Natalia Timofeyuk

Matter densities of 8B and 8Li in a microscopic cluster model and the proton-halo problem of 8B

Abstract The microscopic generator-coordinate method is generalized to the calculation of matter densities in light nuclei having a cluster structure. Multipole components of the proton and neutron densities are calculated for the bound states and narrow resonances of 8B and 8Li, as well as for transitions between these states. The proton densities of 8B and neutron densities of 8Li resemble each other. They suggest that the 2+ and 1+ states belong to a same band while 3+ is of a different nature. For the 2+ ground state, they display long tails which can be related to the slow decrease of the 7Be + p and 7Li + n relative wave functions. The tails of the 8B neutron and 8Li proton densities are much shorter and are sensitive to the choice of the model space. The thicknesses of the proton skin in 8B and of the neutron skin in 8Li are 0.42 and 0.35 fm respectively. The large value of the 8B quadrupole moment is explained without a notion of halo, although the tail of the density contributes by about 20%. Whether the tail of the 8B proton density can help determine the astrophysical S factor for the 7Be(p, γ)8B reaction at zero energy is also discussed.

Microscopic shell-model and cluster-model calculations of the 13C → 12C + n and 8B → 7Be + p vertex constants

Abstract Vertex constants are calculated with an effective nucleon-nucleon interaction and translation-invariant harmonic-oscillator shell-model wave functions consistently obtained with the same interaction. Such calculations are presented for the 13 C → 12 C + n vertex constant for which an accurate experimental value is available, and for the 8 B → 7 Be+p vertex constant, which is crucial for the determination of the 7Be(p,γ)8B reaction at astrophysical energies. Calculations confirm the strong sensitivity of vertex constants to the choice of the effective nucleon-nucleon interaction. A comparison of vertex constants obtained in the shell-model approach and in microscopic two- and three-cluster models with identical nucleon-nucleon potentials shows that the main contribution to the cluster-model vertex constants comes from the shell-model parts of the cluster-model wave functions. The reliability of model calculations of the 7Be(p,γ)8B astrophysical S factor is discussed.

Microscopic calculation of 17Ne and 17N properties in a three-cluster generator-coordinate method

Within the framework of a three-cluster generator-coordinate model, we study several properties of the 17Ne and 17N nuclei. The results of the calculations show that the 17Ne nucleus is well described by 15O+p+p configurations and that the mirror nucleus 17N can be treated as a 15N+n+n system, except in the asymptotic region. The simultaneous investigation of the 17Ne and 17N proton and neutron density distributions as well as of their radii does not support the existence of a proton halo in 17Ne. Allowed β-decay rates of 17Ne and 17N are calculated with α+13N and α+13C configurations representing the 17F and 17O negative-parity states. Both Fermi and Gamow-Teller matrix elements mainly depend on the properties of the final nuclei. We argue that the first-forbidden β-decays of the 17Ne and 17N ground states are independent of possible halo effects in the initial or final nuclei.

Vertex constants and the problem of the nucleon-nucleon potential in the generator coordinate method

Abstract The influence of the nucleon-nucleon potential on the value of the asymptotic normalization coefficient is investigated in the generator coordinate method. In the simple case of 17 O→ 16 O+n, all the potentials considered give overestimated values of the vertex constant.

Soft monopole mode in weakly bound light nuclei

Abstract A low-energy peak appears in the monopole strength functions of light nuclei composed of two weakly bound clusters. It exhausts a significant part of the soft-mode sum rules associated with the relative motion of the clusters, and a non-negligible part of the total sum rules. The peak properties mainly depend on the binding energy of the clusters. This effect is studied both in a simple potential model and in a multichannel microscopic model for nuclei of interest in astro-physics: 7 Li, 7 Be, 8 Li, 8 B and 12 N. The peak value of the 8 B monopole strength function is related to the astrophysical S -factor for the 7 Be(p, γ) 8 B reaction at zero energy.

decay of the halo nucleus

We suggest that in a halo nucleus, the -decay of a core may lead to the formation of a product nucleus with a similar cluster structure. As an example, the -decay of is considered. Calculations of the spectrum within the generator coordinate method (GCM) predict the existence of , and narrow states having a structure and located near the two-neutron threshold. As follows from the calculations, the -decay of to the state mainly occurs when two halo neutrons stay far away from the core.

Densities in a microscopic multicluster model and the proton-halo problem in 8B

Abstract The microscopic cluster model is extended to the calculation of matrix elements of density operators with an exact treatment of angular-momentum and center-of-mass effects. The 8 B and 8 Li densities are studied with the three-cluster version of the model. The similarity between these densities and an analysis of the quadrupole moment do not support the existence of a proton-halo in 8 B.

Threshold effects in the {sup 27}P((3/2){sup +}){yields}{sup 26}Si+p and {sup 27}Mg((3/2){sup +}){yields}{sup 26}Mg+n mirror decays and the stellar reaction {sup 26}Si(p,{gamma}){sup 27}P

Determination of the cross sections for stellar reaction {sup 26}Si(p,{gamma}){sup 27}P using information about the {sup 27}Mg((3/2){sup +}) mirror state can be influenced by mirror symmetry breaking in the {sup 27}P((3/2){sup +}){yields}{sup 26}Si+p and {sup 27}Mg((3/2){sup +}){yields}{sup 26}Mg+n amplitudes, which may arise as a threshold phenomenon due to the coupling to continuum. We study this effect within a multichannel microscopic cluster model. Our calculations confirm that such a possibility exists and suggest that the symmetry breaking in mirror decay amplitudes may become especially large if the mirror decay channels comprise only very small parts of the total wave functions. We use the estimates for mirror symmetry breaking in the {sup 27}P((3/2){sup +}){yields}{sup 26}Si+p and {sup 27}Mg((3/2){sup +}){yields}{sup 26}Mg+n amplitudes to derive the proton width for the astrophysically important {sup 27}P((3/2){sup +}) resonance basing on the asymptotic normalization coefficient for its mirror bound state {sup 27}Mg((3/2){sup +}) which we determine from the {sup 26}Mg(t,d){sup 27}Mg reaction.Determination of the cross sections for stellar reaction 26 Si(p, γ) 27 P using information about the 27 Mg(3/2 + ) mirror state can be influenced by mirror symmetry breaking in the 27 P(3/2 + ) → 26 Si + p and 27 Mg(3/2 + ) → 26 Mg + n amplitudes, which may arise as a threshold phenomenon due to the coupling to continuum. We study this effect within a multichannel microscopic cluster model. Our calculations confirm that such a possibility exists and suggest that the symmetry breaking in mirror decay amplitudes may become especially large if the mirror decay channels comprise only very small parts of the total wave functions. We use the estimates for mirror symmetry breaking in the 27 P(3/2 + ) → 26 Si + p and 27 Mg(3/2 + ) → 26 Mg + n amplitudes to derive the proton width for the astrophysically important 27 P(3/2 + ) resonance basing on the asymptotic normalization coefficient for its mirror bound state 27 Mg(3/2 + ) which we determine from the 26 Mg(t, d) 27 Mg reaction.