The CMS detector at LHC will be equipped with a high precision lead tungstate electromagnetic calorimeter. To ensure the stability of the calorimetric response at a level of a few per mille, every channel of the detector is monitored with a laser system. This system allows to correct for fluctuations in the detector response with high precision, in particular the expected radiation induced changes of the crystal transparency, which are on the level of several per cent at nominal luminosity. Here we report results from long term tests of the system on fully equipped supermodules of the CMS ECAL which are performed after the final step of the construction but before insertion of the modules in CMS. The in‐situ monitoring strategy for the CMS ECAL is discussed in detail and the performance of the full monitoring procedure as achieved in the test beam is illustrated

Bornheim, Adolf

Caltech Authors - Main

The CMS ECAL Laser Monitoring System
Adolf Bornheim and CMS ECAL Collaboration 
 
Citation: AIP Conference Proceedings 867, 408 (2006); doi: 10.1063/1.2396979 
View online: http://dx.doi.org/10.1063/1.2396979 
View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/867?ver=pdfcov 
Published by the AIP Publishing 
 
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The CMS ECAL Laser Monitoring System 
Adolf Bornheim, for the CMS ECAL Collaboration 
California Institute of Technology, 1200 East California Blvd. , Pasadena, CA 91125,  USA 
Abstract. The CMS detector at LHC will be equipped with a high precision lead tungstate 
electromagnetic calorimeter. To ensure the stability of the calorimetric response at a level of a 
few per mille, every channel of the detector is monitored with a laser system. This system allows 
to correct for fluctuations in the detector response with high precision, in particular the expected 
radiation induced changes of the crystal transparency, which are on the level of several per cent 
at nominal luminosity. Here we report results from long term tests of the system on fully 
equipped supermodules of the CMS ECAL which are performed after the final step of the 
construction but before insertion of the modules in CMS. The in-situ monitoring strategy for the 
CMS ECAL is discussed in detail and the performance of the full monitoring procedure as 
achieved in the test beam is illustrated. 
 
Keywords: Lead Tungstate Crystal Calorimeter, CMS ECAL, Monitoring System
PACS: 29.40.Vj, 29.40.Mc, 
INTRODUCTION
One of the main physics goals of the CMS detector at LHC is the search for the 
Higgs boson. In the mass range around 125 GeV, the decay H is of particular 
interest because of its clean final state. This mode however requires a very good 
energy resolution for the electromagnetic calorimeter to take advantage of the 
expected narrow decay width of the Higgs boson in this mass range. The design 
resolution for this calorimeter foresees a constant term in the resolution formula of 
0.55 %. To fulfill this requirement, the CMS experiment features a high resolution 
crystal calorimeter utilizing lead tungstate (PWO) crystals [1]. These crystals do 
experience a very slight, dose rate dependent decrease of their transparency under 
irradiation, which recovers to a large extend in irradiation free periods. In CMS, the 
variation of the crystal transparency under irradiation and the corresponding decrease 
in light out put of the crystals will be monitored with a high precision light monitoring 
system. In this paper we describe the experience with the final design monitoring 
system in the test beam at CERN and discuss the strategy for correcting the variation 
in the crystal response during LHC operation. 
 
 
 
 
408
CP867, XII International Conference on Calorimetry in High Energy Physics, 
edited by S. R. Magill and R. Yoshida 
 © 2006 American Institute of Physics 978-0-7354-0364-2/06/$23.00
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TRANSPARENCY CHANGE OF LEAD TUNGSTATE CRYSTALS 
UNDER IRRADIATION 
 
The mass produced lead tungstate crystals used for the CMS ECAL are radiation hard 
to high integrated dosage, but they experience a dose rate dependent transparency loss, 
as shown in Figure 1 (left). Although there is no damage to the scintillation 
mechanism, color centers are created during irradiation. These radiation induced color 
centers may annihilate at room temperature. During irradiations both annihilation and 
creation processes coexist and the color center density reaches an equilibrium at a 
level depending on the dose rate applied.  
 
