Modern Physics Letters A

It is discussed that the recently observed 0⁺ state at 13.5 MeV in ¹⁶O can be a good candidate of the 4α condensate. The 4α condensed state which is theoretically obtained in the vicinity of the 4α threshold is shown to have a relatively large α decay width, which is in well agreement with the value measured for the 0⁺ state at 13.5 MeV. The analysis of inelastic form factor of electron scattering to the 4α condensed state is made by artificially varying the size of the 4α condensate. Similarly to the case of Hoyle state, only the magnitude of the form factor is found to depend on its size, while the position of maximum and minimum are not changed.

We analyze the inelastic scattering of the α+¹²C system leading to the state in ¹²C at incident energies of E_α=139 MeV ~ 240 MeV using α condensate model wave function, and investigate the affection of the large nuclear radius of on the inelastic angular distribution. It is found that the oscillation pattern in inelastic angular distribution is sensitive to the extent of transition density rather than the nuclear radius of the excited state.

Three-alpha resonance excited states has been theoretically explored by means of the α+α+α microscopic cluster model. The resonance parameters is determined by employing the Complex Scaling Method (CSM). Our model space is extended by including higher partial waves than the RGM+CSM calculation by Pichler¹. Our model has given the third 0⁺ state at 4~5MeV above the three-body threshold but the broad 0⁺ state, which was recently reported experimentally and overlapping with the 2⁺ state², has not been localized between the second and the third 0⁺ states. On the other hands, our model has given the second 2⁺ state about 2MeV and the third at 4~5MeV.

The 3α resonant states of ¹²C are investigated by taking into account the correct boundary condition for three-body resonant states. In order to show how the 3α resonant states having complex eigenvalues contribute to the real energy, we calculated the Continuum Level Density in the Complex Scaling Method.

The second 2⁺ state in the 10 MeV region of ¹²C was investigated by measuring decay-α particles in the ¹²C nucleus excited by inelastic α scattering at 0°. Angular correlation functions were obtained at backward angles from 115° to 255°. Those for 9.64 MeV 3^-, 15.4 MeV 2⁺, and 18.2 MeV 2⁺ states were good agreement with the PWBA calculation. The third 0⁺ state was also investigated. The coincidence spectra at 245° of decay-α particles seem to be consisted of two 0⁺ components.

Experimental method of measuring inelastic proton scattering with high-resolution at forward angles including zero degrees has been developed. An energy resolution of 20 keV and a scattering angle resolution of 0.5 degrees have been achieved as well as low background condition and a reliable background subtraction method. The experimental technique was applied to the ¹²C(p,p′) reaction for studying the property of the second 0⁺ state at 7.7 MeV and a broad bump around E_x~10 MeV, where the presence of a 2⁺ state was reported from the ¹²C(α,α′) measurement. Preliminary spectra and angular distributions are shown.

The spin-orbit splittings of ¹³C and ¹³N corresponding to the ground state and second 0⁺ state (Hoyle state) in ¹²C are investigated within a ¹²C+N model. The agreement of the calculated results with experiments suggests that the and states around E_x = 9 ~ 10 MeV in ¹³C and ¹³N can be interpreted as the spin-orbit partner with the configuration. We also discuss the positive parity states (including ) of ¹³C and ¹³N around the threshold, indicating to have the configuration of the Hoyle state in ¹²C plus a valence nucleon in sd shell.

The cluster structures of the excited states in ¹¹B are studied by analyzing the isoscalar monopole and quadrupole strengths in the ¹¹B(d,d′) reaction at E_d = 200 MeV. The excitation strengths are compared with the predictions by the shell-model and antisymmetrized molecular-dynamics (AMD) calculations. It is found that the large monopole strength for the state at E_x = 8.56 MeV is well described by the AMD calculation and is an evidence for a developed 2α + t cluster structure.

