Research Of Data Compression Algorithm Base On CBM-TOF Trigger-less Data Acquisition System

The Quantum Chromodynamics Phase Diagram (QCD) has been a important research project of nuclear physics. In order to further study the high baryon number density area in Quantum Chromodynamics Phase Diagram (QCD), compression baryon material experiment (CBM) and the construction of SIS 100/300 accelerator had been launched in Antiprotons and Ion Research Center of Germany. As the one of child detectors in CBM,the physical goal of Time of flight (TOF)detector is do particle identification by measuring the speed of them.Due to the particle interactions rate of CBM experiment is very high,the requirements of time resolution in time of flight detector the is very strict. In order to meet the requirements of the time resolution, Multi-gap Resistive Plate Chamber is used to build the main part of the TOF detector and a trigger-less data acquisition system is used to do signal readout.The data acquisition(DAQ) system is development and produced by GSI,its consist by the pre-amplifier and discriminator(PADI)、time to digital converter(GET4-TDC) and readout controller(ROC).In order to make sure that all the particles time information can be detected,during the whole work time the DAQ system will generate large number of time epoch signal and notes signal and place them in the raw data flow.So finally the form of the raw data flower we get will be very complicated, the time information associated with charged particles is hided in the raw data flow.In the paper,a data compression algorithm has been designed that can pick up the time information related to charged particles from the raw data flow generated by the data acquisition system of CBM-TOF. The algorithm has three steps to finish the work,they are validity checking、time ordering and Hit-building & TOT checking. In first step validity checking the data that include time information of charged particles will be picked up from raw data stream,then this kinds of data will be reconstructed into pulse data through second and third step.By compressing the raw data that were derived from beam testing at COSY,The correctness and effectiveness of the algorithm has been proved.For this algorithm, raw data volume was compressed to the maximum extent with a compression ratio of 94.5% and above; in addition, the compression algorithm didn’t loss the useful data including time information of particles.Regarding this algorithm, not only were data flow pressures on trigger-less data acquisition system link relieved, but valuable information was extracted from the raw data both efficiently and rapidly. Therefore, operational requirements for CBM-TOF data preprocessing were basically satisfied, which had a great significance for data reconstitution of TOF detector.