Design of a new time-of-flight detector for isochronous mass spectrometry in the collector ring at FAIR

The masses of exotic nuclei can be measured in ion storage rings by determination of their revolution time in the ring. At the current FRS-Experimental Storage Ring (ESR) facility one method to perform such measurements is the isochronous mass spectrometry (IMS). With the IMS masses of exotic nuclei with lifetimes as short as a few tens of mu s can be measured. To determine these masses the revolution time of the ions in the isochronous ring is measured by a time-of-flight (TOF) detector. To achieve a high mass resolution the performance of the detector is crucial and has been improved significantly. The future Collector Ring (CR) at FAIR will be different compared to the current ESR not only in circumference but also in terms of beam dimensions and intensities. Based on extensive simulations, a new double detector system has been designed for improved IMS at the CR. It is adapted to the beam emittance of the ions in the CR and applies two TOF detectors so that the velocity can be measured in addition for every individual ion. This allows one to obtain correct mass values even for ions which are not perfectly isochronous. Improvements of almost a factor 2 for the timing accuracy with at least 95% detection efficiency will be achieved, even though the active area of the detector had to be increased by a factor of four to adapt to the larger emittance in the CR.