# Introduction

High-precision experiments with exotic nuclei, such as high resolution decay spectroscopy, mass measurements and laser spectroscopy often require low-emittance ion beams with kinetic energies of only a few eV. The same requirement holds for the re-acceleration of short-lived nuclei, e.g. for nuclear reaction studies. At accelerator facilities that produce and separate exotic nuclei in-ﬂight, a low-energy beam of short-lived nuclei can be formed using the gas-stopping technique. After production and separation, the ions are stopped and thermalized in a gas-ﬁlled stopping cell. Typically, such a stopping cell is ﬁlled with a noble gas at a pressure on the order of 100 mbar. The thermalized ions are drawn to the exit side of the cell using electric DC ﬁelds, focused to the exit hole using an radio-frequency (RF) funnel or RF carpet and extracted as a beam with low kinetic energy. For nuclei produced at relativistic energies this technique is particularly challenging because of the large longitudinal and transverse emittance of the ions after production and slowing down. In order to reduce the range straggling of the ions to values that make the stopping in a gas feasible, the ions must ﬁrst be range-bunched using a dispersive magnetic stage and a mono energetic degrader system.

The FRS Ion Catcher serves as a test facility for the Low-Energy Branch (LEB) of the Super-FRS and already now enables a variety of new precision experiments in FAIR Phase-0. At the LEB of the Super-FRS at the Facility for Antiproton and Ion Research (FAIR), projectile and ﬁssion fragments will be slowed down and thermalized in a stopping cell and transferred to the experiments MATS and LaSpec for high-accuracy mass measurements, for in-trap and trap assisted decay spectroscopy and for laser spectroscopy experiments, respectively. The FRS Ion Catcher has been commissioned and its components have been characterized. For the ﬁrst time, a stopping cell for exotic nuclei was operated online at cryogenic temperatures. Using a gas density almost three times higher than ever reached before for a stopping cell with RF ion repelling structures, various $^{238}{ U}$ projectile and ﬁssion fragments were thermalized and extracted with very high eﬃciency. An extraction and ion survival eﬃciencies of more than 60 % have been measured for diﬀerent elements, including noble gases (e.g. radon) and highly reactive elements (e.g. thorium).