Total Absorption Counter
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.
|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|
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)