Mass measurements of fission and projectile fragments, produced via 238 U and 124 Xe primary beams, have been performed with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the Fragment Separator (FRS) Ion Catcher with a mass resolving power (FWHM) of up to 410 000 and an uncertainty of down to 6 × 10 −8. The nuclides were produced and separated in flight with the fragment separator FRS at 300 to 1000 MeV/u and thermalized in a cryogenic stopping cell. The data-analysis procedure was developed to determine with highest accuracy the mass values and the corresponding uncertainties for the most challenging conditions: down to a few events in a spectrum and overlapping distributions, which can be distinguished from a single peak only by a broader peak shape. With this procedure, the resolution of low-lying isomers is increased by a factor of up to 3 compared to standard data analysis. The ground-state masses of 31 short-lived nuclides of 15 different elements with half-lives of down to 17.9 ms and count rates as low as 11 events per nuclide were determined. This is the first direct mass measurement for seven nuclides. The excitation energies and the isomer-to-groundstate ratios of six isomeric states with excitation energies of as little as 280 keV were measured. For nuclides with known mass values, the average relative deviation from the literature values is (4.5 ± 5.3) × 10 −8. The measured two-neutron separation energies and their slopes near and at the N = 126 and Z = 82 shell closures indicate a strong element-dependent binding energy of the first neutron above the closed proton shell Z = 82. The experimental results deviate strongly from the theoretical predictions, especially for N = 126 and N = 127.

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We have developed a stopping cell to be used at the FRS and Super-FRS (Superconducting FRagment Separator) at the GSI Helmholtz Centre for Heavy-Ion Research and the Facility for Antiproton and Ion Research (FAIR), both in Darmstadt, Germany. The cell has a stopping volume with a length of 1 m and a diameter of 25 cm. It is aimed at operation with highdensity helium gas (up to 0.2 mg/cm 3 ). Ours is the first realisation of a stopping cell in which the required purity of the helium stopping gas is ensured by operation at cryogenic temperatures. On the exit side, the ions are guided to the exit hole by an RF carpet with 4 electrodes per mm, operating at a frequency of 5.8 MHz. We present the first commissioning results of the cryogenic stopping cell. Using 219 Rn ions emitted as alpha-decay recoils from a 223 Ra source, a combined ion survival and extraction efficiency between 10 and 25% is measured for helium gas at a temperature of 85 K and with a density up to 0.07 mg/cm 3 (equivalent to a pressure of 430 mbar at room temperature). This density is almost two times higher than demonstrated up to now for RF ion repelling structures in helium gas. Given the operational and design parameters of the system, it is projected that this technology is useful up to a helium gas density of at least 0.2 mg/cm 3 .

A cryogenic stopping cell (CSC) has been commissioned with 238 U projectile fragments produced at 1000 MeV/u. The spatial isotopic separation in flight was performed with the FRS applying a monoenergetic degrader. For the first time, a stopping cell was operated with exotic nuclei at cryogenic temperatures (70 to 100 K). A helium stopping gas density of up to 0.05 mg/cm 3 was used, about two times higher than reached before for a stopping cell with RF ion repelling structures. An overall efficiency of up to 15%, a combined ion survival and extraction efficiency of about 50%, and extraction times of 24 ms were achieved for heavy α-decaying uranium fragments. Mass spectrometry with a multiple-reflection time-of-flight mass spectrometer has demonstrated the excellent cleanliness of the CSC. This setup has opened a new field for the spectroscopy of short-lived nuclei.

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\textit{Objective}. Beams of stable ions have been a well-established tool for radiotherapy for many decades. In the case of ion beam therapy with stable $^{12}$C ions, the positron emitters $^{10,11}$C are produced via projectile and target fragmentation, and their decays enable visualization of the beam via positron emission tomography (PET). However, the PET activity peak matches the Bragg peak only roughly and PET counting statistics is low. These issues can be mitigated by using a short-lived positron emitter as a therapeutic beam. \textit{Approach.} An experiment studying the precision of the measurement of ranges of positron emitting carbon isotopes by means of PET has been performed at the FRS fragment-separator facility of GSI Helmholtzzentrum f"ur Schwerionenforschung GmbH, Germany. The PET scanner used in the experiment is a dual-panel version of a Siemens Biograph mCT PET scanner. \textit{Main results.} High quality in-beam PET images and activity distributions have been measured from the in-flight produced positron emitting isotopes $^{11}$C and $^{10}$C implanted into homogeneous PMMA phantoms. Taking advantage of the high statistics obtained in this experiment, we investigated the time evolution of the uncertainty of the range determined by means of PET during the course of an irradiation, and show that the uncertainty improves with the inverse square root of the number of PET counts. The uncertainty is thus fully determined by the PET counting statistics. During the delivery of 1.6$\times$10$^7$ ions in 4 spills for a total duration of 19.2~s, the PET activity range uncertainty for $^{10}$C, $^{11}$C and $^{12}$C is 0.04, 0.7 and 1.3~mm, respectively. The gain in precision related to the PET counting statistics is thus much larger when going from $^{11}$C to $^{10}$C than when going from $^{12}$C to $^{11}$C. The much better precision for $^{10}$C is due to its much shorter half-life, which, contrary to the case of $^{11}$C, also enables to include the in-spill data in the image formation. \textit{Significance}. Our results can be used to estimate the contribution from PET counting statistics to the precision of range determination in a particular carbon therapy situation, taking into account the irradiation scenario, the required dose and the PET scanner characteristics.

a b s t r a c tA cryogenic stopping cell for stopping energetic radioactive ions and extracting them as a low energy beam was developed. This first ever cryogenically operated stopping cell serves as prototype device for the Low-Energy Branch of the Super-FRS at FAIR. The cell has a stopping volume that is 1 m long and 25 cm in diameter. Ions are guided by a DC field along the length of the stopping cell and by a combined RF and DC fields provided by an RF carpet at the exit-hole side. The ultra-high purity of the stopping gas required for optimum ion survival is reached by cryogenic operation. The design considerations and construction of the cryogenic stopping cell, as well as some performance characteristics, are described in detail. Special attention is given to the cryogenic aspects in the design and construction of the stopping cell and the cryocooler-based cooling system. The cooling system allows the operation of the stopping cell at any desired temperature between about 70 K and room temperature. The cooling system performance in realistic on-line conditions at the FRS Ion Catcher Facility at GSI is discussed. A temperature of 110 K at which efficient ion survival was observed is obtained after 10 h of cooling. A minimum temperature of the stopping gas of 72 K was reached. The expertise gained from the design, construction and performance of the prototype cryogenic stopping cell has allowed the development of a final version for the Low-Energy Branch of the Super-FRS to proceed.