Seminars

Fission is the nuclear process by which a nucleus splits into fragments. Although this reaction was discovered nearly 90 years ago, it remains a major fundamental challenge, both experimentally, as data are difficult to obtain and interpret, and theoretically, as no existing model can yet simultaneously reproduce experimental results, provide reliable predictive power, and offer a consistent physical interpretation of the process.

The theoretical modeling of fission relies primarily on describing the evolution of the system from the initial configuration of the fissioning nucleus to the formation of the final fragments. This is achieved through potential energy surfaces (PES), generated by a set of theoretical calculations representing the atomic nucleus in various relevant states. Calculating these energy surfaces is an essential step, as it allows one to characterize the deformed configurations of the nucleus, identify fission valleys and modes, their associated saddle points, and the shape of the fission barriers that govern the dynamics of the process. These results provide a microscopic foundation for determining key observables such as fragment mass and charge distributions or their kinetic energies.

From the experimental perspective, several SOFIA (Studies On Fission with Aladin) campaigns have been conducted in recent years by the CEA DAM (Bruyères-le-Châtel) to measure fission yields of various fissioning nuclei. During the most recent campaign, around one hundred nuclei were measured, ranging from iridium (Z = 77) to thorium (Z = 90) [1]. These measurements revealed, on one hand, the existence of an asymmetric fission island, for which most fissioning systems produce fragments of unequal mass and charge, and on the other hand, a notable overproduction of krypton isotopes.

In this presentation, these experimental results will be interpreted using a mean-field theoretical framework, specifically the Hartree–Fock–Bogoliubov (HFB) approach under constraints. Within this framework, fission paths are analyzed in terms of shell effects, i.e., quantities related to the sensitivity of the system to its local energy level density. During the process, two types of shell effects can be distinguished: those intrinsic to the fissioning nucleus, and those belonging to the nascent fission fragments formed at large deformations, close to scission.

Then the following fundamental questions will be addressed:

— What governs the fission paths?

— Why do neighboring fissioning nuclei exhibit such differences in charge and mass yields?

— How can the fine details (peaks and dips) in primary fission yields be explained?

— Which shell effects are relevant, and what are their properties?

We will show that shell effects in krypton isotopes play a particularly significant role, consistent with experimental results, while also offering additional point of view on the fission mechanism in this region.

[1] P. Morfouace et al, An asymmetric fission island driven by shell effects in light fragments. Nature, 641:339–344, 2025

Many thousands of different isotopes exist, each of which can differ dramatically from systems with just one less nucleon. The need for nuclear data is large in astrophysics: simulations of phenomena such as nucleosynthesis or neutron star mergers rely on the properties of dense matter at the extremes of isospin, density and temperature; these extremes are today not accessible in the lab.

Energy density functionals (EDFs) are our current best hope to provide this data; an EDF describes a nucleus in terms of its constituent nucleons while the equations remain sufficiently tractable for global application. We are building a new class of models aimed at providing all necessary data to astrophysical applications: the Brussels-Skyrme-on-a-Grid (BSkG). These models accord the nucleus an extreme amount of freedom: nuclear shapes range from spheres and axially symmetric ellipsoids but also exhibit triaxial deformation, reflection asymmetry, non-zero angular momentum or all of these combined! These models do not just describe bulk properties such as masses and radii, but also pseudo-observables that serve as input to reaction models, predictions for dense matter in neutron stars and fission properties.

I will start by introducing the general concepts behind the BSkG models and then discuss the quality of the model w.r.t. nuclear ground state properties, including some comparisons to new experimental data. As second major subject, I will discuss our ongoing effort to predict the fission properties of thousands of unknown isotopes. Finally, I will end the presentation by outlining other research directions that our group is investing in.

El estudio del núcleo atómico tiene un paper central en la física microscópica. Los núcleos contienen -casi- toda la masa del universo; son los protagonistas de la evolución estelar y por lo tanto de la producción de los elementos químicos que nos rodean (nucleosíntes), tanto en ambientes estelares como en supernovas o en fusiones de estrellas de neutrones; son también el locus de manifestaciones de la interacción débil que pueden poner en jaque al modelo estándar de las partículas e interacciones fundamentales (doble desintegración beta sin neutrinos); por no hablar del impacto geotérmico de sus desintegraciones radioactivas y de sus aplicaciones en física médica o en la generación de energía. Desde un punto de vista fundamental, la descripción de la dinámica nuclear supone un enorme desafío, puesto que se trata de un sistema de muchos cuerpos que interactúan fuertemente. La interacción nuclear proviene de las interacciones entre quarks y gluones descritas por la cromodinámica cuántica, por lo que la interacción efectiva resultante es de una gran complejidad. Algo hablaremos de ello. Otra característica de los núcleos es la coexistencia de propiedades que hacen pensar en un sistema de nucleones casi independientes sometidos a un campo promedio, con, en la mayoría de los casos, comportamientos colectivas tales como la superfluidez o la omnipresencia de núcleos deformados, cuya interpretación semiclásica deberíamos reconsiderar. De esto hablaremos algo más. Las fronteras de la física nuclear se sitúan hoy en día por una parte, en el estudio de los núcleos superpesados y por otra  en los núcleos cuyo ratio entre neutrones y protones es muy diferente de la de los que residen en el valle de la estabilidad (por poner un ejemplo, el isótopo estable más pesado del níquel tiene A=64, pero se ha producido artificialmente el que tiene A=82, un núcleo  que es extremadamente“rico” (jerga) en neutrones. El estudio experimental de estos últimos ha puesto de manifiesto una pléyade de nuevos comportamientos, tales como coexistencia de formas o incluso de su entrelazamiento (entanglement). También hablaremos de ello, si el tiempo lo permite. 

Many thousands of different isotopes exist, each of which can differ dramatically from systems with just one less nucleon. The need for nuclear data is large in astrophysics: simulations of phenomena such as nucleosynthesis or neutron star mergers rely on the properties of dense matter at the extremes of isospin, density and temperature; these extremes are today not accessible in the lab.

Energy density functionals (EDFs) are our current best hope to provide this data; an EDF describes a nucleus in terms of its constituent nucleons while the equations remain sufficiently tractable for global application. We are building a new class of models aimed at providing all necessary data to astrophysical applications: the Brussels-Skyrme-on-a-Grid (BSkG). These models accord the nucleus an extreme amount of freedom: nuclear shapes range from spheres and axially symmetric ellipsoids but also exhibit triaxial deformation, reflection asymmetry, non-zero angular momentum or all of these combined! These models do not just describe bulk properties such as masses and radii, but also pseudo-observables that serve as input to reaction models, predictions for dense matter in neutron stars and fission properties.

I will start by introducing the general concepts behind the BSkG models and then discuss the quality of the model w.r.t. nuclear ground state properties, including some comparisons to new experimental data. As second major subject, I will discuss our ongoing effort to predict the fission properties of thousands of unknown isotopes. Finally, I will end the presentation by outlining other research directions that our group is investing in.