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R. Alvarez-Rodriguez

Publications and source records attributed to R. Alvarez-Rodriguez.

At least 19 recordsLinked to original sources
arXiv2013

Nuclear astrophysics with radioactive ions at FAIR

The nucleosynthesis of elements beyond iron is dominated by neutron captures in the s and r processes. However, 32 stable, proton-rich isotopes cannot be formed during those processes, because they are shielded from the s-process flow and r-process beta-decay chains. These nuclei are attributed to the p and rp process. For all those processes, current research in nuclear astrophysics addresses the need for more precise reaction data involving radioactive isotopes. Depending on the particular reaction, direct or inverse kinematics, forward or time-reversed direction are investigated to determine or at least to constrain the desired reaction cross sections. The Facility for Antiproton and Ion Research (FAIR) will offer unique, unprecedented opportunities to investigate many of the important reactions. The high yield of radioactive isotopes, even far away from the valley of stability, allows the investigation of isotopes involved in processes as exotic as the r or rp processes.

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arXiv2012

Three-particle decays of light nuclei resonances

We have studied the three-particle decay of 12C, 9Be and 6Be resonances. These nuclei have been described as three-body systems by means of the complex scaled hyperspherical adiabatic expansion method. The short-distance part of the wave-function is responsible for the energies whereas the information related to the observable decay properties is contained at large distances, which must be computed accurately. As an illustration we show the results for the angular distribution of 9Be and 6Be resonances.

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arXiv2010

Relativistic description of 3He(e,e'p)2H

The Relativistic Distorted-Wave Impulse Approximation is used to describe the $^3$He($e,e^\prime p$)$^2$H process. We describe the $^3$He nucleus within the adiabatic hyperspherical expansion method with realistic nucleon-nucleon interactions. The overlap between the $^3$He and the deuteron wave functions can be accurately computed from a three-body calculation. The nucleons are described by solutions of the Dirac equation with scalar and vector (S-V) potentials. The wave function of the outgoing proton is obtained by solving the Dirac equation with a S-V optical potential fitted to elastic proton scattering data on the residual nucleus. Within this theoretical framework, we compute the cross section of the reaction and other observables like the transverse-longitudinal asymmetry, and compare them with the available experimental data measured at JLab.

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arXiv2010

Few-body decay and recombination in nuclear astrophysics

Three-body continuum problems are investigated for light nuclei of astrophysical relevance. We focus on three-body decays of resonances or recombination via resonances or the continuum background. The concepts of widths, decay mechanisms and dynamic evolution are discussed. We also discuss results for the triple $α$ decay in connection with $2^+$ resonances and density and temperature dependence rates of recombination into light nuclei from $α$-particles and neutrons.

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arXiv2010

Momentum distributions from three-body decaying 9Be and 9B resonances

The complex-rotated hyperspherical adiabatic method is used to study the decay of low-lying $^9$Be and $^9$B resonances into $α$, $α$ and $n$ or $p$. We consider six low-lying resonances of $^9$Be ($1/2^\pm$, $3/2^\pm$ and $5/2^\pm$) and one resonance of $^9$B ($5/2^-$) to compare with. The properties of the resonances at large distances are decisive for the momentum distributions of the three decaying fragments. Systematic detailed energy correlations of Dalitz plots are presented.

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arXiv2010

Structure and three-body decay of $^9$Be resonances

The complex-rotated hyperspherical adiabatic method is used to study the decay of low-lying $^9$Be resonances into one neutron and two $α$-particles. We investigate the six resonances above the break-up threshold and below 6 MeV: $1/2^\pm$, $3/2^\pm$ and $5/2^\pm$. The short-distance properties of each resonance are studied, and the different angular momentum and parity configurations of the $^8$Be and $^5$He two-body substructures are determined. We compute the branching ratio for sequential decay via the $^8$Be ground state which qualitatively is consistent with measurements. We extract the momentum distributions after decay directly into the three-body continuum from the large-distance asymptotic structures. The kinematically complete results are presented as Dalitz plots as well as projections on given neutron and $α$-energy. The distributions are discussed and in most cases found to agree with available experimental data.

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arXiv2008

Three-body decays: structure, decay mechanism and fragment properties

We discuss the three-body decay mechanisms of many-body resonances. R-matrix sequential description is compared with full Faddeev computation. The role of the angular momentum and boson symmetries is also studied. As an illustration we show the computed $α$-particle energy distribution after the decay of 12C(1^+) resonance at 12.7 MeV.

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arXiv2008

Single and low-lying states dominance in two-neutrino double-beta decay

A theoretical analysis of the single-state dominance hypothesis for the two-neutrino double-beta decay rates is performed on the examples of the double-beta decays of 100Mo, 116Cd, and 128Te. We also test the validity of an extended low-lying-state dominance that takes into account the contributions of the low-lying excited states in the intermediate nucleus to the double-beta decay rates. This study has been accomplished for all the double-beta emitters for which we have experimental information on their half-lives. The theoretical framework is a proton-neutron quasiparticle random-phase approximation based on a deformed Skyrme Hartree-Fock mean field with pairing correlations. Our calculations indicate that there are not clear evidences for single- or low-lying-state dominance in the two-neutrino double-beta decay. Finally, we investigate the single electron energy distributions of the outgoing electrons in the double-beta decay processes with an exact treatment of the energy denominators, which could help to a more comprehensive analysis of NEMO-3 data.

