Tile Upgrade Workshop (CERN- February 2008, 8 and 9) Optical components: PMT Blocks Tile Upgrade Workshop (CERN- February 2008, 8 and 9) François Vazeille  Two main possible effects must be considered: particle rates and/or radiations. Iron µ metal 3 in 1 PMT and Divider Plastic support Mixer Small pieces  3 components are considered here: - Light mixers. - PMTs + Dividers (since they were optimized with PMTs). and not the 3in1 cards to be reviewed in front-end electronics.  Passive components, such as the Plastic molded supports (NORYL) are not affected by radiations. 1

Light Mixers Present LHC design • Produced (10 000) and tested in the Optical laboratory of the Faculty of Mathematics and Physics, Charles University in Prague. • Shape and dimensions + drawing optimized by Clermont-Ferrand: - Square cross section + length  The best light uniformity on PMT photocathode. - Length  The best PMT location inside the magnetic shielding. - Final drawing  Special corners for the assembly inside the Plastic NORYL support. • Material: Extruded acrylic PMMA (Polymethyl Metacrylate) PLEXIGLASS XT 2270 clear-transparent (92%) with highly UV-absorbing material  To kill Cerenkov effect of direct particles. 2

 Do we need a new SLHC design? • Drawing: No modification … since same magnetic field and same PMTs. • Material Clear-transparency level before starting SLHC? - Natural ageing? - Ageing due to the light effect (Natural light, Fiber light)? - Ageing due to radiation?  Should be measured on some Light mixers after some LHC years. - Light mixers taken in the worst positions. - Needs access to the corresponding Drawers. 2. UV- dopant amount before starting LHC? as above. 3. SLHC effects: increase of above effects - Transparency decreased  Light yield decrease expected. - UV dopant decreased + 10 times more direct particles  More Cerenkov effect expected. BUT HOW MUCH? 3

• Next actions before taking decisions - Measures after some LHC years: transparency and UV-dopant efficiency. Bibliography works about: - Natural ageing. - Radiation effects on PLEXIGLASS. - UV-dopant loss with time and radiation + direct particles. To complete the bibliography study if there are doubts or missing information by experimental tests: • Intense light bath. • Radiation tests. Comment: Remaking the Light mixers and changing them should a very big job! 4

PMTs Present LHC design • Produced (10 140) and tested in 7 Institutes (Clermont-Ferrand, Dubna, Lisbon, Pisa, Urbana, UTA, Valencia) • Design optimized by Clermont-Ferrand + other Institutes (Pisa, Valencia…)  7 years collaboration with Hamamatsu: PMT R7877  8 stages Metal dynode structure with a metal package. Compactness and stray magnetic field effects. Gain (~105) and linearity (over 16 bits, non-linearity < 2% at the upper range). Dark current…  Do we need a new SLHC design? • Very likely no. • Information from long term studies TJNAF (USA): same PMTs working and tested before/after at Clermont-Fd and used again after several working years in true conditions. Pisa results (see next talk. • A maintenance must be foreseen: replacement of bad PMTs. 5

Dividers Present LHC design • Produced (11 000) and tested by Clermont-Fd  45 mm • Design optimized by Clermont-Ferrand + Chicago (2 ground levels): - Optimization of the PMT working: • Gain and linearity. • Impact of additional continuous current (MB events)  See the performances (next slide). - Minimum of cables: direct connections with PMT and 3in1 card. - No sensitivity to radiations: only passive components. 6

• More on design and performances Choice of resistors of voltage divider  driven by the current flow. Applied rules for a pure resistor Divider: • Rule 1: Pile-up events 100 times the mean anode current Worst case of pile-up events: ~2 µA  voltage Divider current of 200 µA. Warning: Current simulations gave 576 nA as maximum current for Layer 1  There would be an additional safety factor of about 4. • Rule 2: Intense pulses Storage capacitors in the last stages to balance the voltage drop due to large current flow. Performances: from the study of a set of 250 PMTs over the 10 140 requested PMTs. Anode current 100 nA 2 µA 5 µA Gain variation ~ 0% > 1% > 2% 7

• Do we need a new SLHC design? The maximum current will go from 2 to 20 µA  The Rule 1 would be no longer satisfied only a ratio equal to 10 between the current flow and the anode current or 40 if we take the simulation value of about 500 nA.  Bigger sensitivity to MB events and worst linearity behavior, in particular for very high energy events (the best expected events!). - That should affect only a part of the cells: close to the beam. - Using the Laser should be a way to recover the linearity HOW MANY CELLS WOULD BE AFFECTED? SHOULD WE ACCEPT SOME NON-LINEARITY? SHOULD WE CHANGE EVERY DIVIDER OR ONLY A FRACTION OF THEM? • Next actions before taking decisions New simulations (or new analyses of existing simulations) in order to know better the MB currents in every cell. To look at deeper the R&D results of tested PMT/Dividers + Test Bench results. To decide what is acceptable from the Physics goals, knowing that the Laser is a useful tool to solve the problem. 8

- If requested, is it possible to make an upgrade? YES from the design point of view Already made at Clermont-Ferrand by Michel Crouau for LHCb, with several solutions considered to increase the gain on the last dynodes: - Booster Divider (Additional supplies on last stages). - Cockcroft-Walton Divider (Oscillator supplied by a LV). - Transistor Divider (Transistors on last stages). Chosen solutions: less components are added. Divider with 2 Transistors Resistor Divider anode anode  500 µA flowing in the Divider is sufficient to deal with a 100 µA anode current. 9

Chosen Divider in the LHCb experiment for the 100 Hamamatsu multi-anode PMTs. Comment: Remaking the Dividers and changing them should be a very big job! 10

First conclusions 11 Component Is this a component which MUST be upgraded? If yes, why? Program? If no, is this a component that, with high probability, will not need upgrading? Light Mixer Likely no Perhaps the material but not the drawing Bibliography Light tests Tests after some LHC years The proposed program will lead to a certitude PMT No See long term tests Detect bad ones Maintenance Divider No if a compromise is accepted, otherwise yes Increased MB current Working/Linearity affected for: • First cells • High energy jets Deep analysis of Tilecal R&D New look of MB simulations Physical goals New design possible A compromise in between decreased performances on some cells and possible corrections (Laser) 11