRPA) in the coordinate space representation. This model is a natural extension of the RPA model of Shlomo and Bertsch to open-shell nuclei in order to take into account pairing correlations together with the coupling to the continuum. We apply it to the $^{120}$Sn nucleus and show that low-lying excitation modes are significantly influenced by the pairing effects although the effects are marginal in the giant resonance region. The dependence of the pairing effect on the parity of low-lying collective mode is also discussed.

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arXiv2000

Role of deformation in the nonmesonic decay of light hypernuclei

We discuss the nonmesonic decay of deformed p-shell hypernuclei. The Nilsson model with angular momentum projection is employed in order to take into account the deformation effects. The nonmesonic decay rate and the intrinsic $Λ$ asymmetry parameter decrease as a function of the deformation parameter, while the ratio of the neutron- to proton-induced decay rates increases. We find that the deformation effects change these observables by about 10 % for $^9_Λ$Be from the spherical limit.

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arXiv2000

Sub-barrier fusion of drip-line nuclei

We discuss the role of break-up process of a loosely-bound projectile in subbarrier fusion reactions. Coupled-channels calculations are carried out for $^{11}$Be + $^{208}$Pb and $^{4,6}$He + $^{238}$U reactions by discretizing in energy the particle continuum states. Our calculations show that the coupling to the break-up channel has two effects, namely the loss of flux and the dynamical modulation of fusion potential. Their net effects differ depending on the energy region. At energies above the Coulomb barrier, the former effect dominates over the latter and cross sections for complete fusion are hindered compared with the no coupling case. On the other hand, at below the barrier, the latter effect is much larger than the former and complete fusion cross sections are enhanced consequently.

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arXiv2000

Correlation energy of the pairing Hamiltonian

We study the correlation energy associated with the pair fluctuations in BCS theory. We use a schematic two-level pairing model and discuss the behavior of the correlation energy across shell closures, including the even-odd differences. It is shown that the random phase approximation (RPA) to the ground state energy reproduces the exact solutions quite well both in the normal fluid and in the superfluid phases. In marked contrast, other methods to improve the BCS energies such as Lipkin-Nogami method, only work when the strength of a pairing interaction is large. We thus conclude that the RPA approach is better for a systematic theory of nuclear binding energies.

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