1. CMS ECAL Endcaps: Fluence/Dose (500 fb-1)
Immediately behind crystals
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Neutron + charged hadron fluence (cm-2) for E>100 keV
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Absorbed dose (Gy)
CMS ECAL at SLHC 14/ R M Brown - RAL

2. CMS ECAL Endcaps at SLHC: Crystals
Lead tungstate (PbWO4) crystals suffer two types of optical damage under irradiation:
Ionising radiation produces colour centres. The colour centres self-anneal at room temperature. Under irradiation, a dynamic equilibrium is established, thus the density of colour centres and hence the decrease in light yield depends principally on the dose rate (rather than the integrated dose).
The impact on detector performance would be substantial, but manageable at SLHC
Charged hadrons and neutrons. In addition to the ionisation damage, hadrons collide with Pb and W nuclei causing ‘star’ formation. The resulting damage to the crystal lattice produces optical scattering centres and light is lost through Rayleigh scattering. Self annealing is very slow at room temperature and the light yield decreases progressively with increasing dose.
The performance loss at small radii (large ||) is expected to be unacceptable at SLHC
Scintillators with lower-z nuclei are expected to be more resistant to lattice damage.
This has been verified with CeF3
LYSO is under study.
CMS ECAL at SLHC 14/ R M Brown - RAL

3. CMS ECAL Endcaps at SLHC: Photodetectors
The current vacuum phototriodes (VPT) will suffer a significant performance loss at SLHC :
- Degradation of the photocathode
- Radiation darkening of the tube window
There is interest in CMS in developing a new generation of robust photodetectors.
Replacing PbWO4 with a brighter scintillator (LYSO?) would ease this problem.
Silicon devices:
Neutron damage  high leakage currents  amplifier noise?
‘Nuclear counter effect’ (for a simple photodiode, direct sensitivity to shower leakage particles >> sensitivity to scintillation light  high energy tail on energy measurement)
APD?
Vacuum devices:
- Good match to biakali photocathode
- Internal gain not necessary
 Vacuum photodiode?
CMS ECAL at SLHC 14/ R M Brown - RAL

4. Layout of CMS ECAL Endcap Elements
CMS ECAL at SLHC 14/ R M Brown - RAL

5. Partial endcap upgrade for SLHC?
At VPTs:
Dose(h =2.2)/Dose(h =3.0) ~1/10
(neutron fluence ~1/3)
~25% (18/71) Supercrystals are at h >2.2
Only replace detectors/crystals at small radius ?
Very challenging because of complexity of EE construction and high radiation levels
 Needs detailed study h
CMS ECAL at SLHC 14/ R M Brown - RAL