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.

  • After opening the TAC in the summer of 2017 we did not see a significant darkening of TAC glass. We noticed that there was dry grease on the surface to which the PMT was supposed to be attached and that the spring were not positioned correctly to press the PMT against the glass. So the deterioration of the signal is most likely was due to bad optical coupling between the TAC PMT and the lead glass surface.

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)