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@PHDTHESIS{Kuzminchuk:49360,
      author       = {Kuzminchuk, Natalia},
      title        = {{P}erformance studies and improvements of a
                      {T}ime-of-{F}light detector for isochronous mass
                      measurements at the {FRS}-{ESR} facility},
      school       = {Justus-Liebig-Universität Gießen},
      type         = {Dr.},
      reportid     = {GSI-2013-00989},
      pages        = {139},
      year         = {2012},
      note         = {Justus-Liebig-Universität Gießen, Diss., 2012},
      abstract     = {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.},
      keywords     = {Dissertation (GND)},
      cin          = {FRS},
      cid          = {I:(DE-Ds200)FRS-20110310OR124},
      pnm          = {533 - Exotic Nuclei and Nuclear Astrophysics (ENNA)
                      (POF2-533)},
      pid          = {G:(DE-HGF)POF2-533},
      typ          = {PUB:(DE-HGF)11},
      urn          = {urn:nbn:de:hebis:26-opus-87262},
      url          = {https://repository.gsi.de/record/49360},
}