Cross sections for ¹³C(α,α′) reactions at E_α = 400 MeV have been measured to obtain the monopole and quadrupole transition strengths. The excitation strengths are compared with the shell-model predictions. It is suggested that the large monopole strengths observed for the and states in ¹³C are attributed to the cluster structure of those states.

Structures of excited states in ¹¹C and ¹¹B were studied based on variational calculations after spin-parity projection in a framework of antisymmetrized molecular dynamics (AMD). The calculations systematically reproduce experimental data of the transition strengths such as Gamov-Teller(GT) and M1 transitions. A well-developed cluster structure of 2α+³He(t) was found in the state of ¹¹C(¹¹B). We assigned this state to the observed state at 8 MeV. The calculations reproduce the small which has been measured in the recent experiments. It was found that the state has a dilute cluster state, and its features are similar to those of the which is considered to be a gas state of three α clusters.

Spin-isospin excitations in ¹³N have been studied by means of the (³He,t) and (³He,tp) reactions at the bombarding energy of E(³He) = 450 MeV. The zero-degree (³He,t) spectrum is found to be similar to those from the (p,n) reactions at intermediate energies, suggesting a simple direct reaction mechanism at E(³He) = 450 MeV. Three pairs of Gamow-Teller (GT) states with J^π = 1/2^- and 3/2^- have been strongly excited at θ_lab. = 0°. Microscopic structures of the states in ¹³N have been studied by observing decay protons in coincidence with high energy tritons measured at θ_lab. = 0°. The branching ratios for proton decay from the GT states in ¹³N to the final low-lying T=0 states in ¹²C were obtained. The wave functions of the excited states in ¹³N are discussed in the view point of α cluster model.

An examination of the role of the cluster in nuclear reactions is made in the longstanding case of ¹²C+¹²C cluster resonances in ²⁴Mg. A description of both resonant scattering and breakup measurements can be found in terms of shape isomeric cluster structures.

The α+⁶He low-energy reactions and the structural changes of ¹⁰Be in the microscopic α+α+N+N model are studied by the generalized two-center cluster model with the Kohn-Hulthén-Kato variation method. It is found that, in the inelastic scattering to the channel, characteristic enhancements are expected as the results of the parity-dependent non-adiabatic dynamics. In the positive parity state, the enhancement originates from the excited eigenstate generated by the resonance pole of the final channel, while that in the negative parity state is induced by the Landau-Zener level-crossing. The reaction mechanism in breakup of ¹⁰Be into the ⁵He+⁵He and α+⁶He continuums is also studied. The strong enhancement in the breakup of is discussed in connection with the non-adiabatic dynamics.

A systematic analysis of alpha(⁴He)-nucleus elastic scattering is made by using a microscopic optical model potential obtained by the double folding of a complex nucleon-nucleon (NN) effective interaction based on the G-matrix theory. We adopt the so-called JLM interaction as the complex NN interaction and test its applicability to the ⁴He elastic scattering by ¹²C, ¹⁶O, ²⁸Si and ⁴⁰Ca. The experimental cross sections for incident energies ranging from E_Lab = 40 to 240 MeV are well reproduced by the double folding potential up to backward angles. Although modification of the real and imaginary potential strength by about 25% and 35%, respectively, in average is necessary to reproduce the data, the renormalization factors are found to be almost constant with respect to the incident energy and target mass number.

The collective multipole excitations are studied in the framework of relativistic random-phase approximation with the vacuum polarization. First, we show for the nuclear ground state that the leading order of derivative expansion of the effective action arising from the vacuum correction agrees with the exact calculation using the Green function method very well. The derivative expansion makes us easy to perform a fully self-consistent calculation, even for the random-phase approximation. A remarkable effect of the inclusion of the vacuum polarization is the increase of the effective mass m_eff/m_N ~ 0.9, which gives, for all multipole modes, smaller energy-weighted sum rule values than those of the typical relativistic model. Also, the large effective mass constrained by the vacuum polarization can give an excellent agreement with experimental data on the excitation energy for the isoscalar quadrupole resonances. It is shown, further, that the change of the shell structure due to the vacuum polarization plays an important role in the dipole compression modes.

