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S. Ohkubo

Publications and source records attributed to S. Ohkubo.

36 records · Page 2Linked to original sources
arXiv2014

Refractive effects and Airy structure in inelastic $^{16}$O+$^{12}$C rainbow scattering

Inelastic $^{16}$O +$^{12}$C rainbow scattering to the $2^+$ (4.44 MeV) state of $^{12}$C was measured at the incident energies, $E_L$ = 170, 181, 200, 260 and 281 MeV. A systematic analysis of the experimental angular distributions was performed using the coupled channels method with an extended double folding potential derived from realistic wave functions for $^{12}$C and $^{16}$O calculated with a microscopic $α$ cluster model and a finite-range density-dependent nucleon-nucleon force.The coupled channels analysis of the measured inelastic scattering data shows consistently some Airy-like structure in the inelastic scattering cross sections for the first $2^+$ state of $^{12}$C, which is somewhat obscured and still not clearly visible in the measured data. The Airy minimum was identified from the analysis and the systematic energy evolution of the Airy structure was studied. The Airy minimum in inelastic scattering is found to be shifted backward compared with that in elastic scattering.

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arXiv2014

Similarity between nuclear rainbow and meteorological rainbow -- evidence for nuclear ripples

We present evidence for the nuclear ripples superimposed on the Airy structure of the nuclear rainbow, which is similar to the meteorological rainbow. The mechanism of the nuclear ripples is also similar to that of the meteorological rainbow, which is caused by the interference between the externally reflective waves and refractive waves. The nuclear ripple structure was confirmed by analyzing the elastic angular distribution in $^{16}$O+$^{12}$C rainbow scattering at $E_L$=115.9 MeV using the coupled channels method by taking account of coupling to the excited states of $^{12}$C and $^{16}$O with a double folding model derived from a density-dependent effective nucleon-nucleon force with realistic wave functions for $^{12}$C and $^{16}$O. The coupling to the excited states plays the role of creating the external reflection.

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arXiv2014

Evidence for a secondary bow in Newton's zero-order nuclear rainbow

Rainbows are generally considered to be caused by static refraction and reflection. A primary and a secondary rainbow appear due to refraction and internal reflection in a raindrop as explained by Newton. The quantum nuclear rainbow, which is generated by refraction in the nucleus droplet, only has a "primary" rainbow. Here we show for the first time evidence for the existence of a secondary nuclear rainbow generated dynamically by coupling to an excited state without internal reflection. This has been demonstrated for experimental $^{16}$O+$^{12}$C scattering using the coupled channel method with an extended double folding potential derived from microscopic realistic wave functions for $^{12}$C and $^{16}$O.

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arXiv2013

Unification of Airy structure in inelastic $α$ +$^{16}$O scattering and $α$-cluster structure with core excitation in $^{20}$Ne

The Airy structure of the nuclear rainbow and prerainbow in inelastic and elastic $α+^{16}$O scattering is studied with the coupled channel method using a folding potential derived from the microscopic wave functions of $^{16}$O. The theoretical calculations reproduce the characteristic energy evolution of the Airy minimum of the experimental angular distributions. The energy levels with $α$-cluster structure in $^{20}$Ne are reproduced well using the potentials determined from the analysis of scattering. It is shown that the emergence of the $K=0_3^+$ $α$-cluster band with core excitation at 7.19 MeV in $^{20}$Ne is intimately related to the emergence of the prerainbow and rainbow in inelastic scattering to the $^{16}$O($0^+_2$). It is found that the $α$-cluster states with core excitation, the prerainbow and the rainbow in inelastic scattering are understood in a unified way as well as in the case of elastic scattering.

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arXiv2013

Evidence of alpha particle condensation in $^{12}$C and $^{16}$O and Nambu-Goldstone boson

It is suggested that direct evidence of Bose-Einstein condensation of alpha particles is obtained by observing a phase mode (Nambu-Goldstone boson) with long wavelength even when characteristic features such as superfluidity is diffucult to observe. For the 7.65 MeV $0_2^+$ Hoyle state in $^{12}$C and 15.1 MeV $0^+$ state in $^{16}$O, which are candidates for an alpha particle condensate, it is suggested that the emergent band head $0^+$ state of the $K=0_2^+$ rotational band with a very large moment of inertia is considered to be a Nambu-Goldstone boson.

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arXiv2013

Observation of Airy minimum in elastic and inelastic scattering of $^3$He from $^{12}$C at 50.5 and 60 MeV and alpha particle condensation in $^{12}$C

Angular distributions for elastic and inelastic scattering of $^3$He from $^{12}$C were measured at energies 50.5 and 60 MeV. The Airy minimum of the prerainbow scattering was clearly observed in the angular distributions for the 0$_2^+$ (7.65 MeV) state of $^{12}$C (Hoyle state). The experimental results were analyzed with a coupled channels method with double folding potentials derived from the microscopic wave functions for the ground 0$_1^+$, 2$^+$ (4.44 MeV), 3$^-$ (9.64 MeV) and 0$_2^+$ states. The analysis supports the view that the Hoyle state is a three alpha particle condensate with a large radius of dilute matter distribution.

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arXiv2012

Existence of exotic torus configuration in high-spin excited states of $^{40}$Ca

We investigate the possibility of the existence of the exotic torus configuration in the high-spin excited states of $^{40}$Ca. We here consider the spin alignments about the symmetry axis. To this end, we use a three-dimensional cranked Skyrme Hartree-Fock method and search for stable single-particle configurations. We find one stable state with the torus configuration at the total angular momentum $J=$ 60 $\hbar$ and an excitation energy of about 170 MeV in all calculations using various Skyrme interactions. The total angular momentum J=60 $\hbar$ consists of aligned 12 nucleons with the orbital angular momenta $Λ=+4$, +5, and +6 for spin up-down neutrons and protons. The obtained results strongly suggest that a macroscopic amount of circulating current breaking the time-reversal symmetry emerges in the high-spin excited state of $^{40}$Ca.

