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Scientific Report 2007-2009
Particle physics
P6. The Lead Tungstate Crystal Calorimeter of the CMS experiment
The Lead Tungstate crystal calorimeter is a key feature
of the CMS experiment, described elsewhere in this
report (The CMS experiment at the CERN LHC).
The calorimeter is made of 75 848 lead tungstate
(PWO) scintillating crystals. Each crystal has a length
of approximately 23 cm and a truncated pyramid shape
with an average cross section of about 2.2 × 2.2 cm 2 .
Lead tungstate was chosen as scintillating medium after
a long period of R&D, conducted in our laboratories together
with several collaborators around the world. The
choice was due to the fact that PWO has a very short
radiation length (X 0 = 0.89 cm), allowing the construction
of a compact detector, a fast response (most of its
light is emitted in 25 ns), and is radiation tolerant, a
fundamental requirement for LHC. On the other hand
PWO has a relatively low light yield, demanding for an
amplification of its signal. During the R&D phase our
group contributed mainly in clarifying the effect of trivalent
doping on crystals, studying the radiation damage,
and in the definition of instruments and methods for a
reliable measurement of both the light yield and transmission
of PWO crystals. We also developed, in strict
collaboration with italian manufacturers, both the main
supporting structure for the ECAL and the transportation
system. In the first case we had to face with the
requirement of a very light structure to support a weight
of about 100 tons. For the transportation we developed
a dedicated dumped cage instrumented with accelerometers
and a logging device, equipping the driver cabin
with appropriate alarms.
and realized by us.
Photomultipliers cannot be used to measure the light
emitted by PWO, because of the strong 3.8 T magnetic
field produced by the CMS superconducting solenoid.
To overcome this difficulty we employ avalanche photo–
diodes (APDs) in the barrel region and vacuum photo–
triodes in the endcaps (in these regions the predicted
neutron flux is too high for APDs).
The calorimeter has an exceptionally good energy
resolution. The resolution σ (E) for photons whose
energy is higher than about 100 GeV can be considered
constant and is, for ECAL, better than 0.5 %. Such a
result is very important for the discovery of any particle
decaying into two photons, like the Higgs boson. This is
the outcome of a careful design, appropriate machining
of the crystals, good coupling between crystals and
photodetectors, stability and calibration. Such a good
resolution can only be maintained during the lifetime of
the experiment thanks to continuos monitoring of the
crystal transparency, achieved by laser light injected
into each individual crystal by means of an optical
fibre, and periodic calibrations using physics events.
Stability is achieved keeping the whole detector at a
constant temperature of about 18 ◦ C, thanks to thermal
screens and an appropriate cooling system, as well as
operating the photo–detectors at stable voltages. In
particular, the APD gain M is very sensitive to the
magnitude of the bias voltage V , with 1/M (dM/dV )
approximately equal to 3 %/V, requiring a stability
of few tens of mV for biases of the order of 300 V.
The stability of the bias voltage has been achieved
working in strong collaboration with an italian firm,
who developed a specially designed power supply that
has been extensively tested by our group during the
past years.
References
1. S. Chatrchyan et al., ”Performance and Operation of
the CMS Electromagnetic Calorimeter”, arXiv:0910.3423,
CMS-CFT-09-004 (2009).
2. S. Abdullin et al., EPJC 60, 359 (2009)
3. P. Adzic et al. JINST 3, P10007 (2008)
Figure 1: PWO crystals being labeled by a technician.
Crystals are arranged in such a way to form approximately
a cylinder whose axis coincides with the beam
axis. The lateral surface of the detector is called the barrel,
while the two bases are called the end–caps. Crystals
are organized in modular structures providing mechanical
support for them. Half of the modules composing
the ECAL barrel were built in dedicated laboratories in
Roma, after a complex workflow during which all the
parts used to realize the instrument were subjected to a
careful quality control, by automatic machines designed
Authors
L.M. Barone, F.Cavallari 1 , D. del Re, I. Dafinei 1 , M.
Diemoz 1 , E. Di Marco, D. Franci, E. Longo, P. Meridiani,
G. Organtini, A. Palma, F. Pandolfi, R. Paramatti 1 , S.
Rahatlou, C. Rovelli 1 , F. Santanastasio.
http://www.roma1.infn.it/exp/cms
Sapienza Università di Roma 113 Dipartimento di Fisica