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Register a new userExploring manifestation and nature of a dineutron in two-neutron emission using a dynamical dineutron model
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Emission of two neutrons or two protons in reactions and decays is often discussed in terms of dineutron or diproton emission. The discussion often leans intuitively on something described by Migdal-Watson approximation. In this work we propose a way to formalize situations of dineutron emission. It is demonstrated that properly formally defined dineutron emission may reveal properties which are drastically different from those traditionally expected, and properties which are actually observed in three-body decays.
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The low-energy behavior of the strength function for the $1^-$ soft dipole excitation in $^{6}$He is studied theoretically. Use of very large basis sizes and well-grounded extrapolation procedures allows to move to energies as small as 1 keV, at which the low-energy asymptotic behavior of the E1 strength function seems to be achieved. It is found that the low-energy behavior of the strength function is well described in the effective three-body dynamical dineutron model. The astrophysical rate for the $alpha$+$n$+$n rightarrow ^6$He+$gamma$ is calculated. Comparison with the previous calculations is performed.
We investigate how strong a hypothetical 1S0 bound state of two neutrons would affect different observables in the neutron-deuteron reactions. To that aim we extend our momentum space scheme of solving three-nucleon Faddeev equations to incorporate in addition to the deuteron also the 1S0 dineutron bound state. We discuss effects induced by dineutron on the angular distribution of the neutron-deuteron elastic scattering and cross sections of the deuteron breakup. A comparison to the available data for neutron-deuteron total cross sections and elastic scattering angular distributions cannot decisively exclude a possibility that the two neutrons can form 1S0 bound state. However, the strong modifications of a final-state-interaction peak of the neutron-deuteron breakup when changing from negative to positive values of the neutron-neutron scattering length seems to exclude existence of dineutron.
The formation of a dineutron in the nucleus $^{11}$Li is found to be localized to the surface region. The experiment measured the intrinsic momentum of the struck neutron in $^{11}$Li via the $(p,pn)$ knockout reaction at 246 MeV/nucleon. The correlation angle between the two neutrons is, for the first time, measured as a function of the intrinsic neutron momentum. A comparison with reaction calculations reveals the localization of the dineutron at $rsim3.6$ fm. The results also support the density dependence of dineutron formation as deduced from Hartree-Fock-Bogoliubov calculations for nuclear matter.
We investigated the development and breaking of the dineutron correlation in $^{10}$Be by analyzing the elastic and inelastic scatterings with a framework combing the microscopic structure and reaction models. For studying the structure, the $^{10}$Be nucleus was constructed under the assumption of a four-body ($alpha + alpha + n + n$) cluster model. In this work, we focused on the change in the inner structure for the 0$_1^+$, 2$_1^+$, and 2$_2^+$ states when the strength of the spin-orbit interaction is varied. The inner structure, including various physical quantities such as energy, radius, and transition strength, is drastically influenced by the strength of the spin-orbit interaction. In particular, the development and breaking of the dineutron correlation is governed by the spin-orbit strength. The differences in the inner structure can be manifested by applying the obtained wave functions to elastic and inelastic scatterings with a proton target at $E/A =$ 59.4 and 200 MeV. Although the 0$_1^+$ and 2$_1^+$ states are significantly influenced by the spin-orbit strength of the nuclear structure calculation, the elastic and inelastic cross sections are not much affected. On the other hand, the inelastic cross section of the 2$_2^+$ state depends greatly on the spin-orbit strength of the structure calculation. Thus, we discovered a way to measure the degree of the development of dineutron cluster structure based on its sensitivity to the inelastic cross section of the 2$_2^+$ state of $^{10}$Be.
Background$colon$ The $^{29}$F system is located at the lower-N boundary of the island of inversion and is an exotic, weakly bound system. Little is known about this system beyond its two-neutron separation energy ($S_{2n}$) with large uncertainties. A similar situation is found for the low-lying spectrum of its unbound binary subsystem $^{28}$F. Purpose$colon$ To investigate the configuration mixing, matter radius and neutron-neutron correlations in the ground state of $^{29}$F within a three-body model, exploring the possibility of $^{29}$F to be a two-neutron halo nucleus. Method$colon$ The $^{29}$F ground-state wave function is built within the hyperspherical formalism by using an analytical transformed harmonic oscillator basis. The Gogny-Pires-Tourreil (GPT) nn interaction with central, spin-orbit and tensor terms is employed in the present calculations, together with different core$+n$ potentials constrained by the available experimental information on $^{28}$F. Results$colon$ The $^{29}$F ground-state configuration mixing and its matter radius are computed for different choices of the $^{28}$F structure and $S_{2n}$ value. The admixture of d-waves with pf components are found to play an important role, favoring the dominance of dineutron configurations in the wave function. Our computed radii show a mild sensitivity to the $^{27}$F$+n$ potential and $S_{2n}$ values. The relative increase of the matter radius with respect to the $^{27}$F core lies in the range 0.1-0.4 fm depending upon these choices. Conclusions$colon$ Our three-body results for $^{29}$F indicate the presence of a moderate halo structure in its ground state, which is enhanced by larger intruder components. This finding calls for an experimental confirmation.
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