Trigger-less readout of the PANDA electromagnetic calorimeter

The PANDA collaboration at the future FAIR facility at Darmstadt, Germany, will employ antiproton annihilations to investigate resonances in the charmonium mass region. In order to gain high flexibility for physics event selection, a readout system without hardware trigger, i.e. a trigger-less data-acquisition system will be employed and is currently being developed. The event-selection is based on high-level reconstructed information, e.g. invariant mass, secondary vertices, and time correlations, which may be contributed from all sub-detector systems. Prototypes of the PANDA Electromagnetic Calorimeter have been exploited to verify the desired event-selection approach. The prototype readout system includes a dedicated Sampling ADC (SADC), data concentrator (DCON) and compute-node (CN) modules. An optical link connects several SADC digitizers to a DCON module from where the data are streamed to the CN network. Data, collected by the SADCs are processed online in FPGAs by a feature-extraction algorithm. This processing technique guarantees a dead-time free operation and allows including an on-line pile-up recovery process for single detector counting rates up to 1 MHz. The prototype of the readout-chain has been evaluated using pulse generators and high-energy photon beams; the proper event correlation could be verified. The applied FPGA has been irradiated with protons to demonstrate its applicability in the radiation environment of the PANDA experiment.