1. The CMS electromagnetic calorimeter
M. Paganoni
University of Milano Bicocca and INFN
on behalf of the CMS ECAL collaboration
The Xth Vienna Conference on Instrumentation
Vienna, February 16-21, 2004

2. M.Paganoni, Univ. Milano Bicocca and INFN
The LHC collider
pp collider at Ecms=14 TeV
max. Luminosity = 1034 cm-2 s-2
inel= 100 mb  109 events/s
Higgs= 1 pb  10-2 events/s
1 crossing/25 ns (40 MHz)
20 events/crossing  1000 tracks
Doses in ECAL (10 years)
Neutrons: n/cm2
Gammas: 4 – 200 kGy
Extreme conditions for detectors
Granularity (105 107 channels)
Speed of response
DAQ + trigger (109 102 ev/s)
High radiation resistance
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

3. M.Paganoni, Univ. Milano Bicocca and INFN
The CMS experiment
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

4. M.Paganoni, Univ. Milano Bicocca and INFN
Higgs hunting at LHC
Natural width (GeV)
Higgs Mass (GeV) 50
100
200
400
800
0.001
0.004
1.4 30
250
LEP
LHC
H  gg
H ZZ* 4l
H ZZ 4l
H WW or ZZjj
With 100 fb-1 and CMS electromagnetic calorimeter
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

5. Physics requirements
The benchmark is the discovery of a low mass Higgs:
m = 2 E1 E2 (1-cos)
H/mH ≤ at mH ~100 GeV
experimental resolution is crucial
Energy resolution :
a ~ GeV1/2
b < 200 MeV
c ~ 0.005
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

6. The choice of a crystal calorimeter
Excellent energy resolution
Structural compactness, easy assembly
Tower structure, fine transverse granularity
4.5
2.4
NaI(Tl)
BaF2
CsI(Tl)
CsI
CeF3
BGO
PWO r
3.67
4.88
4.53
6.16
7.13
8.26
g/cm3 X0
2.59
2.05
1.85
1.68
1.12
0.89 cm RM
3.4
3.8
2.6
2.2 t
250
0.8/620
1000 20 30
300 15 ns lp
410
220/310
565
310
310/340
480
420 nm
n(lp)
1.56
1.80
2.15
2.29 LY
100%
15%
85% 7% 5%
10%
0.2%
%Nal
  /MeV
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

7. M.Paganoni, Univ. Milano Bicocca and INFN
The choice of PbWO4
Fast scintillation
Small X0 and Rm
Intrinsic radiation hardness
Relatively easy to grow
Massive production capability
Low Light Yield
Strong L.Y. dependence on T
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

8. M.Paganoni, Univ. Milano Bicocca and INFN
CMS ECAL Structure
Preshower (Pb/Si) 3 X0
Barrel: |h|<1.48 , 25.8 X0
61200 PbWO4 crystals, ~22x23x230 mm3
Endcap: < |h| < 3.0 , 25 X0
14648 PbWO4 crystals, 30x30x220 mm3
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

9. M.Paganoni, Univ. Milano Bicocca and INFN
ECAL - barrel
Module (40/50 Submodules)
Submodule (10 crystals)
Submodule
Module
Supermodule (4 modules)
Supermodule
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

10. M.Paganoni, Univ. Milano Bicocca and INFN
ECAL - endcaps
Supercrystal
Dee
138 Supercrystals
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

11. Crystal R&D phase (’95-’98)
Satisfactory and homogeneous properties 98 95
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

12. M.Paganoni, Univ. Milano Bicocca and INFN
Crystal Transparency
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

13. Crystal radiation hardness
Front irradiation. 1.5 Gy. 0,.15 Gy/h
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

14. Uniformity of light collection
all polished
Ra=0.34 m
Ra=0.24 m
Tapered shape of the crystals and high n (2.3) produce non uniformities in the light collection
Uniformity can be improved, at the expense of Light Yield, by depolishing one surface with a given roughness
Max d(LY)/dX0=  0.35%/X0
LY (8X0) = 8 p.e./ MeV (18 C)
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

15. Crystal production (’01-’04)
Technology steps in Bogoroditsk
Barrel
2000
65 mm
Endcap
1999
44 mm
Barrel
1996
32 mm
F = 32 mm
2 in one!
F = 65 mm
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

16. Check of the crystal quality
INFN/ENEA
Rome
CERN
Control and storage to a dbase of:
Dimensions
Transmission
Light Yield and uniformity
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

17. Calorimeter assembly Regional centers in Rome and at CERN
48 modules produced and 8 supermodules assembled
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

