CMS Ecal Laser Monitoring System - California Institute of Technology
Transcript CMS Ecal Laser Monitoring System - California Institute of Technology
CMS ECAL Laser Monitoring System
Compact Muon Solenoid (CMS)
Electromagnetic Calorimeter (ECAL)
• High-resolution, high-granularity scintillating
• 75,848 lead-tungstate (PbWO4) 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
situ calibration and monitoring of the crystals
must be performed.
At the LHC design luminosity, the CMS detector will
be exposed to a very harsh radiation
environment. The PbWO4 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 1034
cm-2s-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.
The Laser Monitoring System
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 Dataflow:
CMS Point 5
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.
Performance from Test Beam Results
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.
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.
Electron Response (APD/PN)
Light Distribution System
Test Beam Data
APD/PN RMS Before
and After Correction
Laser Response (APD/PN)
Energy Resolution Before and After Correction
< 40ns to match ECAL readout
< 3ns for sychronization with LHC
~100 Hz, scan of full ECAL 20min
1 mJ/pulse at monitoring wavelength
(equivalent to 1.3 TeV in dynamic range)
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
• 2 wavelengths per laser
• Pulse width, FWHM
• Pulse jitter
• Pulse rate
• Pulse intensity instability
• Pulse energy
Commissioning at CMS Point 5
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.
Toyoko J. Orimoto, California Institute of Technology, on behalf of the CMS ECAL Group
10th ICATPP Conference on Astroparticle, Particle, Space Physics, Detectors and Medical Physics Applications , Villa Olmo, Como, Italy