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arXiv2011

Linear chain structure of four-$α$ clusters in $^{16}$O

We investigate the linear-chain configurations of four-$α$ clusters in $^{16}$O using a Skyrme cranked Hartree-Fock method and discuss the relationship between the stability of such states and angular momentum. We show the existence of a region of angular momentum (13-18 $\hbar$) where the linear chain configuration is stabilized. For the first time we demonstrate that stable exotic states with a large moment of inertia ($\hbar^2/2Θ$ $\sim$ 0.06-0.08 MeV) can exist.

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arXiv2011

Static and Dynamic Chain Structures in the Mean-Field Theory

We give a brief overview of recent work examining the presence of $α$-clusters in light nuclei within the Skyrme-force Hartree-Fock model. Of special significance are investigations into $α$-chain structures in carbon isotopes and $^{16}$O. Their stability and possible role in fusion reactions are examined in static and time-dependent Hartree-Fock calculations. We find a new type of shape transition in collisions and a centrifugal stabilization of the $4α$ chain state in a limited range of angular momenta. No stabilization is found for the $3α$ chain.

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arXiv2011

Prerainbow Oscillations in $^3$He Scattering from the Hoyle State of $^{12}$c and Alpha Particle Condensation

$^3$He+$^{12}$C scattering is studied in a coupled channel method by using a double folding model with microscopic wave functions of $^{12}$C. Experimental angular distributions in elastic and inelastic scattering to the $2^+$ (4.44 MeV), 0$^+_2$ (7.65 MeV) and 3$^-$ (9.63 MeV) states of $^{12}$C are well reproduced. It is found that the Airy minimum of the prerainbow oscillations for the Hoyle state is considerably shifted to a larger angle due to its dilute density distribution compared with that of the normal ground state in agreement with the idea of $α$ particle condensation.

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arXiv2011

Evidence for higher nodal band states with $^3$He cluster structure in $^{19}$Ne and prerainbows in $^3$He+$^{16}$O scattering

The existence of a higher nodal band state with a $^{3}$He cluster structure, i.e. a vibrational mode in which the inter-cluster relative motion is excited, in $^{19}$Ne in addition to those with the $α$ cluster structure in $^{20}$Ne and the $^{16}$O cluster structure in $^{32}$S, is suggested, which reinforces the importance of the concept of $^3$He-clustering in nuclei. This conclusion was reached by investigating $^{3}$He scattering from $^{16}$O in a wide range of incident energies and prerainbow oscillations.

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arXiv2011

$α$ cluster states in $^{44, 46, 52}$Ti

$α$ decaying states of $^{44, 46, 52}$Ti were investigated with angular correlation functions between $t$ and $α$ with the $^{40, 42, 48}$Ca($^7$Li, $tα$)$^{40, 42, 48}$Ca reactions at E = 26.0 MeV. Many $α$ cluster states were newly observed in the 10 - 15 MeV excitation energy of $^{44}$Ti and their spin-parities were assigned, in which $J^π=7^-$ state was found at 11.95 MeV as a candidate for the member of the $K=0_1^-$ negative parity band. In $^{46}$Ti many $α$ cluster states were also found in the 11 - 17 MeV excitation energy with the $^{42}$Ca($^7$Li, $tα$)$^{42}$Ca reaction, though its strength is weak compared to $^{44}$Ti. No $α$ cluster states were detected for the $^{48}$Ca($^7$Li, $tα$)$^{48}$Ca reaction, in which the number of coincidence events decaying from $^{48}$Ca was very small.

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arXiv2011

$α$-particle condensate states in $^{16}$O

The existence of a rotational band with the $α$+$^{12}$C($0_2^+$) cluster structure, in which three $α$ particles in $^{12}$C($0_2^+$) are locally condensed, is demonstrated near the four-$α$ threshold of $^{16}$O in agreement with experiment. This is achieved by studying structure and scattering for the $α$+$^{12}$C($0_2^+$) system in a unified way. A drastic reduction (quenching) of the moment of the inertia of the $0^+$ state at 15.1 MeV just above the four-$α$ threshold in $^{16}$O suggests that it could be a candidate for the superfluid state in $α$-particle condensation.

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arXiv2011

Evidence for strong refraction of $^3$He in an alpha-particle condensate

We have analyzed $^{3}$He scattering from $^{12}$C at 34.7 and 72 MeV in a coupled channel method with a double folding potential derived from the precise wave functions for the ground 0$^+$ state and $0_2^+$ (7.65 MeV) Hoyle state, which has been suggested to be an $α$ particle condensate. It is found that strong refraction of $^3$He in the Hoyle state can be clearly seen in the experimental angular distribution at {\it low} incident energy region as an Airy minimum of the {\it pre-rainbow oscillations}.

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arXiv2006

Nuclear rainbow scattering and nucleus-nucleus potential

Elastic scattering of alpha-particle and some tightly-bound light nuclei has shown the pattern of rainbow scattering at medium energies, which is due to the refraction of the incident wave by a strongly attractive nucleus-nucleus potential. This review gives an introduction to the physics of the nuclear rainbow based essentially on the optical model description of the elastic scattering. Since the realistic nucleus-nucleus optical potential (OP) is the key to explore this interesting process, an overview of the main methods used to determine the nucleus-nucleus OP is presented. The refractive rainbow-like structures observed in other quasi-elastic scattering reactions have also been discussed. Some evidences for the refractive effect in the elastic scattering of unstable nuclei are presented and perspectives for the future studies are discussed.

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