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					Hall D Photon Beamline-Tagger Review      Jan. 23-25, 2005, Newport News




                           The GlueX Photon Beam


                              presented by
                  Richard Jones, University of Connecticut

              GlueX Tagged Beam Working Group
                           University of Glasgow
                         University of Connecticut
                       Catholic University of America
      Presentation Overview
                  Design requirements
                  Focal plane geometry
                  2D segmentation
                  Scintillating fiber design




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Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006
       I. Design Requirements
                                                                 rates at 3 mA on a 10-4 radiator (108 g/s)
      spectrum coverage
          70% - 75% in 0.1% steps


      energy resolution
          0.1% (9 MeV) r.m.s.


      rate capability
          up to 500 MHz per GeV




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Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006
      Requirements: Channel Count

              1. channel width from energy resolution:
                                            9 MeV

              2. channel width from rate limitations:
                               2 MHz
                                                = 4 MeV                  OR, keep the channel width
                         500 MHz/GeV                                     fixed at 9 MeV and introduce
                                                                         transverse segmentation
              3. additional margins:

                               30% margins                                    100 energy channels


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Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006
      II. Focal Plane Geometry
            1. Focal plane x coordinate is roughly linear in energy
            2. Crossing angle is changing quite rapidly with energy


                                                                              limited mobility



                                                                              nominal
                                                                              position




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Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006
      III. 2D Segmentation
       What is the argument for measuring the out-of-plane scattering angle?

            y
                                                                     9 GeV
                                                                      qy
                                        3qy
                                              (P kick from crystal is negligible)   collimator
                               3 GeV



               This photon cannot get through the collimator but its electron still
                reaches the focal plane and produces a count there.

               By reducing the acceptance of the tagging counters to a region
                                           | qy | < qhalf-collimator
                part of the tagging efficiency lost through collimation is recovered.

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Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006
       Vertical tagger coordinate scale

      Monte Carlo test
          Hall D collimator
          detailed tagger optics
          quadrupole field on


      tagging efficiency
          improved by 30%


      impact on polarization
          negligible




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Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006
      IV. Scintillating Fiber Design
       Design         parameters
            square scintillating fibers

            size 2 mm x 2 mm x 20 mm
                                                                               SiPM sensors
            clear light guide readout                                        clear light fibers
                                                                              scintillating fibers
            aligned along electron direction
                                                                              focal plane
               for reduced background sensitivity                             electron trajectory
            readout with silicon photomultipliers
               (SiPM devices)




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Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006
      Silicon Photomultiplier Fiber Readout
        Excellent characteristics for this application:
            Fast timing (x2 faster than PMT’s)
            Dynamic range factor 1000-10000
            Gain similar to phototube (106)
            Requires no HV (only ~50V bias)
            QE similar to PMT (higher in green, lower in blue)


                                                 l            Device           e    QE   h
        Data reported
         by G. Lolos et.al.                    550 nm          SiPM           0.3   60% 20%
                                                               PMT            1.0    5%  5%
                                               437 nm          SiPM           0.3   45% 15%
                                                               PMT            1.0   25% 25%

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Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006
      Silicon Photomultiplier Fiber Readout
         Expected light yield for axial electrons:

                  total path length in scintillator: 2 cm
                  scintillation light emitted: 104 photons / MeV
                                                                              300 p.e.
                  fiber capture fraction: 5%
                  quantum efficiency: 15%



        One complete electronics chain per energy channel
        Individual SiPM bias voltage settings under computer control
        Conventional counting is possible by enabling all rows

        Vertical alignment requirement is similar to that for collimator
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Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006
      Summary
                 A design for the fine-grained region of the Hall D tagger
                  hodoscope has been found which satisfies the resolution
                  and rate requirements for the GlueX experiment.

                 The design achieves a 30% improvement in tagging
                  efficiency over conventional tagging by configuring the
                  tagging counters as a non-defining collimator slit.

                 Scintillating fibers oriented longitudinally to the electron
                  tranjectories provide adequate light output for good timing
                  and high-efficiency operation, while reducing their sensitivity to
                  off-axis backgrounds.

                 A conceptual design for the readout exists.

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Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006
      Tagging spectrometer: two dipole design
           radiator
                          quadrupole

                                        dipole 1
                                                             dipole 2
                                                                                         full-energy electrons
                                                                         photon beam




                                              focal plane




           final bend angle:              13.4o                             gap width:        3 cm
           dipole magnetic field:         1.5 T                             pole length:      3.1 m
           focal plane length:            9.0 m                             dipole weight:    38 tons
           energy resolution:             5 MeV r.m.s.                      coil power:       30 kW

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Richard Jones, Hall D Beamline-Tagger Review, Newport News, Jan 23-25, 2006

				
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