Performance of Lead Tungstate Crystals for the CMS Electromagnetic Calorimeter

Transcription

1 Performance of Lead Tungstate Crystals for the CMS Electromagnetic Calorimeter Ioan Dafinei I.N.F.N. Sezione di Roma, Rome ITALY (on behalf of CMS ECAL Collaboration)

2 Index 1. PWO general properties 2. PWO properties vs ECAL-CMS requirements 3. survey of PWO performance during ECAL construction 4. Conclusion

3 PWO general properties PbWO 4 (PWO) - studied as phosphor since promoted scintillator early nineties ρ g/cm X 0 cm 0.89 R m cm 2.2 τ ns 15 λ emission nm 420 n@ λ emission LY % of BGO 5 α LY %/ C -2 WO 4 2- blue component ( nm ; <25ns) Pb 2+ intensity (a.u.) L uml aser Exc.QP C Luminescence λ exc =308 nm green component ( nm ; >100ns) emision@400 nm emision@500 nm laser pulse time (s) WO 4 3- ECAL PWO crystals supplier Bogoroditsk Techo Chemical Plant Russia suppressed green component improved radiation hardness

4 PWO properties vs ECAL-CMS requirements ECAL-CMS high refractive index! high stopping power emission spectrum overlapping with spectral sensitivity of APD and VPT (see Q. Ingram s talk) small X 0 small R m high granularity high density and high Z high energy resolution 100GeV BGO PWO CeF large LY APD and VPT LHC Bunch separation 25 ns Luminosity cm-2 s-1 fast radiation hard BGO PWO CeF light production is not affected creation of color centers is quenched by appropriate doping

5 PWO properties vs ECAL-CMS requirements PWO crystals needed for ECAL-CMS Endcap Endcap Barrel Barrel Endcap Xtals point 3º No.of Xtals Xtal size (cm) transv: long: (34 types) (1 type) (Rm) 3 23 (25.8X0) 22 (24.7X0) away from vertex Total weight 67.4 tons 22.9 tons Photosensor Avalanche Photo-Diode (Si) Vacuum Photo-Triode Ioan Dafinei

6 Survey of PWO performance during ECAL construction premise 1 at present (ECAL construction phase), the PWO crystal is taken as it is i.e. no spectacular modifications of its scintillation properties are expected keep the qualification parameters inside the limits defined by ECAL Collaboration find the ways to get the best possible performance from the ECAL modules built with this PWO very strict crystal quality control parallel radiation hardness tests LY characteristics radiation damage (see P.S.Roldan s talk) QC measurements for LY are not routine check but a first step of calibration calibration monitoring

7 Survey of PWO performance during ECAL construction premise 2 (see R-Y.Zhu talk on calibration and monitoring) calibration - defines the ultimate performance of the ECAL - is influenced by a very large number of factors (electronics gain, APD amplification factor, containment efficiency, etc) besides scintillation characteristics of PWO monitoring the change of PWO crystals transparency during ECAL exploitation phase will be monitored in situ with a laser and a correction to particles signal will be applied: where: BeamSignal R = BeamSignal MonitorSignal MonitorSignal : fractional decrease in the beam signal : fractional decrease in the light monitoring signal

8 Survey of PWO performance during ECAL construction premise 3 Doses in 10 years in ECAL HCAL Barrel dose (kgy) (at photodetector) ECAL Endcap ECAL Barrel neutron fluence (/ cm -2 ) (at photodetector) dose (kgy) (at shower maximum in crystal) tests made in: -CERN (l Hopital de Genève) -CERN GIF and H4 -Rome ENEA Casaccia -California Institute of Technology Ioan Dafinei

9 Survey of PWO performance during ECAL construction PWO crystals tests at GIF Gamma Irradiation Facility Irradiation (transversally): Cs 137 (0.662 MeV) ; doserates close to LHC conditions Crystal 1 and 5: 0.20 Gy/h Crystal 2 and 4: 0.26 Gy/h Crystal 3: 0.30 Gy/h Excitation scintillation: luminescence: SPS e beam (50 GeV) UV laser (λ=337±10 nm, 1.55mJ/pulse, t =600 ps) Monitor longitudinal transmission (LED) λ=450±50 nm, t=500 ns Ioan Dafinei

10 Survey of PWO performance during ECAL construction PWO crystals tests at GIF DW3 BEAM DW1 DW2 VETO S2 Ei Crystals scan Crystals box X5 beam size: approx. 3cm ~10m ~2m ~1m Laser fiber crystals are tested in batches of 5 light detectors are standard APDs Lamp fiber LED box Cooling pipes monitoring of different light sources by a PM coupled to a 6 th crystal monitoring of laser by PIN diode monitoring of gain with a constant charge injected to the amplifiers

