•
High-resolution, high-granularity scintillating crystal calorimeter
•
75,848 lead-tungstate (PbWO
4
) crystals
•
Crystals of the short radiation length, small
Molière radius, and speed as a scintillator.
The design energy resolution of the ECAL has a constant term of 0.5%, and to maintain this, in
must be performed.
At the LHC design luminosity, the CMS detector will be exposed to a very harsh radiation environment. The PbWO
4 crystals are radiation hard up to a high integrated dosage, but suffer from dose-rate dependent radiation damage.
Exposure at the level of LHC luminosity (dose-rates of 15 rad/hour at 10 34 cm -2 s -1 ) causes a decrease in crystal transparency due to radiation induced absorption. Although the crystals will self-recover during periods in absence of radiation, this recovery takes places on the order of a week. Therefore, changes in crystal transparency, and therefore calorimeter response, due to radiation damage must be corrected for to maintain the energy resolution of the detector.
The CMS ECAL utilizes a laser monitoring system to monitor the light output of the crystals. With this system, we can measure the change in transparency of each crystal continuously during LHC running, with very high precision.
APD
VPT
The monitoring light source consists of three pairs of lasers (Nd:YLF pump laser and Ti:Sapphire laser), with diagnostics, two 3x1 optical switches, a 1x88 optical switch, a monitor and a PC based controller.
In 2006, a software feedback system was implemented so that the stability of the pulse intensity and
FWHM are maintained at the level of a few percent, with a pulse timing jitter of less than 2 nm observed in laser runs lasting for over 2,000 hours test beam data.
Laser monitoring data will be taken during the LHC “abort gap” events,
3ms every 90 s. Gap events will arrive at the Filter Farm, containing, among other data, the ECAL laser event data, which will be sorted and then analyzed in a PC farm to extract APD/PN values.
The data is then inserted into the OMDS database located at Point 5, and then transferred to the ORCON/ORCOFF database. During the transfer procedure, corrections will be applied.
The laser APD/PN ratios, reference values, and scale factors necessary to implement the transparency correction will be stored in the offline database, and the correction is applied in the offline reconstruction.
The laser monitoring system has been commissioned at a test beam facility at CERN, where the performance was evaluated over periods of months. The stability of the system has been exhibited to be on the order of 0.1%; with such performance, even small changes in transparency can be monitored with precision.
APD/PN RMS Before and After Correction
Test Beam Data
E(t)/E(t
0
) =
[ L(t)/L(t
0
) ] 
Damage Recovery Damage Recovery
Energy Resolution Before and After Correction
Laser Response (APD/PN)
RMS(APD/PN)
The wavelength of the Ti:S laser is tunable, and two wavelengths are available from each laser. Four wavelengths, 440, 495, 709, and 796 nm, are available using the 3x1 optical switch. The selected wavelength is sent to each ECAL element using the 1x88 optical switch.
•
2 wavelengths per laser
•
Pulse width, FWHM
•
Pulse jitter
•
Pulse rate
< 40ns to match ECAL readout
< 3ns for sychronization with LHC
~100 Hz, scan of full ECAL 20min
•
Pulse intensity instability ~few%
•
Pulse energy 1 mJ/pulse at monitoring wavelength
(equivalent to 1.3 TeV in dynamic range)
APD Amplitude Two laser systems
(blue/green and IR/red) have been installed and commissioned in the CMS underground cavern at Point5, and first laser data has been collected.
Event
Pulse Shape
Time sample