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arXiv2008

alpha particle momentum distributions from 12C decaying resonances

The computed $α$ particle momentum distributions from the decay of low-lying $^{12}$C resonances are shown. The wave function of the decaying fragments is computed by means of the complex scaled hyperspherical adiabatic expansion method. The large-distance part of the wave functions is crucial and has to be accurately calculated. We discuss energy distributions, angular distributions and Dalitz plots for the $4^+$, $1^+$ and $4^-$ states of $^{12}$C.

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arXiv2008

Momentum distributions of $α$-particles from decaying low-lying $^{12}$C-resonances

The complex scaled hyperspherical adiabatic expansion method is used to compute momentum and energy distributions of the three $α$-particles emerging from the decay of low-lying $^{12}$C-resonances. The large distance continuum properties of the wave functions are crucial and must be accurately calculated. We discuss separately decays of natural parity states: two $0^+$, one $1^{-}$, three $2^+$, one $3^-$, two $4^+$, one $6^+$, and one of each of unnatural parity, $1^{+}$, $2^-$, $3^+$, $4^-$. The lowest natural parity state of each $J^π$ decays predominantly sequentially via the $^{8}$Be ground state whereas other states including unnatural parity states predominantly decay directly to the continuum. We present Dalitz plots and systematic detailed momentum correlations of the emerging $α$-particles.

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arXiv2008

Distinction between sequential and direct three-body decays

We discuss the three-body decay mechanisms of many-body resonances. Sequential decays proceed via two-body configurations after emission of the third particle. In direct decay all three particles leave simultaneously their interaction regions. The intermediate paths within the interaction regions are not observables and only accessible through models. The momentum distributions carry, apart from polarization, all possible information about decay modes and resonance structure. In this context we discuss detailed results for the decay of the $^{9}$Be($5/2^-$) resonance.

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arXiv2007

Decay of low-lying 12C resonances within a 3alpha cluster model

We compute energy distributions of three $α$-particles emerging from the decay of $^{12}$C resonances by means of the hyperspherical adiabatic expansion method combined with complex scaling. The large distance continuum properties of the wave functions are crucial and must be accurately calculated. The substantial changes from small to large distances determine the decay mechanisms. We illustrate by computing the energy distributions from decays of the $1^{+}$ and $3^-$-resonances in $^{12}$C. These states are dominated by direct and sequential decays into the three-body continuum respectively.

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arXiv2007

Triple charged-particle decays of resonances illustrated by 12C-states

The hyperspherical adiabatic expansion is combined with complex scaling and used to calculate the energy distributions of the particles arising from three-body decaying low-lying $^{12}$C resonances. The large distance continuum properties of the wavefunctions are crucial and must be accurately calculated. The substantial changes from small to large distances determine the decay mechanisms. We illustrate by computing the energy distributions from decays of the $1^{-}$, $2^-$ and $4^{-}$-resonances in $^{12}$C. These states are dominated by sequential ($1^-$), through the $^{8}$Be ground state, and direct ($2^-$, $4^-$) decays.

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arXiv2007

Energy distributions from three-body decaying many-body resonances

We compute energy distributions of three particles emerging from decaying many-body resonances. We reproduce the measured energy distributions from decays of two archetypal states chosen as the lowest $0^{+}$ and $1^{+}$-resonances in $^{12}$C populated in $β$-decays. These states are dominated by sequential, through the $^{8}$Be ground state, and direct decays, respectively. These decay mechanisms are reflected in the ``dynamic'' evolution from small, cluster or shell-model states, to large distances, where the coordinate or momentum space continuum wavefunctions are accurately computed.

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arXiv2007

Structure of low-lying 12C-resonances

The hyperspherical adiabatic expansion is combined with complex scaling and used to calculate low-lying nuclear resonances of $^{12}$C in the $3α$-model. We use Ali-Bodmer potentials and compare results for other potentials $α-α$ with similar $^{8}$Be-properties. A three-body potential is used to adjust the $^{12}$C-resonance positions to desired values extending the applicability of the method to many-body systems decaying into three $α$-particles. For natural choices of three-body potentials we find 14 resonances below the proton separation threshold, i.e. two $0^{+}$, three $2^{+}$, two $4^{+}$, one of each of $1^{\pm}$, $2^{-}$, $3^{\pm}$, $4^{-}$, and $6^{+}$. The partial wave decomposition of each resonance is calculated as function of hyperradius. Strong variation is found from small to large distance. Connection to previous experimental and theoretical results are discussed and agreements as well as disagreements are emphasized.

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arXiv2006

Gamow-Teller strength distributions in Xe isotopes

The energy distributions of the Gamow-Teller strength are studied for even-even Xe isotopes with mass numbers from 124 to 142. A self-consistent microscopic formalism is used to generate the single particle basis, using a deformed Skyrme Hartree-Fock mean field with pairing correlations in BCS approximation. The Gamow-Teller transitions are obtained within a quasiparticle random phase approximation (QRPA) approach using a residual spin-isospin interaction in the particle-hole and particle-particle channels. We then discuss the pairing BCS treatment and the determination of the ph and pp residual interaction coupling constants. We study the GT+ and GT- strength distributions for the equilibrium nuclear shapes, which are an essential information for studies of charge-exchange reactions and double-beta processes involving these isotopes.

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arXiv2006

Effect of deformation on two-neutrino double beta decay matrix elements

We study the effect of deformation on the two-neutrino double beta decay for ground state to ground state transitions in all the nuclei whose half-lives have been measured. Our theoretical framework is a deformed QRPA based in Woods-Saxon or Hartree-Fock mean fields. We are able to reproduce at the same time the main characteristics of the two single beta branches, as well as the double beta matrix elements. We find a suppression of the double beta matrix element with respect to the spherical case when the parent and daughter nuclei have different deformations.

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