11 Survey of PWO performance during ECAL construction PWO crystals tests at GIF Measure R coefficients defined as: R( t) = S S A B ( t) ( t) S S A B ( t ( t = = 0) 0) t=0 (irradiation start) LED irr 59 irr 61 irr 62 where A and B correspond to different ways to produce the light measured by the APD (eg: BEAM and LED) Laser Beam Data analysis: -test pulse correction -temperature correction -normalization (monitor signal) -signal cleaning -normalization to 1 (signal at start) Ioan Dafinei

12 Survey of PWO performance during ECAL construction PWO crystals tests at GIF R BEAM/LED = 1.9 ± 4.2% R BEAM/LASER = 1.6 ± 8.4% R LASER/LED = 1.2 ± 9.6% Crystal to crystal dispersion of R is not significant (inside the errors) Good reproducibility of mesurements Good correlation R LASER/LED vs R BEAM/LED σ R LASER/LED = 9 % σ R BEAM/LED = 5 %

13 Survey of PWO performance during ECAL construction PWO crystals tests at Caltech Caltech γ ray Irradiation Facilities 60 Co 137 Cs studies: luminescence transmittance radiation hardness and recovery light output and decay kinetics Λ = 50 Curie 15, 100, 400 rad/h Λ = 2000 Curie up to 36 Krad/h steps: (1) thermal annealing at 200 C (2) γ-irradiations: 15, 100, 400 and 9k rad/h (until equilibrium) (3) recovery Crystals are kept in dark at 18 C after annealing. Minimum time between annealing and 1 st measurement is 48 hours. (see rad.hard dedicated talk of R-Y. Zhu) Ioan Dafinei

14 Survey of PWO performance during ECAL construction PWO crystals tests at Caltech type I type II type III crystals are hopefully discarded by ECAL rejection criteria (see P.S.Roldan s talk)

15 Survey of PWO performance during ECAL construction PWO crystals tests at Caltech damage and recovery kinetics (see dedicated talk of R-Y.Zhu) ECAL monitoring takes 30 minutes to cover entire calorimeter in situ. Damage and recovery time constants decide whether the monitoring system can adequately track the changes in light yield Longitudinal transmission recovery characteristics type I type II t1= ~40 h ; t2= ~80 d t1= ~30 h ; t2= ~50 d In both cases the fastest component is slower than monitoring time Ioan Dafinei

16 Survey of PWO performance during ECAL construction PWO crystals tests at Caltech Caltech tests generally confirm the results obtained on PWO during R&D and pre-production no variations in either light output or longitudinal transmittance observed 8h after the thermal annealing STABLE no modification under γ-ray exposure of either excitation or emission spectra RELIABLE no very short time recovery components MONITORING OK samples can be divided into 3 types from radiation hardness point of view

17 Survey of PWO performance during ECAL construction studies on PWO crystals for EE Detector Performance Group Average light yield at LHC, versus Eta Starting luminosity (10 33 ) High luminosity(10 34 ) 100% D all total density of the potential color centers 80% 60% 40% Improved radiation tolerance for EE crystals would be of benefit

18 Survey of PWO performance during ECAL construction beam tests at CERN SM0 fixed on the turn table, ready for beam test SPS beam φ 2002 preliminary results η Ioan Dafinei

19 Survey of PWO performance during ECAL construction beam tests at CERN electron beams irradiation: cumulated doses between 2 and 8 Gray S S = α R 0 R 0 Linear relation between signal and monitor! statistics made on the 20 irradiated crystals σ/µ = 6.1% S: scintillation light signal R: monitoring light signal α determined with 3% precision and intrinsic dispersion 5% universality of irradiation slope α (inside 6.1%) Ioan Dafinei

20 Survey of PWO performance during ECAL construction beam tests at CERN 1 st irradiation dose rate: 0.25Gray/h cum.dose: 2.5Gray Behavior under LHC cycle conditions 2 nd irradiation dose rate: 0.18Gray/h cum.dose: 1.8Gray p o w e r recovery c u t same slope for different irradiations good reproducibility during multiple irradiations and recoveries (LHC cycle) same slope for irradiation and recovery Ioan Dafinei

21 Conclusion (1) PWO crystals presently produced at industrial level and used for ECAL modules construction: have optical characteristics (scintillation and transmission) stable enough to guarantee a stability of the final monitoring system at the level of 1.4 have the same linear dependence between signal and monitor during multiple irradiations and recoveries (LHC cycle) allow for good reproducibility of relative intercalibration allow relative inter-calibration at the level of 4.5% from routine quality control data (ACCOS machines)

22 Conclusion (2) Several techniques were put in place aimed at predicting the Lead Tungstate (PWO) crystal performance at the future CMS experiment They converge to the conclusion that the PWO choice for ECAL-CMS was the good one