Developments for Isochronous Mass Spectrometry in the CR at FAIR

At the FAIR facility, the projectile fragment separator Super-FRS will provide beams of exotic nuclei with unprecedented intensity. The new Collector Ring (CR) is optimized to accept large-emittance secondary beams provided by the Super-FRS. High-precision mass measurements of exotic nuclei with life-times as short as a few tens of microseconds will be performed with Isochronous Mass Spectrometry (IMS) at the CR. For these measurements a dual time-of-flight (TOF) detector system is under development (Fig.1). In the detectors, ions passing a thin carbon foil release secondary electrons, which are transported in forward and backward directions to microchannel plates by electric and magnetic fields. For even better performance of the TOF detector, a prototype was optimized for transmission and detection efficiency, rate capability [1] and timing characteristics. Timing characteristics were tested by measuring the coincidence between the backward and forward detectors with a fast sampling digital oscilloscope (40Gs/s). The measurement shows coincident time-of-flight distribution with a standard deviation of 68 ps (Fig 2). The dispersion in the time-of-flight of the secondary electrons due to their initial velocity spread and their spatial distribution after emission from the foil was investigated. The software for the TOF data analysis was extended to determine the time-stamps of online experimental data and offline data acquired with different oscilloscopes and different time discrimination methods. The uncertainty of the time-stamp determination is limited by fluctuations, which are caused by jitter, amplitude and rise time walk effects. The precision of the timing methods was tested using the online experimental data and they agree within about 30 ps, while the resolution of the oscilloscope is 25 ps. The influence of the MCP dead time on the rate capability of the detectors was examined experimentally and compared to a theoretical model [2]. When a large output current is drawn from an MCP, the channel walls near the exit end are charged due to the secondary electron emission.