Specification for broad-band tagging hodoscope

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TAGH-from-magnet-all.jpg

In its standard configuration the fixed-array hodoscope consists of 218 scintillators distributed over 9.1 m behind the focal-plane of the tagger spectrometer. Standard configuration refers to a coherent-peak setting at 9.0 GeV, i.e. the microscope covers the energy range 8.1<Eγ<9.1 GeV.

Tagging range (for a 12 GeV electron beam):

  • Photons between 3.0 GeV and 8.2 GeV: detector positions correspond to 60 MeV steps in Eγ with 30-50% sampling. The counter widths are chosen such that the typical rate per counter is limited to 3-4 MHz at 108 γ/sec on target;
  • Photons between 9.2 GeV and 11.78 GeV: scintillators cover the full energy range, counter widths were calculated such that the typical rate per counter is limited to 1-4 MHz at 108 γ/sec on target.

All counters will be mounted at angles such that they face normal to the path of the scattered electrons.

Scintillators

Material EJ-228
Thickness     6 mm
Height 40 mm (slotted into the light guides, slot depth: 3mm)


Counter # width
[mm]
distance from
focal plane
Eγ
[GeV]
ΔE
[MeV]
1 – 22 21.0 8 / 18 cm 11.43 – 11.78 10.2 – 23.7
23 – 40 16.0 8 / 18 cm 11.01 – 11.43 18.4 – 29.5
41 – 64 10.0 8 / 18 cm 10.46 – 11.01 18.7 – 26.8
65 – 81 8.0 8 / 18 cm 10.05 – 10.46 21.7 – 26.0
82 – 139 5.0 8 / 13 / 18 cm 9.20 – 10.05 16.4 – 22.2
140 – 195 4.0 8 / 13 / 18 cm 7.07 – 8.13 16.6 – 17.2
196 – 274 3.0 8 cm 3.05 – 7.03 13.7 – 20.3
      TAGH-counter-cut.jpg

Light guides

All light guides (UVT PMMA) have cylindrical shape of 120 mm length and 7/8" = 22.2mm diameter to cover the size of the photocathode. A collar at 60mm height with 2 bolts along the slot line ensure proper positioning of the counters: threaded holes for the bolts in the mounting plates are correspondingly oriented.

Photomultipliers

Hamamatsu R9800: 1" tube with 8 dynodes, rise time 1.0 ns, transit time 11 ns, 27% quantum eff., max. anode current 100 μA.
magnetic shields: μ-metal cylinders.
HV dividers and 2-stage preamps (amp. factor 8.5x) developed at Jefferson Lab, they allow for limiting the anode current to 15-20 μA at 4 MHz rate.
Operating voltage: U=900 V resulting in a gain of 5×105, anode current contributes to less than 5% of the active base current.

Readout electronics The new preamp boards have 2-channel output for fADC250 and F1TDC.


Fixed array counter positions

Table moved to Position and Energy Boundaries for all installed Counters