arXiv subjects

Yu. A. Lashko

Publications and source records attributed to Yu. A. Lashko.

13 recordsLinked to original sources

arXiv2025

Many-channel microscopic cluster model of $^{8}$Be: S-factors

We investigate low--energy astrophysical $S$ factors for reactions proceeding through the $^{8}$Be compound system with entrance channels $p+{}^{7}$Li, $n+{}^{7}$Be, and $d+{}^{6}$Li. Using the same microscopic many--channel three--cluster framework as in our previous study of the high--lying $^{8}$Be spectrum, we calculate $S(E)$ for $^{7}$Li($p,\alpha)^{4}$He, $^{7}$Be($n,\alpha)^{4}$He, $^{7}$Be($n,p)^{7}$Li, $^{6}$Li($d,\alpha)^{4}$He, $^{6}$Li($d,p)^{7}$Li, and $^{6}$Li($d,n)^{7}$Be in the energy range relevant for primordial and stellar nucleosynthesis. For the mirror pair $^{7}$Li($p,\alpha)^{4}$He / $^{7}$Be($n,\alpha)^{4}$He and for $^{7}$Be($n,p)^{7}$Li the calculated $S$ factors reproduce both the absolute scale and the low--energy trends of the experimental data within their quoted uncertainties, whereas the absolute $S$ factors for the deuteron--induced channels on $^{6}$Li are underestimated at low energy, consistent with the shifted $^{6}$Li+$d$ threshold and the absence of a broad subthreshold $2^{+}$ structure in the present implementation. A partial--wave analysis identifies the dominant $J^{\pi}$ contributions in each channel and relates them to specific $^{8}$Be resonances, while demonstrating that cluster polarization, previously shown to be crucial for the $^{8}$Be spectrum, is likewise essential for the normalization and energy dependence of several $S$ factors. Evaluating $S(E)$ at appropriate Gamow energies, we obtain a hierarchy of reaction channels that quantifies the relative importance of neutron-- and deuteron--induced processes for the production and destruction of $^{7}$Li and $^{7}$Be.

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arXiv2025

Many-channel microscopic cluster model of $^{8}$Be. I. Formation of high-energy resonance states

The nature and structure of high-energy resonance states in $^{8}$Be, located just below and above the $p+^{7}$Li threshold, are investigated in detail. A microscopic many-cluster and many-channel model is employed to study the formation of these resonances. This model includes three distinct three-cluster configurations: $^{4}$He+$^{3}$H+$p$, $^{4}$He+$^{3}$He+$n$, and $^{4}$He+$d$+$d$, enabling a comprehensive treatment of all major binary decay channels of $^{8}$Be, namely $^{4}$He+$^{4}$He, $p+^{7}$Li, $n+^{7}$Be, and $d+^{6}$Li. The primary focus of our analysis is the structure and dominant decay channels of the twin $1^{+}$, $2^{+}$, $3^{+}$, and $4^{+}$ resonance states. Additionally, we propose and implement a model to clarify how the $2^{+}$ resonance states lying below the $p+^{7}$Li threshold are formed. We demonstrate that these resonances are Feshbach-type states arising due to coupling of the open $^{4}$He+$^{4}$He channel with the closed channels $p+^{7}$Li, $n+^{7}$Be, and $d+^{6}$Li at these energies. Overall, the present approach provides a realistic description of the experimentally observed resonance spectrum near the $^7$Li+$p$ decay threshold, including negative-parity states $1^-$ and $2^-$. Our results are consistent with other microscopic calculations but offer more detailed insight into the internal structure and decay pathways of these resonances.

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arXiv2024

Theoretical analysis of the reactions induced by interaction of $^{6}$Li with nuclei $^{3}$H and $^{3}$He

We determine cross sections and astrophysical S-factors of the reactions generated in collisions between $^{6}$Li and $^{3}$H, and $^{6}$Li and $^{3}$He. A microscopic three-cluster model is employed to study the dynamics of reactions occurring in the mirror nuclei $^{9}$Be and $^{9}$B. In a previous study [Phys. Rev. C {\bf 109}, 045803 (2024)], this model was successfully applied to investigate the resonance structure of $^{9}$Be and $^{9}$B, as well as reactions induced by the interaction of deuterons with $^7$Li and $^7$Be. A fairly good agreement between theoretical results and available experimental data was achieved. To the best of our knowledge, appropriate experimental data for the astrophysical S-factors of the reactions generated by the interaction of $^6$Li with $^3$H and with $^3$He are currently unavailable. Thus, our results can serve as a guideline for future experimental efforts.