18. The photodetectors (barrel)
Avalanche PhotoDiodes developped with Hamamatsu:
Solid state detectors: insensitive to the CMS Magnetic Field
Good matching to PWO4 emission spectrum (Q.E~80%) and Internal Gain (M=50 for V~380V) compensate for poor L.Y.
deff ~ 5 mm (acceptable response to ionizing radiation)
dM/dT ~ -2.4% /0C  dT/T ~ 0.05 K
1/M dM/dV ~ 3%/V  dV/V ~ 10-5
All APD screened for radiation hardness
2 APDs per crystal: 50 mm2 active area
Si3N4, SiO2, contact
p++ photon conversion
p e- acceleration
n e- multiplication
n- e- drift
n++ e- collection
contact g
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

19. The photodetectors (forward)
Vacuum PhotoTriodes (fine mesh) in the EndCap Region
Rather insensitive to Magnetic Field because of the angle
More radiation hard w.r.t APD’s (UV glass window)
Gain 8-10 at 4 T ; Q.E.~ 20% at 420 nm ; area ~ 280 mm2
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

20. M.Paganoni, Univ. Milano Bicocca and INFN
Test of a prototype
Testbeam on 1999 prototype matrix with 30 preproduction crystals and APD:
No sign of rear leakage, with 280 GeV electrons.
Fit Resolution as a function of energy:
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

21. M.Paganoni, Univ. Milano Bicocca and INFN
Energy resolution: a
photostatistics contribution, including :
Light Yield
light collection efficiency
geometrical efficiency of the photodetector
photocatode quantum efficiency
Npe/GeV = 4000 for 0.5 cm2 APD  1.6%
electron current multiplication in APD, contributing
a square root of excess noise factor, F = 2
1.61.4 = 2.25%
Lateral containment (55 matrix)  1.5%
Total stochastic term a = 2.7 %
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

22. M.Paganoni, Univ. Milano Bicocca and INFN
Energy resolution: b
40 ns shaping time, summed over 3x3 or 5x5 channels
Serial noise (p.d. capacitance)  1/t
150 MeV
Parallel noise (p.d. dark current)  t, mostly radiation induced
negligible at the start of the experiment
30 MeV after 1 year at low luminosity
100 MeV after 1 year at high luminosity
Physics pile-up (simulated, with big uncertainties)
low luminosity 30 MeV
high luminosity 100 MeV
Total contribution
low luminosity 155 MeV
high luminosity 210 MeV
Measured at 2003 testbeam (over 3x3) = 180 MeV
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

23. M.Paganoni, Univ. Milano Bicocca and INFN
Energy resolution: c
leakage (front, rear, dead material)
CMS full shower simulation < 0.2 %
system instabilities (designed to be at the permill level t  3 tcal )
temperature stabilization at  0.05 ˚C
(dLY/dT = ˚C ; dM/dT ~ %/˚C)
APD bias stable at  20 mV (dM/dV = 3%/V)
It is the limiting factor at high energies.
Need to keep c  0.5 % :
light collection uniformity
intercalibration by monitor and physics signals at 0.5 % including the radiation damage effect
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

24. M.Paganoni, Univ. Milano Bicocca and INFN
Radiation Damage
[cm] EE
Total dose after 10 years (5x105 pb-1) EB
Dose rates [Gy/h] in ECAL at L=1034cm-2s-1
0.15
[Gy]
The only damage is produced by e.m. radiation on crystal transparency (colour centers)
The damage level depends on the dose rate
An equilibrium is reached after a small dose
Partial damage recovery in few hours
Loss in extracted light (~ 3 % for 0.15 G y/h) can be monitored with a laser 1 2 3 4 5 6 7 8 i n t a l f e r d o w v g h ( m )
T(%)
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

25. Laser monitoring system
Need to monitor crystal transmission continously to follow damage and recovery during LHC cycles
inject laser light in each crystal
follow signals from beam and laser
determine slope laser versus beam
check universality of the slope
Two laser systems: at 440/495 nm (blue, green) and 700/800 nm (red, infrared) with diagnostics on wavelength, pulse shape and intensity
Laser signals injected in the single crystals and in four pin diodes (which have 0.1 % stability)
Full calorimeter measurement foreseen every ~ 1 hour by using LHC gaps (3 ms every 89 ms)
Able to keep inter-calibration coefficients within 0.4 % over two months
Can recover calibration coefficients after power cuts
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

26. Scheme of the laser Absolute linearity study of PN readout electronics
Relative linearity study of APD readout electronics with respect to PN diodes
Beam :
Consistency check of the method
s =0.14%
Stability of the monitoring system achieved at 0.1 % level by normalizing to the PN diodes
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

27. M.Paganoni, Univ. Milano Bicocca and INFN
Read out chain/old
40 MHz clock
High dynamic range (50 MeV  2 TeV)
Radiation hardness
Low noise
Upper-Level VME
Readout Card
(in Counting Room)
Pipeline S
To DAQ
Digital
Trigger S
Energy
Light
Current
Voltage
Bits
PbWO4
Crystal
APD
VPT
Floating-Point
Preamplifier
Fiber
Readout
ADC
On-detector Light-to-Light readout
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

