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)
Ioan Dafinei
INFN Sezione di Roma
Index
1. PWO general properties
2. PWO properties vs ECAL-CMS requirements
3. survey of PWO performance during ECAL construction
4. Conclusion
Ioan Dafinei
INFN Sezione di Roma
PWO general properties
Luminescence
PbWO4 (PWO)
blue component
green component
(400 – 440 nm ; <25ns) (460 – 540 nm ; >100ns)
- studied as phosphor since 1979
- promoted scintillator early nineties
ρ
g/cm
8.26
X0
cm
0.89
3
2.2
Pb2+
intensity (a.u.)
10
0
10
-1
10
-2
10
-3
10
-4
Rm
cm
τ
ns
15
λ emission
nm
420
-
2.29
10
-5
LY
% of BGO
5
10
-6
α LY
%/°C
-2
n@ λ emission
WO42-
ECAL PWO crystals supplier
Bogoroditsk Techo Chemical Plant
Russia
L u mL a ser E xc.QP C
10
WO43-
em ision@ 400 nm
em ision@ 500 nm
laser pulse
λexc=308 nm
-8
10
-7
10
-6
10
-5
tim e (s)
♥suppressed green component
♥improved radiation hardness
Ioan Dafinei
INFN Sezione di Roma
PWO properties vs ECAL-CMS requirements
high stopping power
small X0
small R
m
high granularity
high density and high Z
ECAL-CMS
BGO
PWO
CeF3
1.11
0.89
1.68
2.33
2.2
2.63
high energy resolution
ve ind
i
t
c
a
r
f
e
high r
ex !
large LY
≈ 100GeV
emission spectrum
overlapping with spectral
sensitivity of APD and VPT
APD and VPT
(see Q. Ingram’s talk)
Bunch separation 25 ns
34
-2 -1
LHC Luminosity 10 cm ·s
fast
radiation
hard
BGO
PWO
CeF3
300
15
30
•light production is not affected
•creation of color centers is quenched
by appropriate doping
Ioan Dafinei
INFN Sezione di Roma
PWO properties vs ECAL-CMS requirements
PWO crystals needed for ECAL-CMS
ca
End
cap
d
n
E
p
Barrel
Endcap
61200 (34 types)
14648 (1 type)
2.2 - 2.5 (Rm)
3
23 (25.8X0)
22 (24.7X0)
Total weight
67.4 tons
22.9 tons
Photosensor
Avalanche
Photo-Diode (Si)
Vacuum
Photo-Triode
No.of Xtals
Xtal size (cm)
transv:
long:
el
r
r
Ba
Xtals point 3º
away from vertex
Ioan Dafinei
INFN Sezione di Roma
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
very strict crystal
quality control
parallel radiation
hardness tests
find the ways to get the best possible
performance from the ECAL
modules built with this PWO
LY
characteristics
radiation
damage
calibration
monitoring
(see P.S.Roldan’s talk)
QC measurements for LY are not routine
check but a first step of calibration
Ioan Dafinei
INFN Sezione di Roma
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:
R=
where:
∆ BeamSignal
∆ MonitorSignal
∆ BeamSignal
∆ MonitorSignal
: fractional decrease in the beam signal
: fractional decrease in the light monitoring signal
Ioan Dafinei
INFN Sezione di Roma
Survey of PWO performance during ECAL construction
premise 3
Doses in 10 years in ECAL
HCAL Barrel
ECAL
Endcap
• dose (kGy)
(at photodetector)
0.5
5
2
0.35
1.2
0.2
ECAL Barrel
3
70
20
50
• neutron fluence (/ 1013cm-2)
(at photodetector)
• dose (kGy)
(at shower maximum in crystal)
-CERN (l’Hopital de Genève)
tests made in:
-CERN GIF and H4
-Rome ENEA Casaccia
-California Institute of Technology
Ioan Dafinei
INFN Sezione di Roma
Survey of PWO performance during ECAL construction
PWO crystals tests at GIF
Gamma
Irradiation
Facility
http://cmsdoc.cern.ch/cms/ECAL/GIF/www/
z Irradiation (transversally):
Cs137 (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
z Excitation
scintillation:
SPS e beam (50 GeV)
luminescence:
UV laser
(λ=337±10 nm, 1.55mJ/pulse, t =600 ps)
z Monitor
longitudinal transmission (LED)
λ=450±50 nm, t=500 ns
Ioan Dafinei
INFN Sezione di Roma
Survey of PWO performance during ECAL construction
PWO crystals tests at GIF
VETO
DW1
DW2
BEAM
Crystals box
Laser
fiber
LED box
Lamp fiber
Cooling
pipes
scan
DW3
Crystals
Ei
S2
X5 beam size:
approx. 3cm
~10m
~2m
~1m
• crystals are tested in batches of 5
• light detectors are standard APDs
• monitoring of different light sources
by a PM coupled to a 6th crystal
• monitoring of laser by PIN diode
• monitoring of gain with a constant
charge injected to the amplifiers
Ioan Dafinei
INFN Sezione di Roma
Survey of PWO performance during ECAL construction
