Yu. Guz 13/06/ ECAL LED system : degradation of clear fibers The radiation degradation of clear fibers of the LED system is (still) well pronounced. Therefore the LED system cannot be used in calibration, its usage is limited to: quick control of PMT operation during maintenance at shutdown technical control during data taking (instabilities in contacts etc) (?) cross-check of gain change at HV update After upgrade, at x10 luminosity, the degradation will go faster. -18% -4%

Yu. Guz 13/06/ ECAL LED system – upgrade? If the LED system was stable, it could be used for mid-term (~1-2 month) response following, which in particular makes possible: quickly update HV, to keep L0 level constant perform fine calibration easier and more accurate The LED system can be upgraded into a stable one: replace clear fibers into rad hard ones (e.g. quartz) (we need ~25 km) and replace LEDs to green ones (higher QE will compensate smaller diameter of quartz fiber) technical issues Q: what to do with the 40 cm pieces of clear plastic fiber embedded into each ECAL cell? A: leave them as is, they are useful: their loss of transparency represents that of nearby WLS fibers, so the LED system will, to some extent, follow also the radiation damage of ECAL cells. Q: what is the cost of rad hard quartz fibers? A: Draka SuperElite Super RadHard MMF: <1 CHF/m (maybe cheaper solution can be found) of course there are more open issues for which R&D is necessary

Yu. Guz 13/06/ ECAL LED system – upgrade? If the LED system was stable, it could be used for mid-term (~1-2 month) response following, which in turn makes possible: quickly update HV, to keep L0 level constant perform fine calibration easier and more accurate The LED system can be upgraded into a stable one: replace clear fibers into rad hard ones (e.g. quartz) (we need ~25 km) and replace LEDs to green ones (higher QE will compensate smaller diameter of quartz fiber) technical issues Q: what to do with the 40 cm pieces of clear plastic fiber embedded into each ECAL cell? A: leave them as is, they are useful: their loss of transparency represents that of nearby WLS fibers, so the LED system will, to some extent, follow also the radiation damage of ECAL cells. Q: what is the cost of rad hard quartz fibers? A: Draka SuperElite Super RadHard MMF: <1 CHF/m (maybe cheaper solution can be found) of course there are more open issues for which R&D is necessary BUT THE MAIN QUESTION IS: Q: Do we need it at all, or we are happy with what we have? opinions and comments are welcome