摘要
The Fermilab CKM (E921) experiment studies a rare kaon decay which has a very small branching ratio and can be very hard to separate from background processes.A trigger and DAQ system is required to collecto all necessary unformation for background rejection and to maintain high reliability at high beam rate.The unique challenges have emphasized the following guiding concepts:(1) Collecting background is as important as collecting good events.(2) A DAQ "event" should not be just a "snap shot" of the detector.It should be a short history record of the detector around the candidate event. The hit history provides information to understand temporary detector blindness,which is extremely important to the CKM experiment.(3) The main purpose of the trigger system should not be "knocking down trigger rate" or "throwing out garbage events" .Instead,it should classify the events and select appropriate data collecting straegies among various predefined ones for the given types of the events.The following methodologies are epmployed in the architecture to fulfill the experiment requirements without confronting unnecessary technical difficulties.(1) Continuous digitization near the detector elements is utilized to preserve the data quality.(2) The concept of minimum synchronization is adopted to eliminate the needs of time matching signal paths.(3) A global level 1 trigger performs coincident and veto functions using digital timing information to avoid problems due to signal degrading in long calbes.(4) The DAQ logic allows to collect chronicle records around the interesting events with different levels of detail of ADC information,so that very low energy particles in the veto systems can be best detected.(5) A re-programmable hardware trigger(L2.5)and a software trigger(L3) sitting in the DAQ stream are planned to perform data selection functioins based on full detector data with adjustability.
