Performance of a Prototype MR-TOF-MS for the LEB of the Super-FRS

A multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) [1] will be an important part of the MATS/LaSpec beam-line at the Low-Energy-Branch (LEB) of the Super-FRS at FAIR. The MR-TOF-MS is a fast (∼ms), multi-purpose, non-scanning mass spectrometer with single-ion sensitivity and will be used as a broadband mass spectrometer, as isobar separator or for direct mass measurements of very short-lived nuclei [2]. A prototype MR-TOF-MS has been built and characterized. In the spring of 2009 it was tested at the tandem accelerator of the Maier-Leibnitz-Laboratory Garching (Germany). All important performance characteristics have been studied and are discussed in the following: Mass Resolving Power. The resolving power and the transmission efficiency have been investigated with stable Cs ions. A time-of-flight of 5 ms is sufficient for a resolving power (FWHM) of 100,000, 12 ms lead to a resolving power 200,000 and the limit in resolving power amounts to 320,000. Thus a flight time of 12 ms (corresponding to 256 turns) is a good compromise between resolution and measurement time. For this setting the resolving power at different peak heights has been measured and Gaussianlike peak shapes down to the 1% level have been found. At 0.1% the resolving power still amounts to 20,000. Transmission Efficiency. The transmission efficiency through the whole device for pass-through (zero turns) is 70% and decreases exponentially with the number of turns due to collisions with the residual gas. The decay constant is 670 turns, which results in a transmission efficiency of∼ 50 % for 256 turns. Mass Measurement Accuracy. The mass measurement accuracy has been investigated for an isobaric triplet at mass 78 u (C3H5Cl, C5NH5 and C6H6) for 256 turns. Space charge has been found to be the main source of systematic errors. The impact of space charge effects on the mass measurement accuracy increases linearly with the number of detected ions [3]. Thus mass measurements can be extrapolated to zero space charge. Then, the residual systematic error is less than 1× 10−7. The major contribution of this residual error is known and could be parameterized for further improvement in accuracy. Isobar Separation. Mass separation has been demonstrated for the isobar doublet of CC5H5 and C6H6 (figure 1). The mass difference of the doublet is only 4.164 MeV and the intensity ratio in this experiment was ∼170. The ability to separate such close-lying masses with strongly different abundances will enable experiments with