 
FIGURE 1.  The left plot shows the change in the crystal transparency at different dose rates. Note that 
the transparency change reaches an equilibrium at each rate under continued irradiation. The dose rate 
during the first 100 hours of 15 rad per hours corresponds to the expected dose rate at LHC during low 
luminosity running. Note also that the relative change in the transparency is largest at small dose rates 
and becomes increasingly smaller at larger doserates. The right plot shows the recovery of the 
transparency loss after an irradiation. The recovery can be described by two time constants.  
 
When the irradiation stops, the color center density decreases until it reaches an 
equilibrium again and thus the transparency change recovers. The time constants for 
the damage and recovery process are a crystal property and can vary to some extend. 
The recovery can be described by two time constants. An initial recovery phase takes 
place on a time scale of approximately 10 hours, followed by a slower recovery on the 
time scale of 1000 hours, as illustrated in Figure 1 (right). This has to be compared of 
the typical time constants of the LHC accelerator cycle. The luminosity operation will 
last for about 12 hours, followed by a refill of the machine which takes of the order 
one hour. The interplay between the recover time constants and the LHC operational 
cycle will lead to a distinct pattern in the transparency change, as illustrated in Figure 
2. Since the crystals will not fully recover during the refill period of the accelerator the 
maximal transparency change will be limited. This will reduce the systematic effects 
0.5
0.6
0.7
0.8
0.9
1
1.1
0 100 200 300 400
Time (hours)
 N
or
m
al
iz
ed
 L
ig
ht
 O
ut
pu
t
BTCP-2162R
L.O.= 9.3 p.e./MeV (200 ns, 20.0oC)
dose rate (rad/h):
15 100 500 1000 recovery...
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related to the transparency correction since it will reduce the size of the correction on 
average.   
Test Beam Data
Recovery RecoveryDamage Damage
 
FIGURE 2.  The diagram on the left from an simulation illustrates the interplay between the two time 
constants of the crystal recovery and the operational cycle of the LHC accelerator. Since the recovery 
time is much longer than one LHC cycle, the transparency change will not fully recover during refilling 
of the LHC. This will limit the overall fluctuations of the transparency change. The diagram on the right 
shows two such irradiation cycles mimicked at the test beam by repeatedly irradiating a final design 
CMS ECAL super module. The expected behavior can clearly be seen.    
THE CMS ECAL MONITORING SYSTEM 
The laser monitoring system must track the change in the transparency precisely 
enough to maintain the constant term in the ECAL resolution of 0.55%. This requires 
a measurement of the transparency with an accuracy of better than 0.2%. The 
transparency in each crystal will have to be measured approximately every 30 minutes 
during LHC operation to account for the fast initial change at the beginning of an 
irradiation cycle.  
1 2 3 4 5 6 7 8 9 10 11 12
88.924   sμ
μ
Laser sequence
Laser trigger
Laser latency
119 missing bunches
b.c.
25 ns distant
72 bunches
8 missing bunches
38 missing bunches
39 missing bunches
−30−20−10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
Light
2.97   s
measurement  
   
FIGURE 3.  The diagram illustrates the continuous laser monitoring during LHC operation. Taking 
advantage of a 3 s gap in the LHC bunch train laser pulses are injected at a rate of  100 Hz. 
410
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To achieve this, the monitoring has to be performed while the data taking of the 
detector is ongoing. This can be realized by taking advantage of gaps in the LHC 
bunch train which occur once every 90 s. In Figure 3  the time structure of the LHC 
bunch train is illustrated. The laser system will be synchronized with the LHC clock 
cycle via the CMS trigger system and send laser pulses during the extended gaps. To 
allow this operational mode the laser source has to fulfill a number of design 
parameters. In particular the laser pulse width must be shorter than 40 ns and stable to 
better than 10% to match the shaping of the ECAL electronics. Furthermore must the 
pulse timing be stable to a few ns to allow a synchronization which the LHC clock and 
the detector readout. Finally, the pulse energy must be on the order of 1mJ to provide 
sufficient light to large groups of crystals to ensure that the entire ECAL can be 
monitored in 30 minutes. The pulse energy also has to be stable to better than 10% to 
prevent possible non-linear effects in the reference system against which the signals of 
the individual crystals are compared. To achieve all these performance requirements a 
dual-stage laser system is used [3,4]. Two such laser systems provide two different 
wavelengths, on at the peak of the scintillation light wavelength at 440 nm, one far 
away in the red wavelength range to allow for systematic cross checks.  These two 
systems plus an spare backup system for the blue laser light source distribute the light 
pulses via a system of optical switches to the ECAL super modules, as shown in 
Figure 4. 
  
