Performance studies and improvements of a Time-of-Flight detector for isochronous mass measurements at the FRS-ESR facility

摘要

At GSI Darmstadt the technique of Isochronous Mass Spectrometry (IMS) has been developed for direct mass measurements of exotic nuclides. In this method a cocktail beam of highly-charged ions is produced via projectile fragmentation or fission, separated in the FRagment Separator (FRS) and injected into the Experimental Storage Ring (ESR) operated in an isochronous mode. The mass of the exotic nuclei can be deduced from precise revolution time measurements by a time-of-flight (TOF) detector placed in the ESR. In the detector ions passing a thin foil release secondary electrons, which are transported to two microchannel plate (MCP) detectors in forward and backward directions by electric and magnetic fields. In this work the performance characteristics of the detector were investigated by simulations and by offline and online experiments and significantly improved. In particular the timing performance and the rate capability were measured and enhanced. The detection efficiency improvements developed in previous work were verified and the use of thinner carbon foils to increase the number of turns of the ions in the ring were implemented. This work also forms a basis for the development of a dual detector system for IMS in the collector ring at FAIR.In this work the main contributions to the TOF detector timing such as the transport time of the secondary electrons, the electron transit time through the MCPs and the method of determination of the event time from the MCP signals (event time determination) were analyzed and improved. The timing accuracy of the TOF detector was investigated by coincidence time-of-flight measurements. The timing uncertainty of a single branch of the detector with standard settings was measured in the laboratory with an alpha-source and amounts to sigma(branch)=48 ps. In an online experiment at the ESR using MCPs with 5 µm pore sizes the timing accuracy was measured as sigma(branch)=48 ps with a stable 20^Ne beam and sigma(branch)=45 ps with 238^U fission fragments. Those measurements were performed for the kinetic energy of the secondary electrons (K) equals 700 eV.To improve the transport time of secondary electrons the TOF detector was modified for higher values of electric and magnetic fields. An improved time spread sigma(branch)=37 ps was obtained in the measurements with alpha-particles using MCPs with 10 µm channel diameter for an kinetic energy of 1400 eV of the secondary electrons.The contribution from the transit time through the MCP channels to the time spread was investigated with alpha-particles as a function of different electron yields from the carbon foils. Using a higher thickness of the carbon foil timing is not improved significantly. Therefore, 10 µg/cm^2 is an optimum for the carbon foil thickness in the matter of efficiency and timing. In case of a foil with a Cs-compound on the surface, for which the number of secondary electrons is increased by a factor of 10, the timing was improved to sigma(branch)=27 ps (K=1400 eV).A newly constructed anode design improves the bandwidth of the MCP detector by a factor of 2 leading to a reduction in the width of the MCP signals by a factor of two to an improvement of the rise time by about 20%. The signal shape of the MCP detector influences the determination of the revolution times of the ions in the ring and thus the mass measurement accuracy.Due to the high revolution frequencies of the ions in the ESR (~2 MHz) a high rate capability detector is required. The rate acceptance of the MCP detector was improved in the offline experiments by a factor of 4 due to the larger number of channels of MCPs with 5 µm pore size.At each turn in the ESR the ions pass the foil and lose energy. According to simulations the decrease of the foil thickness by a factor of two allows to double the number of ion revolutions in the ring. To store ions for a longer time in the ESR a thinner carbon foil with a thickness of 10 µg/cm^2 and MCPs with a 5 µm channel diameter were installed in the TOF detector and used for the first time in the online experiments. The results of the experiments measured with 10^Ne^10+ stable beam and 238^U fission fragments were compared to the results of the previous experiments. In the previous experiments a carbon foil with a thickness of 17 µg/cm^2 coated with 10 µg/cm^2 of CsI on both sides, which caused a calculated energy loss of 86 keV (86^As^33+, 386.3 MeV/u) and MCPs with 10 µm pore size were used. For the carbon foil of 10 µg/cm^2 the calculated energy loss is 31 keV, that is a factor of 2.7 less than for the thicker foil. Summing up the results, with thinner carbon foil and higher rate resistance MCPs with 5 µm pore sizes in the TOF detector up to ten times more ion revolutions in the ring were observed. With larger number of turns in the ring one increases the detection efficiency and the mass measurement accuracy.