A compact data acquisition system for multi-detectors: Time encoding for both time and energy measurement

We have developed an acquisition chain that steers the detection of events characterized by the deposited energy and the arrival time. All analog informations are first converted into temporal informations by means of translators plugged inside each detector, then only logical signals (containing: way number, arrival time, energy) are processed by the system (transmission, multiplexing-dispatching) and are coded by a Fast Multi-hit Time to Digital Converter (FTDC) with a 225 ps resolution and a 24 ns dead time. The FTDC runs without trigger and assigns "absolute value" to each temporal signal. A Digital Signal Processor (DSP) processes these values and extracts the events in coincidence according to energy and time gates. So the data are saved on hard disk and the experiment can be reloaded every time it is necessary with different parameters (time and energy windowing). The absolute time saving has the advantage of not making a hardware time adjustment. All processing components are mounted in a dedicated PC board. The system was validated and is currently used for TOHR (French acronym of High Resolution Tomograph), a tomograph developed for high resolution in vivo imaging adapted to small animal models studies. The quadruple coincidences throughput (with 15 detectors) is more than 200,000 events/second. The acquisition control and data saving program is written in Labview.