arXiv subjects

R. Crespo

Publications and source records attributed to R. Crespo.

25 records · Page 2Linked to original sources

arXiv2002

Excitation modes of $^{11}$Li at E$_x \sim$ 1.3 MeV from proton collisions

The cross section for p-$^{11}$Li inelastic scattering at 68 MeV/u is evaluated using the Multiple Scattering expansion of the total Transition amplitude (MST) formalism, and compared with the breakup in the shakeoff approximation. Three different potential models for $^{11}$Li are used to calculate the $^{11}$Li(p,p$'$) continuum excitations, and all show peaks below 3 MeV of excitation energy, both in resonant and some non-resonant channels. In the most realistic model of $^{11}$Li, there is a strong dipole contribution associated with attractive but not a fully-fledged resonant phase shifts, and some evidence for a J$_{nn}^π= 0_2^+$ resonant contribution. These together form a pronounced peak at around 1--2 MeV excitation, in agreement with experiment, and this supports the use of the MST as an adequate formalism to study excited modes of two-neutron nuclear halos.

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arXiv2002

(^{8})B breakup in elastic and transfer reactions

We have studied the transfer reaction $^{14}$N($^7$Be,$^8$B)$^{13}$C at (\mathrm{E}_{\mathrm{lab}}=84) MeV, paying special attention to the effects of the coupling to the continuum in the exit channel. Using the Continuum Discretized Coupled Channels (CDCC) formalism, we find that these effects are important for the description of the elastic scattering observables. However, for the transfer process, differences between the predictions of the differential cross section within the Distorted Wave Born Approximation (DWBA) and the CDCC-Born approximation (CDCC-BA) are found to be negligible. This result supports the use of the DWBA method as a reliable tool to extract the $S_{17}(0)$ factor in this case.

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arXiv2001

Tensor representation of the nucleon-nucleon amplitude

Many approaches to nucleon-nucleus elastic and inelastic scattering are based on the use of the free-space nucleon-nucleon transition amplitude. In calculations where the full spin-dependence of this amplitude is needed, its use is more tractable when it is expressed in terms of irreducible tensor operators of the spins of the interacting nucleons. We present general formulae for this representation which is particularly useful for inelastic scattering studies involving spin-flip transitions of a target nucleon.

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arXiv2000

How unique is the Asymptotic Normalisation Coefficient (ANC) method?

The asymptotic normalisation coefficients (ANC) for the vertex $^{10}$B $\to$ $^9$Be + p is deduced from a set of different proton transfer reactions at different energies. This set should ensure the peripheral character of the reaction and availability of data for the elastic channels. The problems associated with the characteristics of the data and the analysis are discussed. For a subgroup of the set of available data, the uniqueness property of the extracted ANC is fulfilled. However, more measurements are needed before a definite conclusion can be drawn.

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arXiv1999

Probing halo nucleus structure through intermediate energy elastic scattering

This work addresses the question of precisely what features of few body models of halo nuclei are probed by elastic scattering on protons at high centre-of-mass energies. Our treatment is based on a multiple scattering expansion of the proton-projectile transition amplitude in a form which is well adapted to the weakly bound cluster picture of halo nuclei. In the specific case of $^{11}$Li scattering from protons at 800 MeV/u we show that because core recoil effects are significant, scattering crosssections can not, in general, be deduced from knowledge of the total matter density alone. We advocate that the optical potential concept for the scattering of halo nuclei on protons should be avoided and that the multiple scattering series for the full transition amplitude should be used instead.

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arXiv1999

Uncertainties in extracting $S_{17}$ from transfers

The transfer reaction $^7{\rm Be(d,n)}^8{\rm B}$ is modelled for the recent low experimental energies using the DWBA. Tests of the validity of the ANC method for the extraction of the $S_{17}$ astrophysical factor are performed. We show that a peripheral assumption is no longer accurate for center of mass energies greater than approximately 15 MeV. In all cases we found that core deformation and/or excitation effects to be small as long as the optical potential for the entrance channel is not strongly surface peaked. Multistep corrections to the DWBA were found to be small. We also emphasize that the lack of knowledge of the optical potentials for the entrance and outgoing channels induces severe uncertainties in the extracted $S_{17}$ astrophysical factor.

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arXiv1997

Effects of core degrees of freedom in the low energy $^7$Be$(p,γ)^8$B reaction

Quadrupole and octupole couplings to all bound states of $^7$Be are included in describing the capture reaction $^7$Be$(p,γ)^8$B. We verify, contrary to what we had initially stated, that the energy behaviour of the Astrophysical S-factor in the energy range of interest is not significantly sensitive to the core couplings in the g.s. (g.s.) of $^8$B although its overall magnitude is shifted. We find there is some sensitivity to the various deformation models when introducing the nuclear interaction for calculating the scattering states. The various deformation models predict quite different contributions to the quadrupole and magnetic moments but in order to compare with the data the quadrupole and magnetic moment of $^7$Be need to be measured.

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