PWO crystals tests at GIF
Measure R coefficients defined as:
R(t ) =
LED
Laser
irr 59
SA(t ) − SA(t = 0)
SB (t ) − SB (t = 0)
t=0 (irradiation start)
where A and B correspond to different ways
irr 61
to produce the light measured by the APD
irr 62
(eg: BEAM and LED)
Data analysis:
-test pulse correction
-temperature correction
-normalization (monitor signal)
Beam
-signal cleaning
-normalization to 1 (signal at start)
Ioan Dafinei
INFN Sezione di Roma
Survey of PWO performance during ECAL construction
PWO crystals tests at GIF
RBEAM/LED
RBEAM/LASER
RLASER/LED
= 1.9 ± 4.2%
= 1.6 ± 8.4%
= 1.2 ± 9.6%
• Crystal to crystal dispersion of R is not
significant (inside the errors)
• Good reproducibility of mesurements
• Good correlation RLASER/LED vs RBEAM/LED
σR
LASER/LED
σR
BEAM/LED
=9%
=5%
Ioan Dafinei
INFN Sezione di Roma
Survey of PWO performance during ECAL construction
PWO crystals tests at Caltech
studies:
Caltech γ–ray Irradiation Facilities
60Co
137Cs
„
„
„
„
luminescence
transmittance
radiation hardness and recovery
light output and decay kinetics
steps:
(1)
(2)
(3)
Λ = 50 Curie
15, 100, 400 rad/h
Λ = 2000 Curie
up to 36 Krad/h
(see rad.hard dedicated talk of R-Y. Zhu)
thermal annealing at 200°C
γ-irradiations: 15, 100, 400 and 9k rad/h
(until equilibrium)
recovery
Crystals are kept in dark at 18°C after annealing.
Minimum time between annealing and 1st
measurement is 48 hours.
Ioan Dafinei
INFN Sezione di Roma
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)
Ioan Dafinei
INFN Sezione di Roma
Survey of PWO performance during ECAL construction
PWO crystals tests at Caltech
damage and recovery kinetics
ECAL monitoring takes
30 minutes to cover
entire calorimeter in situ.
(see dedicated talk of R-Y.Zhu)
Damage and recovery time constants decide
whether the monitoring system can adequately
track the changes in light yield
Longitudinal transmission (T@440nm) 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
INFN Sezione di Roma
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
E
longitudinalStransmittance
observed 8h
TABL
after the thermal annealing
no modification under γ-ray exposure of
RELIABLE
either excitation or emission spectra
K
O
G
no very short timeRrecovery
components
IN
O
IT
N
O
M
samples can be divided into 3 types from
radiation hardness point of view
Ioan Dafinei
INFN Sezione di Roma
Survey of PWO performance during ECAL construction
Detector Performance Group
studies on PWO crystals for EE
http://agenda.cern.ch/displayLevel.php?fid=4l50
Average light yield at LHC, versus Eta
Starting luminosity (1033)
100%
High luminosity(1034)
Dall
total density of the
potential color centers
80%
60%
40%
Improved radiation
tolerance for EE
crystals would be of
benefit
Ioan Dafinei
INFN Sezione di Roma
Survey of PWO performance during ECAL construction
http://cms-project-h4-testbeam-2003.web.cern.ch
beam tests at CERN
SM0 fixed on the turn table, ready for beam test
SPS beam
φ
2002 preliminary results
η
http://polywww.in2p3.fr/actualites/congres/wach4-2002/
Ioan Dafinei
INFN Sezione di Roma
Survey of PWO performance during ECAL construction
beam tests at CERN
electron beams irradiation: cumulated doses between 2 and 8 Gray
S 
 R
  = α ⋅  
 S0 
 R0 
Linear
relation
between
signal and
monitor!
S: scintillation light signal
R: monitoring light signal
statistics made on
the 20 irradiated
crystals
σ/µ = 6.1%
universality of
irradiation slope α
(inside 6.1%)
α determined with 3% precision
and intrinsic dispersion 5%
Ioan Dafinei
INFN Sezione di Roma
Survey of PWO performance during ECAL construction
beam tests at CERN
1st irradiation
Behavior under LHC cycle conditions
2nd irradiation
dose rate: 0.25Gray/h
cum.dose: 2.5Gray
p
o
w
e
r
dose rate: 0.18Gray/h
cum.dose: 1.8Gray
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
INFN Sezione di Roma
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)
Ioan Dafinei
INFN Sezione di Roma
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
Ioan Dafinei
INFN Sezione di Roma