Abstract
The CMS Electromagnetic Calorimeter utilizes scintillation lead tungstate crystals, with avalanche photodiodes (APD) as photo-detectors in the barrel part. 1224 HV channels bias groups of 50 APD pairs, each at a voltage of about 380 V. The APD gain dependence on the voltage is 3%/V. A stability of better than 60 mV is needed to have negligible impact on the calorimeter energy resolution. Until 2015 manual calibrations were performed yearly. A new calibration system was deployed recently, which satisfies the requirement of low disturbance and high precision. The system is discussed in detail and first operational experience is presented.
Highlights
The CMS Electromagnetic Calorimeter utilizes scintillating lead tungstate crystals, with avalanche photodiodes (APD) as photo-detectors in the barrel part. 1224 High Voltage (HV) channels bias groups of 50 Avalanche Photodiodes (APDs) pairs, each at a voltage of about 380 V
The Electromagnetic Calorimeter (ECAL) energy resolution is very important for several physics analyses at CMS, as for example for the study of the diphoton decay of the Higgs boson [4]
The operational APD gain is achieved with an appropriate High Voltage (HV) system
Summary
A new calibration method of the CMS ECAL Barrel HV system has been presented. The old method required manual intervention to uncable the system in the CMS service cavern and calibrate one by one the HV boards. This results in a lengthy procedure that can only be performed during year-end technical stops, and causes increased mechanical stress on the HV cables. The new calibration system presented here introduces mechanical switches to connect the HV cables to the calibration system and a specific software program cycling through all the channels allowing both to measure the voltage and to recalibrate all the channels singularly. The new system was tested at the beginning of 2016 and both methods provide compatible results
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