sPHENIX HCal discussion

[Edward Kistenev <kistenev AT bnl.gov>]

Here is a proposal on dealing with hot spots at fiber exits for this year test beam (similar proposal was in my earlier mail to Ron). It is based upon assumption (still needs to be totally proven) that it is an artifact of the light (cladding and direct POPOP emission) which is presently able to reach SiPM from the body of scintillator nearly unobstructed. Note that the area around fiber exits has little or no reflective coating (polished away while finishing fibers).

The best would be to add 0.1mm Al Foil in between the tile and SiPM holder with two precise holes keeping just the fibers exposed. Important that foil should be black painted on both sides. The depth of the holder (gap between tile and SiPM should be decreased to 0.65mm (currently it is 0.75mm). 

Here at UTFSM we are also considering the option with 5mm thick black plastic insert glued into the tile edge with two 1mm channels for fibers and 3mm hole for LED. Tile will be cut at a time of grooving, LED will shine into clear plastic (no coating is left after tile cutting). Another option is 5mm extension to the tile but this takes a space and a bit more cumbersome for locating. In the next few days we will produce couple of tiles which I will ship to BNL for measurements. 

I already asked Andrey to add two station of silicon detectors to the box so even if the trigger will still be based upon relatively large scintillators at least hits in the vicinity of the fiber exits could be measured with good resolution. And this will be a good test of hodoscope readiness for the test beam.

Edward

PS. I attach couple of pictures showing modified fiber exits. We were testing how difficult it will be to go down to 4.5 mm depth in tile to put two fibers on top of each other. No problem but we still do not know if plastic will get brittle when it is 7mm thick and nearly cut through - this can be an issue.

Edward Kistenev, PhD

PHENIX Physicist