   
FIGURE 4.  The diagram shows the layout of the CMS ECAL laser monitoring source. Two laser 
systems provide light at 440 nm and 800 nm as primary and secondary monitoring wavelengths. The 
440 nm system has an identical spare system as a backup. A system of optical switches allows to 
distribute the light to one half of a super module at a time. All half super modules of the entire ECAL 
will be scanned once within 30 minutes.  
 
Inside the super module the laser light is distributed via optical fibers and a fanout 
system to each individual crystal. The optical fibers enter the crystals from the front in 
the barrel part of ECAL and from the back in the endcaps. In parallel light is sent to 
Diagnostic
MainQuantronix Ti: Sapphire
( 440 or 495 nm )
Quantronix Nd:YLF  (527 nm)
PIN
PIN
Diagnostic
Main
Diagnostic
Quantronix Nd:YLF  (527 nm)
PIN
PINQuantronix Ti: Sapphire
( 709 or 796 nm )
Diagnostic
Diagnostic
Main
Diagnostic
Quantronix Ti: Sapphire
( 440 or 495 nm )
Quantronix Nd:YLF  (527 nm)
PIN
PIN
1 x 80 
Switch
Optical
Box
PIN
o  o  o  o  o  o
o  o  o  o  o  o
Monitoring
Box
Attenuation
(0−5) x 10dB
0−99%
Optical
Switch
3 x 1
PIN PIN
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reference photo diodes inside the supermodule. The relative difference seen in the 
photodetectors of the ECAL with repect to the reference diodes measures the relative 
transparency change in the crystals. More details of the on-detector part of the 
monitoring system can be found in reference [5]. 
 
 
TRANSPARENCY CORRECTION WITH THE CMS ECAL 
LASER MONITORING SYSTEM 
The feasibility of the radiation damage monitoring with the desired precision has 
been demonstrated in the test beam [6]. One important measure of the performance is 
the stability of the monitoring system response over time in the absence of any 
radiation. This is a measure of the resolution of the system which sets the scale for the 
minimal transparency change which can be detected. The design goal for the CMS 
ECAL monitoring system is to be able to track transparency changes on the level of 
better than 0.2 %. The second benchmark is the ability of the system to correct back 
transparency changes induced by irradiation. This is verified in the test beam by 
irradiating crystals in a high energy electron beam with dose rates similar to the ones 
expected in CMS. During the irradiation the crystals are calibrated with the electron 
beam in regular time intervals, followed by laser monitoring runs during which the 
transparency is measured. The response to the electrons is then corrected using the 
transparency measurements of the laser system. Aside from the stability of the 
monitoring system there are several other issues which impact the performance of this 
correction procedure. The most important one is the relation between the relative 
change of the laser monitoring signal and the relative change of the response to 
electrons. This relation can be described by a linear function for small transparency 
changes, as shown in the left plot of Figure 4.  
2002
2003
2004
2004
(fall)
(spring)
  
 
FIGURE 5.  The left plot shows the linear correlation between the relative loss of the laser monitoring 
response and the relative loss of the response to electrons. The slope  of this linear correlation is quite 
uniform for crystals produced under the same conditions. The plot on the right shows the slope from 28 
crystals produced by BTCP, the main supplier of crystals for the CMS ECAL. The distribution has an 
RMS of about 5 %.      
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Thus, only a scaling factor is needed to determine the correction factor from the 
relative change as measured by the laser system. This scaling factor  is found to be 
the same within about 5 % for crystals produced under the same conditions, as shown 
in the right plot of Figure 4. For a total of 28 crystals the parameter  has been 
measured in various test beam campaigns and found to be the same in all crystals 
within 5 %. There are indications that the orientation of the crystal optical axis with 
respect to the crystal geometrical shape do impact in the scaling factor . At CMS the 
transparency change in the LHC low luminosity running will be on the level of a few 
percent. At this level it is sufficient to know the average value of the parameter  as 
determined in the test beam. Procedures to extract the parameter  in situ using 
physics events are under consideration.  
 
