[Halld-tagger] tagger placement mystery solved

[Richard Jones]

Hello all,

I believe that the following explanation accounts for all observations
reported above.


   1. The microscope is constructed as described above by James. He is
   applying all of the necessary updates to the 3D model to bring it into line
   with the actual detector as assembled.
   2. The beta angle of the assembled detector starts at 12.50o at the
   upstream end and varies continuously to 11.06o at the downstream end, with
   16 equal steps of 0.09o every 6 columns. The total length of the detector
   coverage (102 columns) along the focal plane thus assembled is 100.0 cm
   within precision < 1mm.
   3. Measurements of detector length report a different length than this,
   but comparing apples to apples gives a consistent picture with what was
   built at UConn.
      - When Alex measured it, he was reporting the distance from the
      center of fiber column 2 to the center of fiber column 101. The computed
      value for this length is 97.0 cm, to be compared with his
measured range of
      96.5 - 97.0 cm.
      - When the survey crew measured it, they are reporting the distance
      from the center of fiber column 2 to the center of fiber column 98. The
      computed value for this length is 93.87 cm, to be compared with their
      measured value of 93.82 cm.
   4. The overlap between the TAGH and the TAGM counters at the upstream
   end is explained by different assumptions about the placement of the tagger
   microscope by the one computing the tagger energy bins (Dan Sober) and the
   one responsible for checking the mechanical drawings, physical alignment
   and survey (Tim Whitlatch, Bill Crahen and the UConn crew).
      - The tagger energy bin boundaries (in units of the endpoint energy)
      were computed by Dan Sober (reported on the wiki) and were entered from
      there into ccdb by Richard J. To find these on the gluex wiki,
go to (main
      page) -> photon beam (right-hand menu) -> tagger (right-hand menu) ->
      Analysis of Tagger Field Mapping Data -> Tagger microscope tagged energy
      bin boundaries, based on Dan's map - Richard Jones, June 30, 2014. These
      assumed that the highest energy tagged by the microscope was 9.09 GeV.
      - The mechanical drawings (UConn and Jlab) were based on the
      assumption that the microscope would span the range 8.2 - 9.2
GeV, arranged
      symmetrically around the desired peak region 8.4 - 9.0 GeV. The detector
      offset along the focal plane was computed by the UConn crew on
this basis,
      using Dan’s map analysis. This places the first fiber channel at 9.2 GeV
      (for a 12 GeV beam) not at 9.09 GeV. In the TAGH array, 9.2 GeV falls in
      the acceptance of counter 126. This agrees with the overlap seen by Alex
      Barnes.
      - This places the end of the microscope acceptance at 8.2 GeV, whcih
      lands in TAGH counter 175. However, the TAGH array was not
instrumented up
      to this point until counter 181. This leaves a gap of 6 TAGH counters on
      the low-E end of the microscope where there is no tagger coverage.

I will submit an update to ccdb this evening that will report the correct
TAGM energy values in the offline analysis. Franz Klein is looking into
whether it is practical to install counters in the gap between 175 and 180
in the TAGH numbering scheme. He can use electronics channels currently
allocated to 126 - 131 since those are redundant with the TAGM in the
present alignment scheme.

-Richard Jones
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