Di-neutron cluster structure of ¹¹Li which is suggested strongly by recent experiments is discussed. Recent HFB calculations which show clear di-neutron correlation in surface region of neutron-rich nuclei are also discussed. An approach to di-neutron problem by the use of a hybrid wave function of AMD and di-neutron condensed wave function is proposed.

The low-lying level structure of ³¹Mg has been investigated by the theoretical framework of deformed-basis antisymmetrized molecular dynamics (DAMD) plus generator coordinate method (GCM) with the Gogny D1S force. It is shown that the N=20 magic number is broken and the intruder configuration (neutron 2p3h configuration) dominates the ground state. The observed data such as the magnetic moment and β decay strength are well reproduced.

New treatments are proposed for di-nucleon condensate based on the α condensate wave function. A beautiful mathematical structure is shown for the overlap.

We study the effect of the tensor correlation using a mean-field-type model and a shell model. To treat the tensor correlation in a mean-field-type model, we introduce single-particle states with the parity and charge mixing considering the pseudoscalar and isovector characters of the pion, which mediates the tensor force. We study closed-shell and sub-closed-shell oxygen isotopes and find that a sizable attractive energy from the tensor force is obtained by introducing the parity and charge mixing. We also perform a shell model calculation up to two-particle–two-hole configurations. A large attraction energy is obtained for ¹⁶O when we introduce single-particle wave functions with narrow widths.

We investigate the roles of the tensor correlation on the structures of ⁴He and ⁵He. For ⁴He, we take the high angular momentum states within the 2p2h excitations of the shell model type method to describe the tensor correlation explicitly. We found that the solutions show a good convergence and the obtained wave function has the large tensor correlation. We also found that three 2p2h configurations are largely coupled with (0s)⁴ one to describe the tensor correlation, which is understood from the selectivity of the tensor operator. This tensor correlation is also important to describe the scattering phenomena of the ⁴He+n system including the higher partial waves consistently.

We investigate the tensor and pairing correlations in ¹¹Li based on the ⁹Li+n+n model. For ⁹Li, we perform the configuration mixing with the shell model type wave function to introduce the core polarization caused by the tensor and pairing correlations. For ¹¹Li, we perform the coupled ⁹Li+n+n calculation, in which the couplings between the correlations in ⁹Li and the motion of the last two neutrons emerge Pauli-blocking for the p² configuration of ¹¹Li and increases the s² component to develop the halo structure.

t+t clustering in He isotopes is investigated by using two theoretical approaches. A role of the t+t cluster component in the ground state is examined with AMD triple-S, allowing the wider configuration space containing simultaneously the "t+t+valence neutrons" structure and "⁴He+valence neutrons" structure. We understand the importance of the t+t component even for the ground state. Further, t+t resonances are investigated with RGM type approach. We obtained many t+t states as resonances near to t+t threshold.

Work aimed at probing the spatial configuration of the valence neutrons in two-neutron halo nuclei using the technique of intensity interferometry is described. Following a brief review of the method and its application to our first measurements of the breakup of ⁶He, ¹¹Li and ¹⁴Be, the results of the analysis of a high statistics data set for ⁶He are presented.

Quantum condensates in nuclear matter are treated beyond the mean-field approximation, with the inclusion of cluster formation. The occurrence of a separate binding pole in the four-particle propagator in nuclear matter is investigated with respect to the formation of a condensate of α-like particles (quartetting), which is dependent on temperature and density. Due to Pauli blocking, the formation of an α-like condensate is limited to the low-density region. Consequences for finite nuclei are considered. In particular, excitations of self-conjugate 2n-Z–2n-N nuclei near the n-α-breakup threshold are candidates for quartetting. We review some results and discuss their consequences. Exploratory calculations are performed for the density dependence of the α condensate fraction at zero temperature to address the suppression of the four-particle condensate below nuclear-matter density.