Junior Researcher Webinars 2021 (past)

Monday, 13/12 (16h, CET)
Ab initio computation of Sn and Xe charge densities 
Pierre ARTHUIS (PostDoc at the TU Darmstadt)

Over the past decade, ab initio methods for finite nuclei have expanded their reach towards larger masses and gained both in precision and complexity. While limitations of the interactions and convergence issues had for long restricted the relevance of results in and to the mid-mass domain, those restrictions are now being lifted. In this seminar, after introducing ab initio methods and their recent progress [1], I will detail how we investigated nuclei at the limit of the ab initio mass range in a recent study on Sn and Xe isotopes [2] that focused on charge radii and densities and reproduced scattering results from the SCRIT experiment [3].

References :
[1] H. Hergert, Front. Phys. 8, 379 (2020), https://doi.org/10.3389/fphy.2020.00379
[2] P. Arthuis, C. Barbieri, M. Vorabbi and P. Finelli, Phys. Rev. Lett. 125 18, 182501 (2020),https://doi.org/10.1103/PhysRevLett.125.182501
[3] K. Tsukada, A. Enokizono, T. Ohnishi, K. Adachi, T. Fujita, M. Hara, M. Hori, T. Hori, S. Ichikawa, K. Kurita, K. Matsuda, T. Suda, T. Tamae, M. Togasaki, M. Wakasugi, M. Watanabe, and K. Yamada, Phys. Rev. Lett. 118, 262501 (2017),https://doi.org/10.1103/PhysRevLett.118.262501

Monday, 29/11 (16h, CET)
The NECTAR project and solar cell studies 
Michele SGUAZZIN (PhD student in CENBG-Bordeaux)

Obtaining reliable cross sections for neutron-induced reactions on unstable nuclei is a highly important task and a major challenge. These data are essential for understanding the synthesis of heavy elements in stars and for applications in nuclear technology. However, their measurement is very complicated as both projectile and target are radioactive. The NECTAR (NuclEar reaCTions At storage Rings) project aims to circumvent these problems by using the surrogate reaction method in inverse kinematics., where the nucleus formed in the neutron-induced reaction of interest is produced by a reaction (typically a transfer or an inelastic-scattering reaction) involving a radioactive heavy-ion beam and a stable, light target nucleus. The probabilities as a function of the compound-nucleus excitation energy for gamma- ray emission, neutron emission and fission, which can be measured with the surrogate reaction, are particularly useful to constrain model parameters and to inform more accurate predictions of neutron- induced reaction cross sections.

The objective of the NECTAR project is to combine surrogate reactions with the unique and largely unexplored possibilities at heavy-ion storage rings, in order to measure the decay probabilities with unrivaled accuracy. However, the ultra-high vacuum (UHV) conditions inside the storage rings pose severe constraints to in-ring detection systems.

In this contribution, we will present the conceptual idea of the setup, which will be developed within NECTAR to measure for the first time simultaneously the fission, neutron and gamma-ray emission probabilities at the storage rings of the GSI/FAIR facility. We will also discuss the technical developments that are being carried out towards these measurements, focusing on the use of solar cells to build an in- ring detection system for fission fragments.

In this latter frame, we will explain why solar cells are an appealing alternative to silicon detectors for the detection of heavy ions, and we will present the main results of recent experiments where we have investigated the radiation hardness, energy and time resolution, and UHV compatibility of solar cells.

Monday, 15/11 (16h, CET)
Constraining the Equation of State of dense matter in neutron stars using multi-messenger observations. 
Rahul SOMASUNDARAM (PhD student in IP2I-Lyon)

Neutron stars are one of the most fascinating objects in the universe, exploring matter at the highest densities that we can observe.  Recent  radio, X-ray  and gravitational wave observations have constrained the global properties of neutron stars such as their masses, radii and tidal deformabilities, giving us valuable new  insights  into  the  Equation  of  State of dense  matter.

Simultaneously, there has been significant efforts on the theoretical modeling of the dense matter Equation of State. In this seminar, I will discuss how state-of-the-art calculations exploiting the chiral symmetry of QCD can be used to make predictions for nucleonic matter present in the cores of neutron stars. Additionally, we will explore the plausible existence of exotic, i.e. non-nucleonic matter in neutron stars, and how one could potentially use recent experimental data to detect their presence. 

Monday, 18/10 (16h, CET)
Universal Systems and Effective Field Theories
Lorenzo CONTESSI (PostDoc in CEA)

Universal systems are quantum mechanical objects that can be described similarly despite having different typical sizes, energies, and degrees of freedom. This is possible because they share identical few-body properties and show some separation of scale among observables or with the respective underlying theory. Unitarity is arguably the most common universality class which finds instances in low-energy physics and can be found in hadronic, nuclear, and atomic fields. 

This interconnection between fields is important because allows for the passage of knowledge from one field to another with special benefit to the fields on which very little is experimentally known.  What is the “best” theory to describe the emergent phenomena maintaining interdisciplinarity is not a simple question to be answered. However, effective field theories have proven in the years to be the most natural framework for this search since their systematic improvability and the connection to the underlying physics of the various universal systems. In this seminar, I will talk about various universal systems, especially in the unitarity universality class, with a particular focus on how they can be described, how to pass the knowledge from one field to another, and on the many-body emergent phenomena.