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arXiv2023

Many-channel microscopic theory of resonance states and scattering processes in $^{9}$Be and $^{9}$B

We present a many-channel microscopic model that extends the three-cluster model previously formulated in \cite{2009NPA...V37}. This extended model incorporates multiple three-cluster configurations, which are subsequently reduced to a comprehensive set of binary channels. These channels dictate the dynamics of various nuclear processes and the resonance structure of a compound nucleus across a broad energy spectrum. The application of this model focuses on investigating the nature of high-energy resonance states in $^{9}$Be and $^{9}$B, as well as the astrophysical $S$-factors for the reactions $^{7}$Li$(d,n)αα$ and $^{7}$Be$(d,p)αα$, particularly pertinent to the cosmological lithium problem. Parameterization of resonance states is performed across a wide range of total angular momenta and includes states of both positive and negative parity. Dominant decay channels are identified for each resonance state. Detailed analysis of astrophysical $S$ factors resulting from deuteron interactions with $^{7}$Li and $^{7}$Be is conducted within an energy range from zero to 2 MeV. Four exit channels in $^{9}$Be ($^{8}$Be($0^{+}$)$+n$, $^{8}$Be($2^{+}$)$+n$, $^{5}$He($3/2^{-}$)$+α$, $^{5}$He($1/2^{-}$)$+α$) and four in $^{9}$B ($^{8}$Be($0^{+}$)$+p$, $^{8}$Be($2^{+}$)$+p$, $^{5}$Li($3/2^{-}$)$+α$, $^{5}$Li($1/2^{-}$)$+α$) are considered. A clear hierarchy of reactions is established for the energy range 0$\leq E<$1.0 MeV. Notably, reactions $^{7}$Li$+d=^{8}$Be($0^{+}$)$+n$ and $^{7}$Be$+d=^{8}$Be($0^{+}$)$+p$ substantially dominate over all other reactions within this energy range. The model satisfactory describes the experimental astrophysical $S$ factors for these reactions.

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arXiv2021

Structure of the ground and excited states in $_Λ^{9}$Be nucleus

We investigate properties of bound and resonance states in the $_Λ^{9}$Be nucleus. To reveal the nature of these states, we use a three-cluster $2α+Λ$ microscopic model. The model incorporates Gaussian and oscillator basis functions and reduces a three-cluster Schrödinger equation to a two-body like many-channel problem with the two-cluster subsystems ($_Λ^{5}$He and $^8$Be) being in a bound or a pseudo-bound state. Influence of the cluster polarization on the energy and widths of resonance states in $_Λ^{9}$Be and on elastic and inelastic $_Λ^{5}$He+$α$ scattering is analyzed.

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arXiv2019

Properties of a potential energy matrix in oscillator basis

Matrix elements of potential energy are examined in detail. We consider a model problem - a particle in a central potential. The most popular forms of central potential are taken up, namely, square-well potential, Gaussian, Yukawa and exponential potentials. We study eigenvalues and eigenfunctions of the potential energy matrix constructed with oscillator functions. It is demonstrated that eigenvalues coincide with the potential energy in coordinate space at some specific discrete points. We establish approximate values for these points. It is also shown that the eigenfunctions of the potential energy matrix are the expansion coefficients of the spherical Bessel functions in a harmonic oscillator basis. We also demonstrate a close relation between the separable approximation and $L^{2}$ basis (J-matrix) method for the quantum theory of scattering.

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arXiv2019

Influence of the Pauli principle on two-cluster potential energy

We study effects of the Pauli principle on the potential energy of two-cluster systems. The object of the investigation is the lightest nuclei of p-shell with a dominant $α$-cluster channel. For this aim we construct matrix elements of two-cluster potential energy between cluster oscillator functions with and without full antisymmetrization. Eigenvalues and eigenfunctions of the potential energy matrix are studied in detail. Eigenfunctions of the potential energy operator are presented in oscillator, coordinate and momentum spaces. We demonstrate that the Pauli principle affects more strongly the eigenfunctions than the eigenvalues of the matrix and leads to the formation of resonance and trapped states.

