Isospin transport phenomena in nuclear reactions in the Fermi energy range

This thesis work has been done within the NUCL-EX group of the INFN group II. The work has consisted of the preparation and the first measurement of the coupled apparatus INDRA-FAZIA, in the identification of data obtained in the previous measurement with FAZIA and in the analysis of data that were already calibrated in a precedent measurement with INDRA coupled with the spectrometer VAMOS. The FAZIA apparatus was built with a collaboration that involves more than 10 institutions in six different countries. A long phase of research and development was necessary to obtain what today can be considered one of the best apparatus for measurements of charged particles emitted in nuclear reactions. The measurement made with INDRA-VAMOS was analyzed in order to verify the isospin effects (N/Z content) in peripheral and semi-peripheral reactions Ca(40,48)+Ca(40,48) at 35 MeV/u. In these collisions, the transport models predict the formation of a low-density neck between two hot fragments kinematically similar to the projectile (PLF) and target (TLF). The isotopic identification of PLF provided by VAMOS, together with those of light charged particles (LCP) revealed in coincidence with INDRA allow, through their correlation, to reconstruct the primary fragment. During the reconstruction phase, the mass number (A) is made without the emitted neutron contribution, because these are not detected. To estimate the excitation of the source is necessary to make some assumption about the evaporated neutrons, to obtain the most realistic estimate. Finally, in order to extrapolate information about the energy symmetry term of the Nuclear Equation of State, it is shown the study of the width of isotopic distributions. The preliminary result obtained is the trend of the symmetry energy coefficient (Csym) as a function of the charge of the reconstructed primary fragment, which is dependent on the surface term, present in the liquid drop model of the nucleus

^3_Λ H studies in relativistic ion-ion collisions: matter radius and production mechanisms

In the exploration of nuclear physics, hypernuclei stand as unique entities, introducing strangeness into the nuclear landscape and extending it to reveal new structural phenomena. Investigating their internal composition gives access to the hyperon-nucleon and hyperon-hyperon interactions, which are challenging to study directly (e.g., by elastic scattering) due to the short lifetime of hyperons. A better understanding of baryon interactions, including hyperons, improves the knowledge on the nuclear equation of state and, consequently, the inner core structure of neutron stars. Among hypernuclei, the hypertriton (^3_Λ H), and specifically its size, not measured so far, has been indicated as a key probe to understand the nucleosynthesis mechanisms in relativistic heavy-ion collisions. This thesis focuses on ^3_Λ H produced in relativistic ion-ion collision at GSI/SIS18 energies (up to 2 AGeV), in order to access its matter radius and possible production mechanisms. In the first part of the thesis, the concept of a new accepted experiment that will be performed in 2025 at the R^3B setup in GSI using ^{12}C+^{12}C collisions at 1.9 AGeV is detailed. The experiment aims at the first determination of the ^3_Λ H size, predicted to be a halo hypernucleus, through interaction cross section measurements. To achieve that, a new experimental method to extract the interaction cross section of hypernuclei with a target nucleus, sensitive to their matter radii, was developed. A precision of 15% or better in the interaction cross section can be achieved, allowing extraction of the unknown ^3_Λ H matter radius and assessing its halo or non-halo character. In addition, realistic GEANT4 simulations have been performed in order to optimize the design of the experimental setup, including the main detector, the mini-HYDRA (HYpernuclei Decay at R^3B Apparatus) time-projection chamber, and to assess the feasibility of the experiment. Finally, the design and validation of a new detector, the HYDRA plastic wall, is presented, which is intended to be used as a trigger in the measurement. The second part of the thesis focuses on the production mechanisms of ^3_Λ H in heavy-ion collisions at the HADES setup in GSI. Here, the production is explored by analyzing existing datasets, taken in 2019 and 2012, with different collision energies, i.e., Ag+Ag at 1.58 AGeV and 1.23 AGeV, and Au+Au at 1.23 AGeV. While the first set is exactly at the strangeness production threshold from elementary nucleon-nucleon collisions (1.58 GeV) the others are below it. The data analysis identified clearly the ^3_Λ H signal from the invariant mass of its decay products, π^-+{}^3He, for both the high and low energy datasets: the significance level for the peaks are 18.27, 5.16, and 4.00 for the Ag+Ag at 1.58 AGeV, 1.23 AGeV, and Au+Au at 1.23 AGeV, respectively. Following that, the associated production cross-sections at and below the strangeness production threshold are extracted and the production cross section ratio of low-to-high energy from the Ag+Ag dataset amounts to 0.30±0.08(stat.)±0.03(sys.). These findings indicate contributions from additional production mechanisms for hypernuclei that need to be further investigated by comparing the experimental results with predictions from transport models

Method to evidence hypernuclear halos from a two-target interaction cross section measurement

We present a two-target measurement method to determine the interaction cross section of hypernuclei with a target nucleus. The method allows to extract from two independent measurements the production cross section of a given hypernucleus as well as its interaction cross section on a specific target. The latter is then further analyzed to deduce the matter radius of the hypernucleus. The sensitivity of the method has been investigated for the specific case of the lightest hyperhalo candidate hypertriton ($$^3_\Lambda$$H) produced from $$^{12}$$C+$$^{12}$$C collisions at 1.9 GeV/nucleon. Furthermore, its feasibility is demonstrated by detailed simulations for realistic experimental conditions at GSI/FAIR, using a dedicated HYDRA (HYpernuclei Decay at R$$^3$$B Apparatus) time-projection chamber prototype. A precision of 15% or better in the interaction cross section can be achieved, allowing an extraction of the unknown $$^3_\Lambda$$H matter radius and assessing its halo or non-halo character

A new Time-of-flight detector for the R 3 B setup

© 2022, The Author(s).We present the design, prototype developments and test results of the new time-of-flight detector (ToFD) which is part of the R3B experimental setup at GSI and FAIR, Darmstadt, Germany. The ToFD detector is able to detect heavy-ion residues of all charges at relativistic energies with a relative energy precision σΔE/ ΔE of up to 1% and a time precision of up to 14 ps (sigma). Together with an elaborate particle-tracking system, the full identification of relativistic ions from hydrogen up to uranium in mass and nuclear charge is possible.11Nsciescopu