PERFORMANCE OF THE MONITORING SYSTEM 
 
In Figure 5 we show an example of an irradiation cycle in the test beam. A final 
design super module was irradiated with 120 GeV electrons at does rates equivalent to 
four times the expected does rate at LHC in low luminosity operation. During the 
irradiation the crystals exposed to the beam were calibrated repeatedly with the 
electron beam. In parallel the transparency was monitored with the laser monitoring 
system. Over a period of  six hours we observe a decrease of the raw response to 
electrons of about 2 %.  
 
FIGURE 6.  The raw ECAL response to 120 GeVelectrons during an irradiation period on a super 
module in the test beam. The response is the sum of nine crystals centered around the crystal hit by the 
beam. The beam rate was set such that the radiation level corresponds to four times the expected 
radiation level at the LHC. The crystal response drops by about 2 percent and then approaches a 
plateau. The data has then be corrected using the laser monitoring data. The individual calibration 
points are within 0.15 % of the initial calibration.   
  
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After applying a correction based on the measurements from the laser monitoring 
system the individual calibration points are within 0.15 % of the initial calibration 
before irradiation [7]. This value is achieved for a set of four crystals all irradiated in 
the same was as shown in the example in Figure 5. For this correction the average 
value for the parameter  as determined from Figure 4 was used. This is to be 
compared to the design value for the constant term of the energy resolution of the 
CMS ECAL which is 0.55 %. The effect of the transparency change correction is well 
below other effects contributing to the overall constant term, in particular the 
intercalibration to be performed in situ [8].  
SUMMARY
   A final design laser monitoring system of the CMS ECAL has been tested in the 
test beam. With this monitoring system the radiation induced transparency change in 
the crystals of the CMS ECAL can be measured and corrected for. The residual effect 
after the correction of the transparency change is found to be less than 0.15 %. This is 
well within design specifications and negligible compared to other contributions to the 
constant term of the resolution of 0.55 %. 
ACKNOWLEDGMENTS 
 
I would like to thank the entire ECAL collaboration for their outstanding effort, R.-Y. 
Zhu and the CALTECH team for their superb work on the laser system and the 
organizers of the XII International Conference on Calorimetry in High Energy Physics 
at Chicago. 
REFERENCES
1.  “The Electromagnetic Calorimeter Technical Design Report”, CMS Collaboration , CERN/LHCC 97-33, 1997 
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Zhu, IEEE Trans. Nucl. Sci. NS-47 (2000). 
3. “Performance of the Monitoring Light Source for the CMS Lead Tungstate Crystal Calorimeter ”, IEEE Trans. 
Nucl. Sci.Vol. 52 No. 4 (2005) 1123--1130. 
4. “ECAL Monitoring Light Source at H4”, D. Bailleux, A. Bornheim, L. Y. Zhang, K. J. Zhu, R.-Y. Zhu, and D. 
Liu, CMS IN 2003/045. 
5. “Laser monitoring system for the CMS lead tungstate crystal calorimeter”,  M. Anfreville et. al., to appear in 
Nucl. Instr. And Meth. 
6. “The  CMS ECal laser monitoring system.”, A. Bornheim, in proceedings of the 9th ICATPP Conference on 
Astroparticle, Particle, Space Physics, Detectors and Medical Physics Applications, Como, Italy. 
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8. “CMS ECAL Calibration Strategy”, G. Daskalakis, these Proceedings
 
 
 
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The CMS ECAL Laser Monitoring System

English

Bornheim, A

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