The CMS ECAL Laser Monitoring System IPRD 2008 11th Topical Seminar On Innovative Particle and Radiation Detectors Adi Bornheim California Institute of Technology On behalf of the CMS ECAL Collaboration Siena,
90 Tons of Lead-Tungstate 2
Introduction CMS is building a high resolution Crystal Calorimeter (ECAL) to be operated at LHC in a very harsh radiation environment. Barrel resolution design goal : Current effective resolution goal : 2.5% / E 0.55% 0.2 / E <0.5% for energies above 100 GeV Calibrating and maintaining the calibration of this device will be very challenging. Hadronic environment makes physics calibration more challenging PWO4 Crystals change transparency under radiation. The damage is significant (few % - up to ~5 % for CMS ECAL barrel radiation levels) compared to the desired constant term (0.5 %). The dynamics of the transparency change is fast (few hours) compared to the time scale needed for a calibration with physics events (days - weeks - month). Compensate by monitoring the change with a laser monitoring system. 3
Radiation Effects on PWO4 Transparency Radiation Reduces transmittance in the blue and green, peak of PWO4 emission spectrum Effect is dose rate dependent. Monitoring relative loss of PWO4 transmittance with pulsed laser light. For the expected dose rate at CMS barrel (15 rad/hour), typical transmittance loss is at a level of up to ~5%. Almost no effect in the red wavelength range. Monitor with red light to separate out possible variations in the light distribution system and the readout chain. Approx. PWO emission spectrum 4
PWO4 Transparency Change Characteristics Crystal light yield changes under irradiation. Change is dose rate dependent. Crystal light yield change under irradiation is linearly correlated with longitudinal transmittance (transparency). Magnitude of the transparency change is crystal dependent. Transparency change recovers at room temperature. Recovery time is crystal dependent with two time constants, one of few 10 hours and one >1000 hours. 5
Damage and Recovery in a LHC Cycle Simulation Test Beam Data Depends on : Radiation level (η, Luminosity) Crystal characteristics Damage Recovery Damage Recovery Damage-recovery cycle in sync with the ~12 hour LHC fill cycle 6
In-Situ Monitoring & LHC Bunch Train Abort Gap Abort gaps occur at ~10 khz - Laser pulses at ~100 Hz Use ~1% of gaps. Measure transparency of all crystals from one half-module at a time. 600 laser shots for one transparency measurement. Laser pulse latency ~4 µs Scan entire ECAL every 20 minutes 7
On-Detector Monitoring System APD PN APD Very stable PN-diodes used as reference system Each Level-1 Fan-out is seen by 2 PN diodes, each PN diode sees 2 Level-1 Fan-out, 10 PN diodes per SM SM are illuminated one half at a time, 88 LM for full ECAL Barrel crystals front illuminated, endcap rear illuminated Precision charge pulsing system for electronics calibration 4 VPT 8
Ti:Sapphire Laser with Two Wavelengths Nd:YLF Pump Tunable Ti:S 10 kw 440 nm 500 nm 700 nm 800 nm 100 mw 9
Long Term Laser Performance Changes in the pulse shape of the laser are the larges systematic effect. Precise control of laser performance is of utmost importance. 10
Light Distribution System 1.015 1.01 1.005 1.0 0.995 0.99 0.985 11
Endcap LED Pulser System ECAL EE equipped with additional LED pulser system. Provides additional wavelength (~600 nm) and allows high rate pulsing. Complementary to laser system. 12
Monitoring System Performance - Stability Stability : RMS of the measured APD/PN values over a period of 4 weeks. Mean before and after correction for laser pulse shape changes : 0.180 % Peak before and after correction for laser pulse shape changes : ~0.170 % 0.088 % ~0.05 % Typically ~0.1 % long term stability in real environment. This includes the stability of the entire readout chain - temperature, HV, etc. We can measure the crystal transparency with better than 0.1 %. 13
Laser Light Loss Electron Signal Loss # Crystals Dispersion of α for 35 BTCP crystals signal from Beam (e-) slopea: α = 1.55 a 2002 2003 mean = 1.538 2004 (fall) σ/mean 6% 2004 (spring) 2006 (center) 2006 (neighbor) signal from Laser α About 100 α-parameter extractions with final hardware on 35 barrel and 41 endcap crystals have been performed. At startup use same parameters for all crystals from one producer. An in-situ determination of α is under consideration. 14
Correcting Transparency Change Single irradiation cycle effects : Transparency change can be corrected to better than 0.1 % Energy resolution is fully restored to pre-irradiation level 15
Laser Monitoring Online Data Flow ECAL DAQ LaserSystem EMTC Other Subdetectors ECAL TTCci Calibration Events CMS Global Trigger Storage Offline Managers LaserFarm Disk Buffer Readout Units Online PCs HLT Filter Filter Farm Laser data acquisition fully integrated in the CMS DAQ and data flow. 16
Running experience in CMS Quasi Online Analysis Plots CMS Online DQM Plots Raw data rate for laser monitoring is 4 Mb/s. Data is sampled by DQM and analysed in ECAL Laser Farm detail in a quasi-online fashion. Aim to have transparency corrections available on the level of 0.1% at the end of a LHC fill. So far mostly used for commissioning, debugging and timing studies of ECAL. Stability studies are under way. Regular data taking has commenced, aiming to monitor at design precision. 17
Summary CMS ECAL Laser Monitoring System has been installed and commissioned in the final detector. All performance criterions have so far been achieved. Next step is to validate the stability of ECAL response to the level of 0.1 % over several weeks without irradiation. Then, operating the system and follow the crystal transparency on the level of 0.1% over 10 years. 18