arXiv subjects

Filomena Nunes

Publications and source records attributed to Filomena Nunes.

6 recordsLinked to original sources

arXiv2025

Wavefunction-Based Emulation of Coupled-Channels Scattering with Non-Affinely Parametrized Interactions

Physics based emulators offer a fast and reliable replacement for an exact solution of the scattering problem in nuclear physics. Previous work developed a reduced-basis emulator for single-channel elastic scattering using an optical potential. Since many reactions of interest can be cast as a coupled-channel problem, the purpose of this work is to extend the RBM to a coupled-channel framework (CC-RBM). Although the framework derived is general, in this work we apply it to reactions where the Hamiltonian coupling term comes from assuming a rotational structure model for the target. From a set of training coupled-channel wavefunctions, we perform a singular value decomposition to obtain a reduced set of basis wavefunctions, and then solve the extended (Petrov-)Galerkin equations. In addition, the empirical interpolation method is used to expand the potentials. We apply the CC-RBM method to elastic and inelastic scattering of neutrons on 48Ca including a quadrupole coupling to populate the first 2+ state, and neutrons on 208Pb, including an octupole coupling to populate its first 3- state. We demonstrate that the CC-RBM calculated cross sections match those obtained using traditional finite-difference methods. We show that the CC-RBM results can reliably reproduce the nuclear scattering cross sections at different energy regimes. The computational accuracy versus time plots demonstrate that the CC-RBM method efficiently increases precision with increasing basis size. Most importantly, for the precisions required in reaction calculations (a percent on the cross section), we find the CC-RBM method offers roughly one and a half orders of magnitude gain in computational speed compared to the traditional coupled-channels solver. However, we also discuss how this scaling becomes less favorable, the larger the number of channels included in the coupled-channel set.

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arXiv2020

Properties of a separable representation of optical potentials

Background: Separable interactions have a long history in nuclear physics. In the last few years, separable expansions have been used to represent the optical potential between a nucleon (proton or neutron) and a target. Purpose: We explore the non-local properties of these separable optical potentials as well as their convergence behavior. Method: For a couple of cases, we use the generalized Ersnt-Shakin-Thaler scheme to generate separable interactions starting from local optical potentials. We study the variation of the interaction with energy range and rank. Results: We find that, overall the off-diagonal behavior of the converged separable interaction deviates from the Gaussian form assumed by Perey and Buck. However, in the region surrounding the maximum depth the Gaussian form works quite well. Focusing on this region, we study potentials describing neutron elastic scattering on $^{16}$O and $^{48}$Ca for beam energies in the range of $ E=$10-50 MeV and explore several measures of non-locality of the separable interactions. Conclusions: When the energy range considered for generating the separable interaction is $0\le E_{range}\le 50$ MeV, the resulting non-locality is large and target dependent. Contrarily, the nonlocality obtained including larger energy ranges in the separable procedure is independent of the target and other details of the original local potential. We find that, even when including in the expansion many support points with energy ranges $0\le E_{range}\le 2400$ MeV, the resulting potential retains non-local behavior. Connections with microscopic optical potentials as well as other transformations used in the nucleon-nucleon domain are made.

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arXiv2018

Non-local interactions in the $(d,p)$ surrogate method for $(n,γ)$ reactions

Single-neutron transfer reactions populating states in the continuum are interesting both for structure and astrophysics. In their description often global optical potentials are used for the nucleon-target interactions, and these interactions are typically local. In our work, we study the effects of nonlocality in $(d,p)$ reactions populating continuum states. This work is similar to that of [1] but now for transfer to the continuum. A theory for computing cross sections for inclusive processes $A (d,p) X$ was explored in [2]. Therein, local optical potentials were used to describe the nucleon-target effective interaction. The goal of the present work is to extend the theory developed in [2] to investigate the effects of including nonlocality in the effective interaction on the relevant reaction observables. We implement the R--matrix method to solve the non--local equations both for the nucleon wavefunctions and the propagator. We then apply the method to systematically study the inclusive process of $(d,p)$ on $^{16}$O, $^{40}$Ca, $^{48}$Ca and $^{208}$Pb at 10, 20 and 50 MeV. We compare the results obtained when non--local interactions are used with those obtained when local equivalent interactions are included. We find that nonlocality affects different pieces of the model in complex ways. Depending on the beam energy and the target, the non-elastic breakup can either increase or decrease. While the non-elastic transfer cross section for each final spin state can change considerably, the main prediction of the model [2], namely the shape of the spin distributions, remains largely unaltered by nonlocality. [1] L. J. Titus and F. M. Nunes. Testing the Perey effect. Phys. Rev. C, 89:034609, Mar 2014. [2] G. Potel, F. M. Nunes, and I. J. Thompson. Establishing a theory for deuteron-induced surrogate reactions. Phys. Rev. C, 92:034611, Sep 2015.

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arXiv2016

White Paper on Nuclear Astrophysics

This white paper informs the nuclear astrophysics community and funding agencies about the scientific directions and priorities of the field and provides input from this community for the 2015 Nuclear Science Long Range Plan. It summarizes the outcome of the nuclear astrophysics town meeting that was held on August 21-23, 2014 in College Station at the campus of Texas A&M University in preparation of the NSAC Nuclear Science Long Range Plan. It also reflects the outcome of an earlier town meeting of the nuclear astrophysics community organized by the Joint Institute for Nuclear Astrophysics (JINA) on October 9- 10, 2012 Detroit, Michigan, with the purpose of developing a vision for nuclear astrophysics in light of the recent NRC decadal surveys in nuclear physics (NP2010) and astronomy (ASTRO2010). The white paper is furthermore informed by the town meeting of the Association of Research at University Nuclear Accelerators (ARUNA) that took place at the University of Notre Dame on June 12-13, 2014. In summary we find that nuclear astrophysics is a modern and vibrant field addressing fundamental science questions at the intersection of nuclear physics and astrophysics. These questions relate to the origin of the elements, the nuclear engines that drive life and death of stars, and the properties of dense matter. A broad range of nuclear accelerator facilities, astronomical observatories, theory efforts, and computational capabilities are needed. With the developments outlined in this white paper, answers to long standing key questions are well within reach in the coming decade.

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arXiv2011

FRIB Theory Users Group Report: Joint ATLAS-HRIBF-NSCL-FRIB SuperUsers Meeting 18-20 August 2011

The FRIB (Facility for Rare Isotope Beams) Theory Users Group participated in the Joint ATLAS-HRIBF-NSCL-FRIB SuperUsers Meeting, hosted by Michigan State University August 18-20, 2011. Prior to the meeting a survey of the FRIB Theory Users Group was conducted to assess the health of the low-energy nuclear theory community and to identify perceived areas of need, in anticipation of the new demands on theory that will accompany FRIB. Meeting discussions focused on survey results and on possible responses. These discussions are summarized here.

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arXiv2003

Reaction models to probe the structure of light exotic nuclei

We review here theoretical models for describing various types of reactions involving light nuclei on the driplines. Structure features to be extracted from the analysis of such reaction data, as well as those that need to be incorporated in the reaction models for an adequate description of the processes, are also under focus. The major unsolved theoretical issues are discussed, along with some suggestions for future directions of the field.

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