Transcript CMS Ecal Laser Monitoring System

CMS ECAL Laser Monitoring System
Overview
Test Beam Studies
Compact Muon Solenoid (CMS)
Electromagnetic Calorimeter (ECAL)
• High-resolution, high-granularity scintillating
crystal calorimeter
• 75,848 lead-tungstate (PbWO4) crystals
• Crystals of the short radiation length, small
Molière radius, and fast speed as a scintillator.
Irradiation of PbWO4 crystals results in the
formation of color centers which absorb
and scatter light, reducing the transparency
of
the
crystal.
Simultaneously,
the
scintillation mechanism of the crystal is
unaffected by irradiation, resulting in the
crystal’s response to the laser monitoring
system
being
different
than
its
electromagnetic shower response.
The laser monitoring system has been commissioned at a test beam
facility at CERN, where the performance was evaluated over the period
of months. A stability on the order of 0.1% has been demonstrated for
the monitoring system, allowing even small changes in transparency to
be monitored with precision and the dynamics and characteristics of
crystal transparency changes to be studied.
At the LHC design luminosity, the CMS detector will
be exposed to a harsh radiation environment
(dose-rates of 15 rad/hour at 1034 cm-2s-1). The
PbWO4 crystals are radiation hard, but suffer from
dose-rate dependent radiation damage.
Radiation causes a degradation 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.
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 has been commissioned in situ and is now running at
CMS Point 5.
According to radiation damage models, the relationship between the
crystal response to the laser monitoring system (
) and to
electromagnetic showers (
) can be described by a power law:
Traditionally, values of  have been measured by
fitting plots like the one to the left (the fit
method). Here, events incident in a single
crystal are grouped in small time intervals and
the crystal response is fit, determining a mean
and spread. These data points are compared
with the corresponding laser measurements.
The Laser Monitoring System
APD
VPT
The monitoring light source consists of three pairs of lasers (Nd:YLF
pump laser and Ti:Sapphire laser), with diagnostics, a 3x1 optical
switches, a 1x88 optical switch, a monitor and a PC based controller.
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. Laser pulses of the selected
wavelength is sent to each ECAL element using the 1x88 optical switch.
Comparison between  values extracted
using the fit method and using the
minimization method yield consistent
values. Systematic uncertainties of
minimization method are appreciably
smaller.
 four xtal minimization
< 40ns to match ECAL readout
< 3ns for synchronization with LHC
~100 Hz, scan of full ECAL in 20min
~few%
1 mJ/pulse at monitoring wavelength
(equivalent to 1.3 TeV in dynamic range)
Dispersion of  for 33 BTCP endcap crystals
# Crystals
Laser Specifications:
• 2 wavelengths per laser
• Pulse width, FWHM
• Pulse jitter
• Pulse rate
• Pulse intensity instability
• Pulse energy
In the 2006 and 2007 test beams, a new
method was considered, where the energy
resolution of all events explicitly minimized
as a function of  (the minimization
method). In 2007, a wide beam-spot was
used to irradiate four crystals at once with
a mono-energetic electron beam, and the
energy response resolution was minimized
w.r.t.
the four crystals’  values
simultaneously. This approach can be
generalized to collision data, where
electromagnetic resonances can be used
to measure alpha in situ.
 fit method
The design energy resolution of the ECAL has a
constant term of 0.5%, and to maintain this,
calibration and monitoring of the crystals must
be performed in situ at the LHC.
mean = 1.514
 measurements for 33 endcap (left) and 35 barrel (right) crystals. The
mean values of the distributions are identical. Systematics for endcap
measurements still under investigation.
Timing (1 Unit =25 ns)
Data from the laser monitoring
is also used for ECAL
commissioning. For
instance, the laser is used to
understand the timing of the
ECAL signal. The plot on
the right shows the crystalby-crystal timing variations
from laser data. The lower
plot shows the ECAL signal
timing from LHC beam shots
(from , after laser timing
corrections.
Timing (1 Unit =25 ns)
Commissioning at CMS
Results of correction procedure using measured  values for 120 GeV/c
electron beam. The correction fully recovers the pre-irradiation
resolution.
Christopher S. Rogan, California Institute of Technology, on behalf of the CMS ECAL Group