Total Absorption Counter

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CLAS Total absorption counter used for PrimEx

The table below summarizes the information I obtained from various sources about the TAC counter that GlueX borrowed from CLAS.

Table: Properties of CLAS TAC counter
Property (source) Value
Material (from Mahbub) SF-5 Lead Glass
Composition by weigth (Web) 55% PbO, 38% SiO2, 5% K2O, 1% Na2O
Length (from Mahbub) 40cm
Height (from Mahbub) 20cm
Width (from Mahbub) 20cm
Weight of lead-glass block 65 kg
Density (Web) 4.08 g/cm^3
Radiation Length of material (Web) ~25mm
Critical Energy (Web) 15.8 MeV
Refractive index (Web) 1.67270
PMT (from Mahbub) 5 inch Hamamatsu R1250 PMT


Observed problems

After using Hall B lead glass TAC between 2016 and 2017 the light output significantly decreased. The radiation hardness of this lead glass is highly unlikely to exceed 10^5 rad. We are not going to be able to significantly reduce the radiation dose received by TAC since we need to accumulate meaningful statistics in the pair spectrometer with a reasonable converter thickness that does not distort the PS acceptance shape with respect to the production converter. Therefore, it is important for GlueX to have a total absorption counter made of material with radiation hardness of ~10^6 rad or better.

Options considered earlier

  • lead-glass counter array:
    • efficiency ~100%
    • to avoid radiation damage: rate < 100kHz using radiator of 10-5 X0 and ~100 pA beam
  • Cerenkov counter (higher current - but probably lower efficiency)