# Dr Alexis Diaz-Torres

### Biography

I grew up in rural Cuba and competed in the Cuban Team at the 19th International Junior Physics Olympiad in Bad Ischl (Austria) in 1988. I studied Physics at KLTE (Hungary), InSTEC (Cuba) and received my PhD in Theoretical Nuclear Physics from the Justus-Liebig University in Giessen (Germany) in 2000. After that, I gained research experience at Surrey (UK), Goethe University in Frankfurt am Main (Germany), ANU (Australia) and the European Centre for Theoretical Studies in Nuclear Physics and Related Areas (Italy). I was appointed as a Senior Lecturer at Surrey in 2016, and promoted to Reader in 2020 with effect from 1 January 2021. My scientific interests include the physics of nuclear reactions, which is crucial for a better comprehension of the creation of the chemical elements in the universe and is therefore at the core of science programmes at new generation nuclear research facilities. I use quantum dynamical methods for describing nuclear reactions and associated measurements. I am a fellow of the Institute of Physics and the Higher Education Academy.

### Areas of specialism

### University roles and responsibilities

- Leader of the Maths & Physics Programmes
- Module Leader of Mathematical and Computational Physics (PHY1038)
- Module Leader of Maths and Physics MPhys Project (PHYM060)

### Research

### Research interests

These include Quantum Dynamics; Theory of Open Quantum Systems; Nuclear Theory; Few-Body & Many-Body Physics; Super-Heavy Elements Research; Nuclear Molecules; Nuclear Astrophysics.

### Research projects

This project will be founded by a Leverhulme Research Project Grant (£200k) over 36 months from 1 September 2020. New basic studies of chemical element creation in the universe will be carried out. I will set up a team [1 PDRA, 1 PhD student and myself in collaboration with Prof. Peter Saalfrank (University of Potsdam, Germany), Dr. Adriana Palffy (Max-Planck Institute for Nuclear Physics, Germany) and Dr. Gilbert Gosselin (CEA, France)] to develop a new model based on the theory of open quantum systems, allowing us to characterise quantum mechanically fusion reactions of atomic nuclei in dense stellar plasma for the first time. This will expand our knowledge of nucleosynthesis, as accelerator experiments on Earth cannot probe important effects on element creation caused by stellar plasma. The new model will provide a breakthrough in our understanding of the origin of the elements and, therefore, of life itself.

- *OWL: a code for the two-center shell model with spherical Woods-Saxon potentials*, A. Diaz-Torres, Computer Physics Communications **224** (2018) 381-386.

- *PLATYPUS: a code for reaction dynamics of weakly bound nuclei at near-barrier energies within a classical dynamical model*, A. Diaz-Torres, Computer Physics Communications **182** (2011) 1100-1104.

### Supervision

## Postgraduate research supervision

**Former PhD students (as primary supervisor):**

- Dr. Maddalena Boselli (ECT* & University of Trento). The thesis can be downloaded from http://eprints-phd.biblio.unitn.it/1852/

**Current PhD students (as primary supervisor):**

- Terence Vockerodt

- Rafael Van den Bossche

- Iain Lee

Terry and Rafael are working on exciting research projects about reaction dynamics of complex atomic nuclei in slow collisions. They already have peer-reviewed articles, for instance, Vockerodt & Diaz-Torres, PRC 100 (2019) 034606; Van den Bossche & Diaz-Torres, PRC 100 (2019) 044604. Iain is working on a research project about modelling fusion of atomic nuclei in dense stellar plasma using the theory of open quantum systems.

### My teaching

I am currently lecturing the Mathematical and Computational Physics module (PHY1038) in Semester 2 for year-1 students. I am also teaching Computer Lab of the Essential Maths module (PHY1034) in Semester 1 for year-1 students as well as Computer Lab Projects of the modules PHY2071/73 in Semester 2 for year-2 students. Other teaching activities include small group tutorials in Semester 1 and the supervision of both BSc Final Year Projects (PHY3002) in Semester 2 and MSc Physics Dissertation Projects (PHYM021) over the summer.

Linking teaching and research is one of my main priorities. For instance, doing this, two BSc final year projects turned into peer-reviewed articles: e.g., Diaz-Torres & Quraishi, PRC 97 (2018) 024611: Lee & Diaz-Torres, JPG 47 (2020) 015101.

### My publications

### Highlights

Most of my publications can be downloaded from ResearchGate (https://www.researchgate.net/profile/Alexis_Diaz-Torres2). Please find below some ePrints at Surrey. A complete list of my publications can be found on ORCID (https://orcid.org/0000-0001-6234-9353).

