Reactions with Radioactive Relativistic Beams

At the high-energy branch of the Super-FRS, reaction studies in inverse kinematics will be performed by the R3B collaboration using the full energy range available from the Super-FRS without further stopping, cooling or accumulation, that is, 200–2000 AMeV. The research program planned covers a wide variety of scattering experiments such as heavy-ion induced electromagnetic excitation, knockout and breakup reactions, light-ion induced elastic, inelastic and quasi-free scattering together with relativistic nucleus-nucleus collisions experiments. Such experiments will investigate properties of isospin asymmetric nuclei and nuclear matter of relevance to nuclear physics and nuclear astrophysics. UK-led topics include studies of quasi-free scattering reactions (QFS) and the nuclear equation of state (EOS).

The energy domain available to R3B is unique in the world for RIB reaction studies and has several experimental advantages. For example: in scattering experiments at these energies the nuclear transparency is a maximum which allows for maximally unbiased single-particle or cluster knockout reactions like quasi-free scattering; the reaction mechanism can be well disentangled from structural features; and it is fully feasible to perform full ion-by-ion tracking to thus achieve the highest resolution. Relativistic nucleus-nucleus collisions experiments at these energies will allow asymmetric nuclear matter to be explored up to 2-3 times normal nuclear-matter density. The energy and the acceptance of the experiment corresponds fully to the characteristics of the RIB production at FAIR, meaning that R3B will be the prime project to perform reaction studies of the isospin frontier opened up by the Super-FRS.

R3B is being designed and constructed by a large international collaboration of 50 institutes. The UK holds several key collaboration positions. Lemmon is Chair of the Technical Board, a member of the Management Board and Coordinator of the Si Tracker Working Group. Labiche is Co-Coordinator of the NuSTAR Simulations Working Group. Lazarus is Co-Coordinator of the Joint R3B/EXL Front End Electronics (FEE) and Data Acquisition Working Group. Catford, Chartier, Freer, Lemmon and Woods are on the R3B Collaboration Board/Steering Committee. In order to manage the UK R3B project a structure comprising a management board and a technical board has been set up.

The R3B collaboration has designed an experimental setup capable of fully benefiting from the Super-FRS beams with the characteristics inherent to the in-flight production method. Located at the focal plane of the high-energy branch of the Super-FRS, R3B is a versatile fixed-target detector system with high efficiency, acceptance, and resolution for kinematically complete measurements of reactions with high-energy radioactive beams, even at very low beam intensities down to one ion per second. Detectors will track and identify the beam onto a reaction target, typically liquid hydrogen, C or Pb, as well as after the target. The forward-going charged products and neutrons will be tracked, identified and momentum-analysed in a superconducting dipole magnet, time-of-flight walls and a neutron time-of-flight wall. Light charged particles and gamma rays from the target region will be measured by the target recoil detector. This detector surrounds the target volume and consists of two main elements: the Si tracker and the calorimeter. The Si tracker is a double-layer silicon micro-vertex tracker, providing precise tracking and vertex determination as well as energy and multiplicity measurement with high efficiency and acceptance. This will be surrounded by a scintillator-based calorimeter called CALIFA (CALorimeter for In-Flight emitted gAmma detection) for the full-energy detection of the light-ion ejectiles and gamma-ray detection. The target recoil detector is essential for a large part of the physics programme of R3B and, in particular, the light-ion induced studies such as quasi-free scattering that the UK is leading within the collaboration. The UK will concentrate its effort on the target recoil detector. Specifically it will lead the design and construction of the Si tracker and its associated front-end electronics. It will also be responsible for the overall mechanical design for the target recoil detector and the target region in general. Specifically, the integration of the liquid hydrogen target, the Si tracker and the calorimeter will be led by the UK. The UK will also perform simulations of the performance of this combined detector system.

Justification The target-recoil detector is vital to the physics programme of R3B and in particular to the UK-led studies of QFS. The UK is in a very strong leadership position within the collaboration both for the design and construction of this detector and for the physics that it enables. The UK institutions involved have unique facilities and expertise within the collaboration which make it a natural choice to lead this project. These include the UK mechanical design, electronics and DAQ teams and the Liverpool Semiconductor Detector Centre. In particular, the Liverpool Semiconductor Detector Centre (LSC) has facilities unavailable elsewhere within the international R3B collaboration. There is invaluable expertise at the LSC in the assembly of complex Si-microstrip detectors, including the ATLAS tracker endcap, the LHCb velo and the ALPHA tracker (anti-hydrogen experiment at CERN).