Initial results for automatic calibration of the LabPET II front-end detector module

An automatic calibration process for the LabPET II detector front-end module has been developed. By aiming at sub-millimetric spatial resolution, an unprecedented channel density is reached. The new detector front-end module is based on an application-specific integrated circuit (ASIC) implementing a Time-over-Threshold (ToT) scheme to extract both energy and time information. Consequently manual channel calibration becomes a tedious task and an automatic calibration strategy adapted to the ToT scheme is required to cope with the huge number and complexity of detector channels. The calibration process involves four main tasks: Clock Phase Adjustment, Photopeak Alignment, Timing Optimization and Energy Correction. To grant an easy and frequent adjustment, the calibration routine must be automatic and real-time. It was implemented in VHDL and C on a Microblaze microprocessor core embedded in a field programmable gate array (FPGA). Misalignment between channels were reduced after calibration from 17% FWHM to 6% FWHM.