Quasi-Free scattering (QFS) reactions have been a powerful structure probe using proton beams on stable target, and can be used nowadays to study exotic nuclei provided a proton target is available. For this reason, the IRFU team is building a liquid Hydrogen (LH2) target dedicated to experiments with the R3B or the AGATA setup at GSI and, later on, FAIR. This target may serve several physics programs:
1) the spectroscopy of exotic nuclei; 2) the study of nucleon-nucleon correlations, in particular short-range correlations 3(SRC); 3) the production and study of exotic hypernuclei.
These studies will benefit from the IRFU expertise for the R&D and the building of cryogenic targets: in collaboration with the instrumentation divisions of IRFU, the DACM (Cryogenics-Magnetism), DEDIP (detection, electronics), DIS (System Engineering) the group has displayed a set of programs to optimize the reaction targets used for the experiments at GANIL (CHyMENE project), at RIKEN (MINOS, on-going). This expertise will be applied for the future R&D of the COCOTIER project, developed for the nuclear structure studies at FAIR.
Spectroscopy of exotic nuclei
The excitation spectrum of a nucleus is very sensitive to the underlying ground state structure and can be probed via gamma, missing-mass and invariant-mass spectroscopy techniques. The SEASTAR program at RIBF led by the IRFU team is a recent successful example of gamma spectroscopy of very neutron-rich nuclei with mass A≤110 using a thick (10-15 cm) proton target (cf MINOS-reactions). The combination of a thick liquid Hydrogen target, a vertex tracker, and a high-resolution gamma array as AGATA will open the possibility to explore new regions of the nuclides chart, for example for masses A ≥ 110, as well as odd-mass nuclei where the increasing level density makes the measurement more demanding in terms of gamma-decay energy resolution.
The presence of a high momentum tail (k > kF, with kF ~ 250 MeV/c) in the nucleon momentum distribution, and the back-to-back emission of nucleons with such high momenta have been observed in proton and electron induced QFS reactions and interpreted in terms of short-range correlations (SRC). SRC, i.e. the combination of the attraction at intermediate range and repulsion at short range of nucleon-nucleon potentials give rise to a spatially compact configuration (~1-1.5 fm) of nucleons with high relative momentum. SRC offer a unique laboratory benchmark for the short-range part of NN interaction that plays an important role in calculations involving nuclear matter at high density and momentum, such as neutron stars. Exploring the N/Z dependence of the isospin content of such pairs will allow testing if the current description of SRC as short-distance and high-relative momentum nucleon pairs is correct and what is the role of the different terms of the nuclear interaction (central, tensor) at different relative momenta (and therefore distances). Exploring the N/Z dependence, e.g. along the Oxygen isotopic chain, will be possible only in inverse kinematics impinging a high-intensity (>103 pps) and high-energy (>1 GeV/u) radioactive-ion beam on a proton target.
Production of exotic hypernuclei
Hyperons (Λ, Σ, Ξ, Ω) are baryons containing at least one strange quark, unlike nucleons (proton or neutron) only composed of u and d valence quarks. Interestingly, it was discovered that hyperons can form bound systems with nucleons and create short-lived hypernuclei. The investigation of hypernuclei provides a practical method to study the fundamental hyperon-nucleon (YN) and hyperon-hyperon (YY) interactions in nuclear matter at low energies since the very short Λ lifetime of 263 ps makes it technically extremely difficult to use it directly as a projectile for scattering or capture experiments. Heavy-ion beam induced reaction is a very promising way to produce light exotic hypernuclei, as already proved by the HypHI collaboration at GSI. Simulations done at DPhN show that the use of a thick LH2 target offers the advantage to reduce the background from Carbon-induced reactions without significantly reducing the Hypernuclei production cross section.
The IRFU team has engaged to build a thick (>5 cm) LH2 for R3B by June 2019 (expected beginning of 2019 beam time for FAIR phase 0). The detailed features of such a target (thickness, diameter) will be fixed by the end of 2017. Its construction is financed by the ANR grant COCOTIER (Oct. 2017- 2021, PI A. Corsi) dedicated to the study of SRC. In parallel to the construction of the LH2 target, detailed simulations for the detection setup for the study of SRC will be carried on.
Both SRC and Hypernuclei studies have similar requirements in terms of detection setup; therefore a common development may be envisaged. A challenging technical solution consisting in identifying and tracking particles momenta by inserting a time-projection chamber in GLAD dipole magnet is currently under study.
The IRFU team is also supporting other experiments proposed by the R3B collaboration to the G-PAC demanding the use of the LH2 target. Gamma spectroscopy experiments exploiting the LH2 target will be planned beyond 2023 once the AGATA array is installed at FAIR.
Contact : A. Corsi IRFU, DPhN