Technical design report for the cryogenic stopping cell of the Super-FRS at FAIR

Precision experiments with thermalized exotic nuclei will be possible at the super-conducting fragment separator Super-FRS at the Facility for Antiproton and Ion Research (FAIR). In the Early Science/First Science programs of FAIR, they will be performed at the focal plane FHF1 of the Super-FRS (in front of the High-Energy Branch) and, at a later stage during First Science++, also at the Low-Energy Branch. Exotic nuclei will be produced in flight and separated in the Super-FRS and their momentum spread will be reduced by energy-bunching. The ions will be further slowed down in a homogeneous degrader, thermalized in a gas-filled stopping cell, and extracted and transferred to the experimental setups. The stopping cell is, thus, a key device for experiments with thermalized exotic nuclei, and its performance characteristics will have a strong impact on the range of nuclides available and their yields, since its stopping and extraction efficiencies and extraction times strongly influence the rate of the extracted nuclei and put a limit on their lifetimes. In combination with a multiple-reflection time-of-flight mass spectrometer, the stopping cell will enable the measurement of masses, branching ratios, e.g., β-delayed (multi-)neutron emission probabilities, and lifetimes, as well as the in-cell production of exotic nuclei by multi-nucleon transfer with primary and secondary beams. Moreover, it will be a tool for the absolute calibration of the particle identification in the Super-FRS. Furthermore, using the combination of accurate mass determination with the PID of the Super-FRS on an event-by-event basis, completely new experimental possibilities will become available, such as the identification of millisecond isomers at the Super-FRS and the measurement of the dependence of the isomer-to-ground-state ratios on the production mechanism. These studies will be pursued in the context of the Super-FRS Experiment Collaboration. At the Low-Energy Branch, high-accuracy mass measurements, in-trap conversion electron and alpha spectroscopy, and trap-assisted spectroscopy will be performed with MATS (Precision Measurements of very short-lived nuclei using an Advanced Trapping System for highly charged ions). Measurements of nuclear spins, magnetic dipole and electric quadrupole moments, and root-mean-square charge radii will be carried out using collinear laser spectroscopy on ions and atoms and beta-NMR experiments with LaSpec (Laser Spectroscopy of short-lived nuclei). These experiments will address a wide scientific field ranging from nuclear structure and nuclear astrophysics to tests of the weak interaction and of the Standard Model. For the Super-FRS, a stopping cell with an areal density of 20mg/cm2 is required, with a cross-section of the stopping volume of 200cm2, a high extraction efficiency that is element-independent, an extraction time on the order of 10 ms, and a rate capability of 107 ions/s up to a nuclear charge of Z=92. Furthermore, the stopping cell needs to deliver bunches of ions with high purity. No existing stopping cell is capable of reaching these performance characteristics simultaneously. To fulfill these requirements, a novel concept for gas-filled stopping cells has been developed. It is based on the cryogenic stopping cell of the FRS Ion Catcher, and, in addition, it implements the two-stage extraction of the thermalized ions in a direction orthogonal to the incoming ion beam (high-areal-density orthogonal-extraction cryogenic stopping cell, HADO-CSC). This will boost all performance characteristics of the stopping cell and thus remove the performance bottleneck of present stopping cells for the thermalization of exotic nuclei produced at relativistic energies.