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arXiv2019

How the antisymmetrization affects a cluster-cluster interaction: two-cluster systems

We study effects of the antisymmetrization on the potential energy of two-cluster systems. The object of the investigation is the lightest nuclei of p-shell with a dominant alpha-cluster channel. For this aim we construct matrix elements of two-cluster potential energy between cluster oscillator functions with and without full antisymmetrization. Eigenvalues and eigenfunctions of the potential energy matrix are studied in detail. Eigenfunctions of the potential energy operator are presented in oscillator, coordinate and momentum spaces. We demonstrate that the Pauli principle affects more strongly the eigenfunctions than the eigenvalues of the matrix and leads to the formation of resonance and trapped states.

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arXiv2018

Two- and three-cluster decays of light nuclei with the hyperspherical harmonics

We consider a set of three-cluster systems ($^{4}$He, $^{7}$Li, $^{7}$Be, $^{8}$Be, $^{10}$Be) within a microscopic model which involves the hyperspherical harmonics to represent intercluster motion. We selected such three-cluster systems which have at least one binary channel. Our aim is to study whether the hyperspherical harmonics are able and under what conditions to describe two-body channel(s) (nondemocratic motion) or they are suitable for describing three-cluster continuum only (democratic motion). It is demonstrated that a rather restricted number of the hyperspherical harmonics allows us to describe bound states and scattering states in two-body continuum for a three-cluster system.

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arXiv2015

Dynamics of two-cluster systems in phase space

We present a phase-space representation of quantum state vectors for two-cluster systems. Density distributions in the Fock--Bargmann space are constructed for bound and resonance states of $^{6,7}$Li and $^{7,8}$Be, provided that all these nuclei are treated within a microscopic two-cluster model. The density distribution in the phase space is compared with those in the coordinate and momentum representations. Bound states realize themselves in a compact area of the phase space, as also do narrow resonance states. We establish the quantitative boundaries of this region in the phase space for the nuclei under consideration. Quantum trajectories are demonstrated to approach their classical limit with increasing energy.

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arXiv2003

6He+6He clustering of 12Be in a microscopic algebraic approach

The norm kernel of the A=12 system composed of two 6He clusters, and the L=0 basis functions (in the SU(3) and angular momentum-coupled schemes) are analytically obtained in the Fock--Bargmann space. The norm kernel has a diagonal form in the former basis, but the asymptotic conditions are naturally defined in the latter one. The system is a good illustration for the method of projection of the norm kernel to the basis functions in the presence of SU(3) degeneracy that was proposed by the authors. The coupled-channel problem is considered in the Algebraic Version of the resonating-group method, with the multiple decay thresholds being properly accounted for. The structure of the ground state of 12Be obtained in the approximation of zero-range nuclear force is compared with the shell-model predictions. In the continuum part of the spectrum, the S-matrix is constructed, the asymptotic normalization coefficients are deduced and their energy dependence is analyzed.

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arXiv2002

Norm kernels and the closeness relation for Pauli-allowed basis functions

The norm kernel of the generator-coordinate method is shown to be a symmetric kernel of an integral equation with eigenfunctions defined in the Fock--Bargmann space and forming a complete set of orthonormalized states (classified with the use of SU(3) symmetry indices) satisfying the Pauli exclusion principle. This interpretation allows to develop a method which, even in the presence of the SU(3) degeneracy, provides for a consistent way to introduce additional quantum numbers for the classification of the basis states. In order to set the asymptotic boundary conditions for the expansion coefficients of a wave function in the SU(3) basis, a complementary basis of functions with partial angular momenta as good quantum numbers is needed. Norm kernels of the binary systems 6He+p, 6He+n, 6He+4He, and 8He+4He are considered in detail.

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arXiv2001

Spectrum of 10Be in the basis of the leading representation of the SU(3) group

A realization of the approximation of the SU(3) leading representation with the microscopic Hamiltonian and a nucleon-nucleon interaction is presented in detail. An effective Hamiltonian reproducing results of calculations with some known potentials is constructed. It is shown that its structure is quite similar to that of the triaxial rotator, and the wave functions in the Elliott's scheme are linear combinations of Wigner's D-functions although they should be properly normalized.

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