The data acquisition system for a Compton and pair-creation sensitive instrument

A data acquisition system has been developed for the prototype of the Medium Energy Gamma-ray Astronomy (MEGA) telescope. It reads out two different types of detectors: The first type, called tracker, is a stack of Silicon strip detectors. The second is a calorimeter consisting of CsI(Tl) bars coupled to Silicon PIN-photodiode arrays. Analyzable photon interactions in the detectors cause either coincident events in both detector types or hits in multiple subsequent strip detectors. Once such a pattern is detected, all channels of both detector types are read out. This leads to data from 11328 channels, which is reduced to the channels containing interactions and written to the computer hard disk. The whole system is designed around the front-end chip TA1, which is used for both types of detectors. It is a charge sensitive device including discriminators for trigger generation. The triggers from all chips are evaluated by our coincidence electronics, a self-made VME-bus card. Nearly all logic of this card is implemented in a programmable logic device. An additional RAM forms a lookup table for trigger patterns that should start the readout of the whole detector. Commercially available VME modules perform the analog to digital conversion and generate the readout sequence. A single-board computer takes care of the instrument control, the data reduction and storage, collects housekeeping data etc. Its software is written in C++, so that a high modularity and several abstraction layers can he achieved easily. The system showed stable operation over several weeks during laboratory measurements as well as during beam tests.