Nuclear Physics Division

In December 2019, the NFS (Neutrons For Science) facility received its first proton beams, delivered by the linear accelerator of the new Spiral2 facility at the GANIL. On the fringes of the progressive commissioning of the accelerator in 2020, short beam periods were used to successfully test several NFS components. The first experiments are planned at the facility in the fall of 2021.

A first beam of protons accelerated up to 33 MeV was sent in December 2019 to the NFS irradiation station (Figure 1), coupled to a pneumatic transfer system aiming at transporting the irradiated samples to a measurement station. The production cross-sections of different nuclei (obtained through irradiation of iron and copper samples) were thus measured. The results of this test are in agreement with previously published data. The irradiation and measuring device, built and operated by physicists from the NPI laboratory in Rez (Czech Republic), will be used in the future for novel measurements of reaction cross sections by activation.

Meteorites are bombarded throughout their journey by cosmic radiation. This cosmic ray exposure (CRE) is a formidable footprint of their history, provided of course that we know how to decipher it. The interaction of cosmic radiation with the atomic nuclei constituting the meteorite will produce so-called cosmogenic isotopes, very often radioactive. Measurements of activities, once the meteorite is found on earth, associated with a model can allow us to go back to its pre-atmospheric size, its CRE ages, its terrestrial age, and even to better understand this cosmic ray flux. This type of model is based on a key ingredient: the elementary cross sections of isotope production. For the first time, these have been provided only by the INCL nuclear reaction code developed at Irfu in the framework of a study of iron meteorites [1], thus increasing the precision of the analyses.

INCL (Liège intranuclear cascade) is a simulation code known for its ability to model light particle-nucleus interactions. It is used in very various fields, such as proton therapy, neutron sources, radioactive ion beams or ADS's (Accelerator Driven Systems). In order to extend its capabilities in the field of higher energy reactions, in connection with cosmic rays or with the study of hypernuclei, a team of physicists led by Irfu has recently developed a new version of the code involving strange particles. This work was at the heart of a recently defended thesis (2019) and the new possibilities offered by this code were published in early 2020 in the journal Physical Review C [1].
 

The 2020 edition of the Large Hadron Collider Physics Conference (LHCP) took place from 25 to 30 May 2020. Due to the COVID-19 pandemic, the conference, originally planned to be hosted in Paris, was held entirely online. The ALICE collaboration presented new results showing how charmed particles - those containing quarks, the elementary components of matter, known as c - can act as "messengers" for the plasma of quarks and gluons, which is believed to have existed in the primordial Universe and can be recreated during heavy ion collisions in the Large Hadron Collider (LHC). By studying the charmed particles, scientists can learn more about hadrons, particles in which quarks are bound together by gluons, and about quark-gluon plasma, a state of matter in which quarks and gluons are not confined within hadrons. These new results are the result of an analysis conducted as part of a thesis currently underway at the DPhN.

After more than four years of research and development, design and manufacturing work, the MFT (Muon Forward Tracker), a new detector that will equip the ALICE experiment at the LHC, has seen its construction finalized and is currently under commissioning at CERN. In order to limit as far as possible the amount of material crossed by the particles, the conception of this detector has required the development of many innovative techniques and procedures, particularly in the integration of silicon sensors on flexible hybrid circuits called ladders, for which Irfu was responsible within the project. It took two years to manufacture the 500 ladders of the MFT, and a very long sequence of operations was the subject of numerous studies under the responsibility of the Irfu Antenna team at CERN. The production of these ladders has just been successfully completed and it is therefore time to make a short assessment

In December 2019, the NFS (Neutrons For Science) facility received its first proton beams, delivered by the linear accelerator of the new Spiral2 facility at the GANIL. On the fringes of the progressive commissioning of the accelerator in 2020, short beam periods were used to successfully test several NFS components. The first experiments are planned at the facility in the fall of 2021.

A first beam of protons accelerated up to 33 MeV was sent in December 2019 to the NFS irradiation station (Figure 1), coupled to a pneumatic transfer system aiming at transporting the irradiated samples to a measurement station. The production cross-sections of different nuclei (obtained through irradiation of iron and copper samples) were thus measured. The results of this test are in agreement with previously published data. The irradiation and measuring device, built and operated by physicists from the NPI laboratory in Rez (Czech Republic), will be used in the future for novel measurements of reaction cross sections by activation.

The spectroscopy of a mendelevium isotope, ²⁵¹Md composed of 101 protons and 150 neutrons, reveals a surprise: when it rotates, it behaves exactly like a lawrencium isotope made of 103 protons and 152 neutrons. The experiment carried out at the University of Jyväskylä in Finland required the most advanced tools to study these rare and ephemeral nuclei: filtering and identification of the nuclei, gamma ray and electron detection. Is this completely unexpected similarity the result of chance, or is it related to the properties of strong interaction? The investigation continued with the theoreticians to try to understand this singularity. The results have just been published in the journal Physical Review C.

The complexity of the atomic nucleus reflects a multi-component character of the « nuclear force » that holds protons and neutrons together. Proper separation and characterization of each of these components represents a challenge for both theoretical and experimental nuclear structure studies. The tin isotopes (nuclei with Z=50 protons and a number of neutrons depending on the isotope) provide ideal opportunities to study the competition between two of the nuclear force components: the so-called pairing, related to the marked tendency of protons and neutrons to form pairs in the nuclear matter, and the so-called quadrupole interaction term, describing the natural susceptibility of nuclei to adopt deformed shapes. Though of a different nature, these two interaction terms contribute to the goal of achieving an optimum organization of nucleons in the atomic nucleus that will minimize its energy.  Previous works have demonstrated that a shift of balance between these two components takes place when approaching tin-100, and this observation provided important constraints for theoretical descriptions of this so-called “doubly-magic” nucleus. Having the same numbers of protons and neutrons (Z=N=50), ¹⁰⁰Sn is a key nucleus to validate model descriptions of exotic nuclei.