N. S. Zelenskaya

Mechanisms of sequential particle transfer and characteristics of light neutron-excess and oriented nuclei

The procedure for evaluating the second-order corrections to the matrix elements of the reaction A(x, y)B, which are obtained using the method of distorted waves with a finite radius of intercluster interaction (DWBAFR), is developed. It is based on the assumption of a virtual cluster structure of light nuclei and uses integral equations for a four-body problem in the Alt-Grassberger-Sandhas formalism. These corrections are related with the mechanisms of sequential particles transfer. The latter are represented by the quadrangle diagrams. Their matrix elements are summed up coherently with those given by the pole and triangle diagrams which were calculated by using DWBAFR. The computer code QUADRO is written for the numerical implementation of the method proposed. The statistical tensors of nucleus B formed in the reaction A(x, y)B at incident particle energies of about 10 MeV/nucleon in the center of mass frame are determined. Specific calculations allowed for description of both the experimental cross sections (0-rank statistical tensors) of various reactions (including those where nucleus B has some excess neutrons) and polarized characteristics of nucleus B* (in the case of the latter produced in the exited state). A two-neutron periphery of nuclei 6He, 10Be, 12B (both in dineutron and cigarlike configurations) is restored by analyzing the differential cross sections of elastic alpha-6He-scattering and 9Be(d, p)10Be and 10B(t, p)12B reactions. It is shown that the structure of neutron peripheries is fundamentally different for these nuclei and its feature depends on the way those neutron-excess nuclei are formed: in 6He both configurations contribute to a two-neutron halo, while in 10Be there is a barely noticeable one-neutron halo, and in 12B there is a “dineutron skin”. Orientation characteristics of nuclei B* are calculated. Their comparison with experimental data made it possible to draw important conclusions about a contribution to the statistical tensors of nucleus B* coming from the two-step mechanisms and its impact on the properties of oriented light nuclei, including their polarization. Finally, a simplified method for calculating the matrix elements of mechanisms, which take into account sequential particle transfer, is proposed. It is demonstrated to be correct by evaluating a contribution of the corresponding corrections to the total amplitude of the reaction.

Mechanism of independent neutron transfer in elastic α 6He scattering and structure of the 6He nuclear wave function

The presence of two topologically different configurations, a dineutron and a cigar-like one, in the spatial wave function for the neutron-rich nucleus 6He is predicted. In elastic α6He scattering, the dineutron configuration leads to the appearance of backward maxima in the cross section for elastic α6He scattering. The cigar-like configuration is responsible for the scattering mechanism involving independent sequential neutron transfer and the formation of the 5He-5He virtual system. In order to estimate the contribution of this mechanism, the matrix element for the two-step sequential particle transfer is calculated by a method that takes into account delay in the interaction and which is realized in the QUADRO code. Calculations performed on this basis make it possible to estimate the weight of the cigar-like component in the wave function for the 6He nucleus.

Manifestations of a dineutron cluster in elastic α6He scattering

A consistent theoretical analysis of elastic α6He scattering is performed with allowance for the correction associated with the exchange of a dineutron cluster. It is shown that the inclusion of the exchange mechanism, along with that of potential scattering, makes it possible to reproduce the measured cross section over the entire range of the 6He scattering angle. Concurrently, one can also obtain information about the spatial distribution of the dineutron cluster and the α particle in the ground state of the 6He nucleus.

Neutron periphery in light nuclei

A spatial configuration of light nuclei that involves two excess neutrons (6He, 10Be, and 12B) is studied by analyzing cross sections for various reactions on these nuclei: 6He(α, α)6He, 9Be(d, p)10Be, and 10B(t, p)12B. Pole dineutron-transfer mechanisms (dineutron configuration of the neutron periphery) and second-order mechanisms involving sequential neutron transfer (cigar-like configuration of the neutron periphery) are taken into account in the calculations. It is shown that the neutron periphery is drastically different in the nuclei in question: in 6He, there is a halo receiving a contribution from both configurations; in 10Be, there is an only slightly noticeable halo featuring a contribution of practically one neutron from the cigar-like configuration; and, in 12B, there is a neutron skin.

