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 mass-to-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.

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

Research on radioactive ion beams produced with in-flight separation of relativistic beams has advanced significantly over the past decades, with contributions to nuclear physics, nuclear astrophysics, atomic physics, and other fields. Central to these advancements are improved production, separation, and identification methods.The FRS Ion Catcher at GSI/FAIRexemplifies these technological advancements. The system facilitates high-precision experiments by efficiently stopping and extracting exotic nuclei as ions and making these available at thermal energies. High-energy synchrotron beams enhance the system’s capabilities, enabling unique experimental techniques such as multi-step reactions, mean range bunching, and optimized stopping, as well as novel measurement methods for observables such as beta-delayed neutron emission probabilities. The FRS Ion Catcher has already contributed to various scientific fields, and the future with the Super-FRS at FAIR promises to extend research to even more exotic nuclei and new applications. 

High-Precision Experiments with Trapped Radioactive Ions Produced at Relativistic Energies

In the vicinity of gas-filled stopping cells vacuum pressures are typically too high for operation of conventional electrostatic beam lines. In this high pressure environmen t radio frequency quadrupoles (RFQ) can provide an efficent ion transport. Advantages of RFQ beam lines are their high transmission efficiencies of nearly 100%, ability to operate at high pressures, compactness ( ≈ 1m length) and diverse range of applications, e.g. cooling, bunching, mass separa tion and beam diagnosis. Such an RFQ based beam line [1] for the prototype cryogenic stopping cell (CSC) [2] has been commissioned successfully at the FRS Ion Catcher at GSI [3]. Recently, the identification of ions in the diagnostics unit (DU) of the RFQ beam line was done by α spectroscopy, but this way of identification is only suitable for α decayed nuclei. In order to improve the identification capabilities of the DU the extraction RFQ of the CSC was modified to operate as an RF-DC mass filter [4]. Thus it is possible to identify ions by their mass-to-charge ratio directly behind the stopping cell. Fig. 1 shows a mass spectrum of ions produced by a discharge ion source mounted inside the CSC. The mass spectrum shows ions from the electrode material of the discharge source (Fe, Cr etc.) and from a Ra recoil ion source. The mass filter can provide mass-to-charge information of ions between 30 u/e and250 u/e. This mass spectrum has a resolving power of 10, while achieving a transmission of80 %. Besides mass identification, the extraction RFQ can be used as a mass filter, transmitting a narrow mass window only and reducing the number of transmitted ions down to the required mass region. As seen in Fig. 1, if the mass filter is tuned for transmission of mass 140 u, the main ion current (mass 65 u) is surpressed by four orders of magnitude. The mass resolution can be increased further, but as shown in Fig. 2 higher resolving powers lead to reduced transmission. Despite the high pressures the mass-filter provides mass selective ion transport with resolving powers of up to150 at residual gas pressures of 8.4 ·10−2 mbar. The performance of the mass filter at high residual gas pressures is of special importance, as the buffer gas emerging from the CSC is pumped away in the extraction region and therefore the areal density of the CSC is determing the residual gas pressure in the DU. The rough mass identification of ions directly behind the CSC allows systematic investigations of the performance of the CSC. Furthermore the transmission of the downstream ∗Work supported by the BMBF under contracts no. 05P12RGFN8, b y the state of Hesse (LOEWE Center HICforFAIR), by JLU Giessen and GSI under the JLU-GSI strategic Helmholtz partnership agre ement Figure 1: Mass spectrum of ions produced by a discharge ion source inside the CSC measured using the extraction RFQ mass filter

An RFQ based beam line and mass filter to improve identification capabilities at the diagnostics unit of the prototype CSC for the LEB

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An RFQ based beam line and mass filter to improve identification capabilities at the diagnostics unit of the prototype CSC for the LEB