Monday, 04/10 (16h, CET)
Properties of neutron star crust within nuclear physics uncertainties
Guilherme GRAMS (PostDoc in IP2I-Lyon)

A compressible liquid drop model (CLDM) is used to correlate uncertainties associated with the properties of the neutron star (NS) crust, with uncertainties associated with Chiral Effective Field theory (χEFT) predictions for the properties of homogeneous neutron matter and nuclear matter. We also study the impact of the surface, curvature, and Coulomb energies on the crustal properties. Fits to experimental nuclear masses are employed to further constrain the CLDM, and we find that they disfavor some of the χEFT Hamiltonians. These fits also reveal how the curvature energy alters the correlation between the surface energy the bulk symmetry energy.

Properties of the NS star crust against nuclear uncertainties are then analyzed, and we show that their impact vary from one observable to another: i) the finite size models impact the crust composition (A, Z) and have negligible influence on others quantities, ii) the largest uncertainties for the asymmetries Icl and Ye, as well as the volume fraction u, are induced by the Hamiltonians alone, iii) the largest uncertainties for the matter composition in the densest regions of the crust, as well as the precise location of the crust-core transition, are related to the Hamiltonians as well as by the surface energy isospin asymmetry parameter (psurf). As a consequence, the crust moment of inertia is also largely impacted by the choice of Hamiltonian as well as by the parameter psurf . The uncertainties induced by the loss function used for the fit to finite nuclei (∆E versus ∆E/A) as well as by the in-medium nucleon mass m∗ are much smaller.

Finally, we analyze the impact of these nuclear uncertainties on the NS mass-radius relations within a unified approach and with the condition to reach 2M⊙. In the analysis of the macroscopic NS properties, the Hamiltonians are the main source of uncertainties.

Monday, 20/09 (16h, CET)
Understanding 22Na cosmic abundance by measuring lifetimes in 23Mg
Chloé FOUGÈRES (GANIL PhD Student)

Simulations of novae explosive nucleosynthesis predict the production of the radionuclide 22Na. Its half life of 2.6 yr makes it a very interesting astronomical observable by allowing space and time correlations with the astrophysical object. This radionuclide should bring constraints on nova models. It may also help to explain abnormal 22Ne abundance observed in presolar grains and in cosmic rays. The gamma-ray line at 1.275 MeV associated with its β decay has not been observed yet by the gamma-ray space observatories [1]. New generations of gamma-ray telescopes like the european e-ASTROGAM intrusment, are expected in the coming decades. Hence accurate predicted yields of 22Na are required. Within novae thermal range, the main destruction reaction 22Na(p,γ)23Mg has been found dominated by a resonance at 0.213 MeV corresponding to 23Mg at Ex = 7.786 MeV. However the measured strengths of this resonance are in disagreement [2, 3].

An experiment was performed at the GANIL facility to measure the lifetime of the key state at Ex=7.786 MeV with an expected resolution of 1 fs. The principle of the experiment was close to the one of Ref. [4]. With a beam energy of 4.6 MeV/u, the reaction 3He(24Mg,α)23Mg populated the state of interest. This reaction was tagged thanks to particle detectors (spectrometer VAMOS++, silicon detector SPIDER) and gamma tracking spectrometer AGATA. The state of interest decayed either by gamma deexcitation or proton emission. The expected time resolution of 1 fs was made possible with AGATA high space and energy resolutions. Several Doppler based methods were used to analyse the lineshape of gamma peaks. SPIDER detector gave information about the proton branching ratio.

The analysis procedures and preliminary results will be presented. Emitted light particles were identified within SPIDER data and Doppler shifted gamma rays from 23Mg excited states were measured with AGATA. Gamma spectra are improved by imposing a coincidence with 4He measured with VAMOS. It ensures to suppress feeding from higher states. A new method based on Doppler correction leads to estimate the velocity of the gamma emitting 23Mg ions and lifetimes of 23Mg excited states. Proton decays from unbound levels in 23Mg are also identified, leading to an upper value for the branching ratio of interest. With an higher precision on the lifetime at Ex = 7.786 MeV, a new value of 22Na(p,γ)23Mg resonance strength ωγ is extracted by using also the most accurate measurement in BRp [5]. The contribution of this new resonance strength to the thermonuclear 22Na(p,γ)23Mg rate is calculated and compared with the latest results [2]. Using the astrophysical code MESA [6] on nova bursts, the impact of the new rate on the predicted 22Na production will be discussed.

[1] R. Diehl, Astro. Rev. 9:3,1-54 (2013).
[2] A.L. Sallaska et al., Phys. Rev. Let. 105, 152501 (2010).
[3] F. Stegmuller et al., Nuc. Phy. A 601, 168-180 (1996).
[4] O.S. Kirsebom et al., Phys. Rev. C 93, 025802 (2016).
[5] M. Friedman et al., Phys. Rev. C 101, 052802(R) (2020).
[6] B. Paxton et al., APJS 208, 4 (2013).