28. M.Paganoni, Univ. Milano Bicocca and INFN
Read out chain/new
Major revision imposed by budget (cost of optical links) and possible thanks to 0.25 mm CMOS rad hard technology
trigger and data storage on detector
read out of data only on receipt of a L1 trigger (3 ms)
3 Gigabit Optical Links each trigger tower (25 crystals)
Reduction of about a factor 8 in the number of data links
Simplification of off-detector electronics
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

29. Electronics - pictures
MGPA card
trigger tower
LVR card
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

30. M.Paganoni, Univ. Milano Bicocca and INFN
Cooling system
Power ~ 2.5 W/channel !!
Cooling requirements:
DT ~  C over 3 x calibration time
DT ~ 0.2 0C uniformity within a supermodule
2.5 W/channel to be taken directly from electronics chips (to avoid convection)
Need a good thermal coupling between the electronics and the cooling water
Need to minimize the heat flow to the APD
Stainless steel cooling pipes embedded in Al, thermally coupled with gap pad / gap filler (Berquist) to the electronics components
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

31. M.Paganoni, Univ. Milano Bicocca and INFN
Cooling system
Mean = K
RMS = K
Test to validate the cooling system
(switch off and on electronics)
successful
trigger tower concept
cooling bar production
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

32. M.Paganoni, Univ. Milano Bicocca and INFN
Cooling : pictures
Housing with gap pad
Mother Board on the Grid
Mother Board installation
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

33. M.Paganoni, Univ. Milano Bicocca and INFN
Cooling : pictures
VFE and LVR cards on the cooling bars
Trigger tower on the cooling bars
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

34. M.Paganoni, Univ. Milano Bicocca and INFN
Trigger tower
Trigger
GOH
FENIX
FPGA
VFE
QPLL
CCU
Readout
GOH
LVR
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

35. M.Paganoni, Univ. Milano Bicocca and INFN
Testbeams
2002 testbeam : 100 channels with old electronics, cooling and monitoring systems not final
Calibration table + hodoscope
Behaviour of APDs, HV, LV, VFE electronics
Universality of irradiation behaviour
Check of pre-calibration from LAB measurements
2003 testbeam : 50 channels with the new electronics, final cooling and laser monitoring
noise level, auto-gain switching
stability of cooling and monitoring systems
2004 testbeam : 1 supermodule
Final system test and complete calibration
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

36. Testbeam: laser monitoring
Dispersion of a for 19 crystals
Use of same a coefficient for
all crystals possible !
s/m = 6.3%
on top of a % effect
signal from Laser
signal from Beam (e-) a
slope :
a = 1.55
Follow up of intercalibration during irradiation with 0.4 % accuracy
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

37. Testbeam: precalibration M.Paganoni, Univ. Milano Bicocca and INFN
Comparison of Light Yield
(LAB - BEAM)/BEAM
97 crystals :
RMS = 4.57 %
Sigma = 4.40 %
Obtain crystal intercalibration from measurements of light yield in the LAB
Put crystals on electron BEAM and check intercalibration
Compare LAB and BEAM
LY/LYref
Intercalibration can be obtained from LAB with a precision of 4.5 % !
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

38. Testbeam: energy resolution
Good energy resolution achievable with 0.25 mm electronics after correlated noise subtraction
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

39. M.Paganoni, Univ. Milano Bicocca and INFN
The calibration at LHC
Goal : follow the evolution of intercalibration at few permill level (absolute energy scale from Z -> ee)
Starting point : calibration on beam for as many modules as possible (2% accuracy) and laboratory measurements (Light Yield,transparency, APD multiplication and electronics gain) for the others (5% accuracy)
Calibrate every month with physical events (W  e n ; Z  ee), at 0.3% level
Follow up of the radiation damage with the laser monitoring (0.4% precision)
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

40. M.Paganoni, Univ. Milano Bicocca and INFN
The achieved goals
Increase of the light yield, while keeping a fast response and longitudinal response uniformity
Development of a solid state photodetector with gain (APD) to be operated in a high magnetic field
Radiation hardness of crystals and electronics
Fast growing of crystals and mechanical assembly
Design and validation of the cooling system and the monitoring system
Test of the pre-calibration procedure and of the follow-up of the radiation damage
Validation of the new electronics
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN

41. ….and the next challenges
End the production of crystals and check their quality
Assemble the whole detector
Calibrate on beam as many supermodules as possible
Monitor the calorimeter during the precalibration and all the data taking at the few permill level
Vienna, VCI04, 20/2/2004
M.Paganoni, Univ. Milano Bicocca and INFN