LED Calibration Systems for CALICE Hadron Calorimeter

This paper presents several versions of the calibration and monitoring system for the scintillator tile hadron calorimeter for the ILC. The first Calibration and Monitoring Board (CMB) is used to calibrate all 7608 SiPMs of a 1 m3 hadron calorimeter prototype. Each CMB has 12 LEDs and each LED illuminates 18 tiles with SiPMs through 18 optical fibers. The pulse is 10 ns wide and delivers a signal equivalent to 70 MIPs to each SiPM. The new AHCAL engineering prototype offers both embedded LED driver (1 LED per 1 tile) and an external calibration board (1 LED per many tiles, up to 72). The embedded LED driver circuit was tuned for shorter pulses and produce nice single photo-electron spectrum and can also saturate the SiPM. A newer version of external calibrating system uses sinusoidal signal generated by a Quasi-Resonant LED driver. The system generates ∼3.5 ns optical pulse with high intensity (0.4 nJ) and low EM noise. The QRLED driver with a special notched optical fibre can saturate 12 SiPMs at once with a signal up to 200 MIPs per tile. Next version will be improved to generate longer pulses (∼5 ns), therefore a higher light intensity. Our development includes an optical distribution through a notched fibre, which shines equally from 12, 24 or 72 points. The light in the final AHCAL prototype will be routed from single LED by 3 fibers having 24 notches each, illuminating row of 72 tiles at once.