αγ angular correlations in the inelastic scattering of α particles on 24Mg at Eα = 30.3 MeV

The results from measuring the angular dependence of differential α-scattering cross sections on 24Mg with final nucleus formation in the ground and three lower excited states are presented, along with the double differential 24Mg(α, αγ)24Mg(2+, 1.369 MeV) reaction cross sections at Eα = 30.3 MeV. The spin-tensor components of the density matrix of the 24Mg nucleus in the 2+(1.369 MeV) state are reconstructed and its orientation characteristics are determined. The experimental results are compared to theoretical results under the assumption of collective excitation and mechanisms for compound nucleus formation.

Investigation of the mechanism of inelastic deuteron scattering on 12C at E d = 15.3 MeV by the method of dγ angular correlations

The results are presented that were obtained by measuring the differential cross sections for the reaction 12C(d,d) 12C occurring at Ed = 15.3 MeV and leading to the production of a 12C nucleus in the ground and the first excited state. The energy dependences of the differential reaction cross sections were measured for three angles of deuteron emission in the range of projectile-deuteron energies Ed between 12 and 15.3 MeV. The double-differential cross sections for the reaction in question were measured for the 2+ state of the 12C nucleus at 4.44 MeV, and the angular dependences of the even spin-tensor components of the density matrix were determined, along with the angular dependences of the populations of magnetic sublevels and the components of the tensors of multipole-moment orientation. These experimental results are compared with their theoretical counterparts obtained under the assumption of various reaction mechanisms, including collective interaction, heavy-particle stripping, a two-step mechanism that takes into account the delay in the interaction, and the mechanism of compound-nucleus formation.

Angular correlations in the inelastic-scattering process 24Mg(d, dγ)24Mg at Ed = 15.3 MeV

The double-differential cross sections for the reaction 24Mg(d, dγ1.369)24Mg at the projectiledeuteron energy of Ed = 15.3 MeV weremeasured for deuteron emission angles in the forward hemisphere. All even spin-tensor components of the density matrix for the 2+ state of the 24Mg nucleus at 1.369 MeV were reconstructed, and its orientation properties were determined. These experimental results were compared with the results of calculations based on various versions of the coupled-channel method.

Tensor polarization of 6Li(2.186 MeV, 3+) states produced in the reaction 9Be(p, α)6Li* at 40 MeV

A consistent theoretical analysis of polarization moments tkq is performed for the 6Li(2.186 MeV, 3+) states produced in the reaction 9Be(p, α)6Li*. The analysis is based on the distorted-wave Born approximation implemented for finite-range forces with allowance for spin-orbit interaction. The calculated tensor moments are compared with relevant experimental data. Particle-particle angular-correlation functions and tensor moments are shown to provide radically new information about reaction mechanisms and about nuclear interactions.

Determination of the final nucleus density matrix by measuring the angular correlation of charged particles and γ-quanta in various planes

Abstract The measurements of the angular correlation function of charged particles and γ-quanta emitted by a final even-even nucleus in state J f in different planes of γ-quantum ejection with respect to the reaction plane have been shown to offer the unique possibility of restoring all the even-rank components of the model-less density matrix in the transitions with J f = L ( L ) is the multipolarity of the γ-transition). The minimum number of planes in which the angular correlation function should be measured to restore the density matrix has been found. The formulas relating the components of the model-less density matrix to the polarization characteristics of an even-even nucleus in a given state have been obtained.

Structure of the spatial periphery of the isotopes 9,11 Li

The cross sections for the (t, p) reactions on the lithium isotopes 9,11Li were calculated within a theoretical approach based on employing integral equations of the four-body problem in the Alt—Grassberger-Sandhas formalism and the multiparticle shell model. This made it possible to determine the wave functions for the relative motion of various clusters and the nuclear core and to calculate, on their basis, the root-mean-square radii of nuclei of the isotopes 9,11Li and the spatial structure of their neutron periphery. It is shown that the 9Li nucleus has virtually no neutron halo. The 11Li nucleus is a Borromean halo nucleus. The two-neutron periphery of this nucleus manifests itself in both spatial configurations, a dineutron and a cigar one, the respective root-mean-square radii being large (about 6.5 to 6.9 fm).