arXiv subjects

Souichi Ishikawa

Publications and source records attributed to Souichi Ishikawa.

5 recordsLinked to original sources

arXiv2025

Low-energy ${}^{12}$C continuum states in three-$α$ model

The electric multipole strength distributions for transitions from the ${}^{12}\mathrm{C}(0_1^+)$ ground state to $3α$ ($0^+$, $1^-$, $2^+$, and $3^-$) continuum states are studied in terms of $3α$ model. Several sets of the $3α$ Hamiltonian are introduced phenomenologically with conventional $α$-$α$ interaction potentials and $3α$ potentials with different range parameters. The transition strength distributions are obtained from wave functions of $3α$ bound- and continuum states calculated by the Faddeev three-body formalism. From these strength distributions, the resonance parameters (energy, $3α$-decay width, and transition strength) of $3α$ resonance states are extracted. Dependencies of these observables on interaction potential models are studied to examine the introduced interaction models. Using the transition strength distributions, excitation-energy spectra in the ${}^{12}\mathrm{C}(α,α^\prime)3α$ reaction are calculated to compare with experimental data.

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arXiv2020

Study of three-neutron bound and continuum states

The three-neutron ($3n$) system is studied by numerical calculations with the Faddeev three-body formalism for a realistic nucleon-nucleon (NN) potential. A response function for the transition from ${}^3\mathrm{H}$ to $3n$ continuum states by an isospin excitation operator is calculated, from which no evidence of $3n$ resonance state is found. Different methods to extrapolate the $3n$ energy from bound state energies with an extra attractive effect to the NN potential are examined. While extrapolations with attractive effects by enhanced NN potentials or three-body potentials result the non-existence of $3n$ resonance states, one by external trapping potentials leads to a positive $3n$ energy, which may be considered as a resonance state. It is found that this contradiction is due to a general defect of the trapping method.

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arXiv2017

Spin-isospin excitation of ${}^3$He with three-proton final state

Spin-isospin excitation of ${}^3$He nucleus by proton-induced charge-exchange reaction, ${}^3\mathrm{He}(p,n)ppp$, at forward neutron scattering angle is studied in a plane wave impulse approximation (PWIA). In PWIA, cross sections of the reaction is written in terms of proton-neutron scattering amplitudes and response functions of the transition from ${}^{3}$He to three-proton state by spin-isospin transition operators. The response functions are calculated with realistic nucleon-nucleon potential models using a Faddeev three-body method. Calculated cross sections agree with available experimental data in substance. Possible effects arising from the uncertainty of proton-neutron amplitudes and three-nucleon interactions in three-proton system are examined.

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arXiv2016

Monopole transition strength function of ${}^{12}$C in three-$α$ model

The energy level structure of ${}^{12}$C nucleus at a few MeV above the three-$α$ threshold is still unsatisfactory known. For instance, most microscopic calculations predicted that there exist one $0^+$-state in this energy region besides the well known Hoyle state, while some experimental and theoretical studies show the existing of two $0^+$-states. In this paper, I will take a three-$α$-boson (3$α$) model for bound and continuum states in ${}^{12}$C, and study a transition process from the ${}^{12}$C($0_1^+$) ground state to 3$α$ $0^+$ continuum states by the electric monopole ($E0$) operator. The strength distribution of the process will be calculated as a function of $3α$ energy using the Faddeev three-body theory. The Hamiltonian for the $3α$ system consists of two- and three-$α$ potentials, and some three-$α$ potentials with different range parameters will be examined. Results of the strength function show a double-peaked bump at low energy region, which can be considered as two $0^+$-states. The peak at higher energy may originate from a 3$α$ resonant state. However, it is unlikely that the peak at the lower energy is related to a resonant state, which suggests that it may be due to so called "ghost anomaly". Distributions of decaying particles are also calculated.

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arXiv2014

Decay and structure of the Hoyle state

The first $0^+$ resonant state of the $^{12}$C nucleus ${}^{12}$C$(0_2^+)$, so called the Hoyle state, is investigated in a three-$α$-particle (3-$α$) model. A wave function for the photodisintegration reaction of a $^{12}$C bound state to 3-$α$ final states is defined and calculated by the Faddeev three-body formalism, in which three-body bound- and continuum states are treated consistently. From the wave function at the Hoyle state energy, I calculated distributions of outgoing $α$-particles and density distributions at interior region of the Hoyle state. Results show that a process through a two-$α$ resonant state is dominant in the decay and contributions of the rest process are very small, less than 1 \%. There appear some peaks in the interior density distribution corresponding to configurations of an equilateral- and an isosceles triangles. It turns out that these results are obtained independently of the choice of $α$-particle interaction models, when they are made to reproduce the Hoyle state energy.

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