A Time-of-Flight-Based Isobar Separator and Mass Spectrometer for the LEB

At the Low-Energy-Branch (LEB) of the Super-FRS at FAIR, precision measurements of very short-lived nuclei will be performed. For these experiments (MATS, LASPEC), the nuclei have to be stopped, cooled, separated and measured as fast and efficiently as possible. To achieve this goal, a multi-purpose, non-scanning mass spectrometer with single-ion sensitivity, a multiple-reflection timeof-flight mass spectrometer (MR-TOF-MS), has been developed [1,2]. Applications. The MR-TOF-MS will be positioned behind the gas-filled stopping cell at the LEB, where it can be used as a broadband mass spectrometer, an isobar separator or a high-precision mass spectrometer: (i) The broadband mode will be used for optimization of the range and range-compression [3] in the Super-FRS and the stopping and extraction from the gas cell. A precise tuning of the range and the range compression is essential for experiments at MATS and LASPEC, because the range of the nuclei produced in the Super-FRS is 1000 times higher than the areal weight of the gas cell. (ii) Isobaric contamination that is produced by secondary reactions in the degraders or by charge-exchange reactions [4] in the gas cell can be orders of magnitude larger than the ions of interest. In particular for fission fragments, the abundance of contaminants transmitted through the Super-FRS can be up to two orders of magnitude higher than that of the nuclei of interest. To remove these ions, the isobar separator mode is required, in which up to 10 isobaric ions/s can be handled. (iii) The high-precision mode will enable mass measurements with an accuracy of 10 to 10 in about 2 ms. Thus the life-time of the ions to be measured is limited only by the extraction-time from the gas cell. The mass spectrometer is non-scanning and all isobaric ions can be measured simultaneously. Developments. The start of the time-of-flight measurement in the MR-TOF-MS is given by the injection of ions from an RF trap. The performance of the mass spectrometer is significantly determined by the characteristics of the injected ion population. An injection trap system has been developed to provide cooled ion bunches of low emittance. It consists of three stages for fast ion cooling, while avoiding collisional losses during ion ejection. A fast-switching square wave RF source has been developed. It allows to switch off the RF during ejection in order to avoid a massdependens of the ion energies. The trap system has been set up and commissioned. Time-of-flight peak widths of about