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: Monitor of the laser light intensity Attenuate the light intensity according to needs Provide the trigger to DAQ corrected for laser time jiitter Laser is used for: Monitor PMT stability Measure PMT linearity 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: (e electron charge, f excess noise fraction) In our case the coherence of laser light makes it substantially more complicate: 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.