Why we need an improved
LASER calibration system
TileCal calibration system
The Laser box
Q-switched DPSS (diode-pumped solid state) LASER
The laser box is the control system for laser:
1) Monitor of the laser light intensity
2) Attenuate the light intensity according
to needs
3) Provide the trigger to DAQ corrected for
laser time jiitter
Laser is used for:
1) Monitor PMT stability
2) Measure PMT linearity
3) Monitor PMT stability during LHC stacks
Beam expander (light distribution)
This system should deliver a fixed and constant fraction of laser light to each Drawer (48 PMT)
But The light sent to PMT is not stable in time.
The monitoring of PMT stability becomes problematic.
A new system has been designed in Pisa.
Instabilities of the present system
Drift of laser with time
Instability of channels
connected to a clear fibre
from the patch pannel
Diffusive sphere (being tested in our lab)
Light OUT
(clear fiber bundle)
Light IN
(liquid fiber)
This system would provide a stable distribution of light to PMT but at the price of
Large inefficiencies. We would need a Laser more powerful than what we have.
THE STABILITY OF PMT GAIN (THE PISA METHOD)
The method developed in Pisa exploits the distribution of photoelectrons, a well
known technique:
G
1 Var(q)
ef Mean(q)
(e electron charge, f excess noise fraction)
In our case the coherence of laser light makes it substantially more complicate:
1 Var(q)

G
 Mean(q)  K
ef Mean(q)
the linear relation between V/E and E
in the case of coherent laser light, for
two value of K, is shown in figure.
We need to measure K and G in all
PMT
Gain stability
This is an example of time evolution of PMT gain. The statistical precision of the gain is
better than 0.5%.
This method, developed in Pisa is the most precise to follow the evolution of PMT gain
since it is insensitive to changes of laser light.
With this procedure we can follow the evolution of Calorimeter response in between
Cs scans. With Cs scans we have a very precise and careful measure of the e.m. scale
of the detector, but the scans are rather invasive and can be run only in shutdowns.
Saturation (I)
Saturation (II)
Set up a new sistem to be tested in
bat 175 at CERN
(CERN, LIP,Clermont, Pisa)
• New laser: better intensity and time stability:
Cost 40000 (Pisa contributes with 10000)
• New light mixer (Pisa, LIP)
• New patch pannel to distribute light (LIP)
• New light control system with diodes (Clermont)
• The set-up should be ready by summer 2010
Tested in 2010 and 2011 and installed during the
2011 shut down.