Isochronous Mass Spectrometry has been developed to measure masses of exotic nuclei with lifetimes as short as a few tens of microseconds at the FRS-ESR facility at GSI. For measurement of the ions revolution frequencies, a time-of-flight detector is used. Secondary electrons released from a thin carbon foil at each passage of the stored ion through the detector are transported to micro-channel-plates (MCP) by electric and magnetic fields.
This time-of-flight detector, currently installed in the ESR to measure the masses of ions in the isochronous mode, was investigated in this work by experiments and realistic simulations. The detector efficiency was optimized off line with a-particles and electrons and tested on line with a stable Nickel beam. All stages of the detector from the creation of secondary electrons to the final timing signals were examined. The typical number of secondary electrons released per ion can be estimated within a factor of two using an empirical formula. The formula incorporates the target properties andthe electronic stopping power of the ion. Typical average electron numbers for mass measurements in the isochronous mode range from 1 to 10 electrons. The transport of the electrons from the foil to the MCP was calculated for the first time using the 3-dimensional geometry of the detector. The simulation helped to understand the transport of the electrons in the detector and thus optimize the detection efficiency while preserving the timing performance. With the calculate settings the detection efficiency and also the detection duration on one MCP detector side were significantly improved (factor of 2).
The detection efficiency of the MCP in dependence of the average number of secondary electrons was also examined in the experiment. The detection efficiency of the MCP detector for a Ni-projectile at 372 MeV/u was estimated to be about 88%. In addition saturation effects of the MCPs were examined . The saturation effect is a dead time effect, which happens because of the large recharge time of the micro channels (1-10 ms) compared to the measurement time (400 ms). This effects causes the decrease of the gain and so the amplitude of the signals. This also will have the effect of decrease of the detection efficiency. The dependence of the detection efficiency of the thickness ofcarbon foils was measured and yields an excellent detection efficiency for foil thickness down to 10 µg/cm². Using thinner foils will allow longer observation times in the ring due to less energy loss and straggling. By taking all these effect into account it is possible to describe the behaviour of the detection efficiency and the amplitude in dependence of the turn number. This new knowledge makes it possible to further improve the detector in the ESR and within the ILIMA project to develop a new dual detector system for the CR at FAIR.
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