Separation of atomic and molecular ions by ion mobility with an RF carpet

Gas-filled stopping cells are used at accelerator laboratories for the thermalization of high-energy radioactive ion beams. Common challenges of many stopping cells are a high molecular background of extracted ions and limitations of extraction efficiency due to space-charge effects. At the FRS Ion Catcher at GSI, a new technique for removal of ionized molecules prior to their extraction out of the stopping cell has been developed. This technique utilizes the RF carpet for the separation of atomic ions from molecular contaminant ions through their difference in ion mobility. Results from the successful implementation and test during an experiment with a 600~MeV/u $^{124}$Xe primary beam are presented. Suppression of molecular contaminants by three orders of magnitude has been demonstrated. Essentially background-free measurement conditions with less than $1~\%$ of background events within a mass-to-charge range of 25 u/e have been achieved. The technique can also be used to reduce the space-charge effects at the extraction nozzle and in the downstream beamline, thus ensuring high efficiency of ion transport and highly-accurate measurements under space-charge-free conditions.Comment: 8 pages, 4 figure

The science case of the FRS Ion Catcher for FAIR Phase-0

The FRS Ion Catcher at GSI enables precision experiments with thermalized projectile and fission fragments. At the same time it serves as a test facility for the Low-Energy Branch of the Super-FRS at FAIR. The FRS Ion Catcher has been commissioned and its performance has been characterized in five experiments with 238U and 124Xe projectile and fission fragments produced at energies in the range from 300 to 1000 MeV/u. High and almost element-independent efficiencies for the thermalization of short-lived nuclides produced at relativistic energies have been obtained. High-accuracy mass measurements of more than 30 projectile and fission fragments have been performed with a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) at mass resolving powers of up to 410,000, with production cross sections down to the microbarn-level, and at rates down to a few ions per hour. The versatility of the MR-TOF-MS for isomer research has been demonstrated by the measurement of various isomers, determination of excitation energies and the production of a pure isomeric beam. Recently, several instrumental upgrades have been implemented at the FRS Ion Catcher. New experiments will be carried out during FAIR Phase-0 at GSI, including direct mass measurements of neutron-deficient nuclides below 100Sn and neutron-rich nuclides below 208Pb, measurement of β-delayed neutron emission probabilities and reaction studies with multi-nucleon transfer.Peer reviewe

Removal of molecular contamination in low-energy RIBs by the isolation-dissociation-isolation method

Experiments with low-energy rare ion beams often suffer from a large amount of molecular contaminant ions. We present the simple isolation-dissociation-isolation method to suppress this kind of contamination. The method can be applied to almost all types of low-energy beamlines. In a first step, a coarse isolation of the massto-charge ratio of interest is performed, e.g. by a dipole magnet. In a second step, the ions are dissociated. The last step is again a coarse isolation of the mass-to-charge ratio around the ion of interest. The method was tested at the FRS Ion Catcher at GSI with a radioactive ion source installed inside the cryogenic stopping cell as well as with relativistic ions delivered by the synchrotron SIS-18 and stopped in the cryogenic stopping cell. The isolation and dissociation, here collision-induced dissociation, have been implemented in a gas-filled RFQ beamline. A reduction of molecular contamination by more than 4 orders of magnitude was achieved.peerReviewe