### Publications

presented. The model is based on two spherical Wood-Saxon potentials and

the potential separable expansion method. It describes the single-particle

motion in low-energy nuclear collisions, and is useful for characterizing a

broad range of phenomena from fusion to nuclear molecular structures.

wave-packet dynamics,Physical Review C 97 (5) 055802 pp. 055802-1 - 055802-8 American Physical Society

Low-energy collisions are described in the body-fixed reference frame using wave-packet dynamics

within a nuclear molecular picture. A collective Hamiltonian drives the time propagation of the

wave-packet through the collective potential-energy landscape. The fusion imaginary potential for

specific dinuclear configurations is crucial for understanding the appearance of resonances in the

fusion cross section. The theoretical sub-barrier fusion cross sections explain some observed resonant

structures in the astrophysical S-factor. These cross sections monotonically decline towards

stellar energies. The structures in the data that are not explained are possibly due to cluster effects

in the nuclear molecule, which are to be included in the present approach.

A classical dynamical model that treats breakup stochastically is presented for low energy reactions of weakly bound nuclei. The three-dimensional model allows a consistent calculation of breakup, incomplete, and complete fusion cross sections. The model is assessed by comparing the breakup observables with continuum discretized coupled-channel quantum mechanical predictions, which are found to be in reasonable agreement. Through the model, it is demonstrated that the breakup probability of the projectile as a function of its distance from the target is of primary importance for understanding complete and incomplete fusion at energies near the Coulomb barrier.

A classical dynamical model that treats breakup stochastically is presented for low energy reactions of weakly bound nuclei. The three-dimensional model allows a consistent calculation of breakup, incomplete, and complete fusion cross sections. The model is assessed by comparing the breakup observables with continuum discretized coupled-channel quantum mechanical predictions, which are found to be in reasonable agreement. Through the model, it is demonstrated that the breakup probability of the projectile as a function of its distance from the target is of primary importance for understanding complete and incomplete fusion at energies near the Coulomb barrier.

_{12}C+

_{12}C is reported. Lowenergy

collisions are described in the body-fixed reference frame using wave-packet dynamics within a

nuclear molecular picture. In contrast to conventional methods, such as the potential model and the

coupled-channels approach, these new calculations reveal three resonant structures in the S-factor,

explaining some structures observed in the data. The structures in the data that are not explained are

possibly due to cluster effects in the nuclear molecule, which need to be included in the new approach.

^{184}Re from the

^{9}Be +

^{181}Ta reaction,Physical Review C 99 (5) 054617 pp. 054617-1 - 054617-6 American Physical Society

^{184}R in the incomplete fusion of the

^{9}Be +

^{181}Ta system were measured at

energies around the Coulomb barrier, using online activation followed by offline ³-ray spectroscopy method.

The PLATYPUS code that is based on a classical dynamical model is employed to analyze the measurements. By

applying a phenomenological approach, model calculation managed to fairly reproduce the order of magnitude

of the yield ratios at above barrier energies. Through the study, it is shown that the PLATYPUS code in conjunction

with a phenomenological analysis can provide a reasonable explanation of isomer yield ratios resulted from

incomplete fusion of weakly bound projectiles.

^{16}O and

^{154}Sm with excited states at Coulomb energies using a theoretical dynamical model. The two-body Schrödinger equation is solved using the time-dependent wave-packet coupled-channels method. The wave function of the collective motion and excitations are visualized in both position and momentum space, providing a detailed mechanism of the reaction. We benchmark our calculations of the energy-resolved fusion transmission coefficients with those from the time-independent coupled-channels method. The present results are in good agreement with the time-independent results for a wide range of energies and angular momenta, demonstrating both the reliability of the quantum wave-packet dynamical approach for fusion and its usefulness for providing additional insights into fusion dynamics.

intermediate mass target 89Y has been extended to energies greater than the fusion barrier, by implementing

off-line characteristic ³ -ray detection techniques. The available experimental data for the 9Be + 89Y reaction

system were compared with the theoretical predictions, using the PLATYPUS code that is based on a classical

dynamical model. By introducing the breakup probability that deduced in the literature from the fitting of the

experimental data, the model managed to reproduce the CF cross sections of 9Be beam with targets of different

atomic mass. Through the study, it is revealed that the extended CF excitation function for the 9Be + 89Y system

is consistent with the systematical behavior that the prompt-breakup probability at above-barrier energies is

roughly independent of the target in the reactions induced by the same weakly bound projectiles.

increased by more than a factor of ten, resulting in 4000

presently known isotopes of 118 elements. This considerable progress we owe to the discovery of new reaction types along with the development of powerful accelerators and experimental techniques for separation and identification of reaction products. Model predictions indicate that still about

4000 further nuclides are waiting for their discovery. The vastest unexplored territory is located on the neutron-rich side in the upper half of the chart of nuclides and hides the answers to some of the most fundamental questions of nuclear physics like the limits of nuclear stability, element

synthesis in the universe or stellar evolution. The access to these nuclei is presently limited by available beam intensities and/or the lack of appropriate methods for their production and identification. The latter concerns particularly new neutron-rich isotopes of transuranium and superheavy

elements. To extend this area, the hope is presently based on multinucleon transfer reactions and on the application of fusion reactions with radioactive ion beams. But how promising are these approaches? Based on a survey of present-day knowledge, we will treat the questions where we currently are

on our journey towards new territory on the chart of nuclides, how the chances are to gain new territory in the future and which challenges we will have to face.