REAL-TIME DATA ACQUISITION-SYSTEM FOR VERY-HIGH-ENERGY GAMMA-RAY ASTRONOMY EXPERIMENT

An experiment is being conducted at Pachmarhi (MP) to search for TeV gamma rays from celestial sources like: Cygnus X-3, Crab Pulsar, Cas-γ, Her X-1 etc. The detection of an extensive air shower intiated by TeV γ-rays or more abundant cosmic-ray photons in the Earth's atmosphere is through the Cherenkov light from the secondaries (e+ and e-). It is identified by multiplicity logic of the signals from an array of parabolic mirrors (or a group of mirrors called a bank) at the foci of which a fast (RCA 8575) phototube is placed. Whenever such an event occurs, we record the time of occurrence correct to a microsecond, the scalers which record various counting rates, the latch information signifying which mirrors or banks have triggered during the event, the pulse height in each phototube to be read from a CAMAC ADC module and the relative time delays between various mirrors or banks from a CAMAC TDC module. The event rate is in the range of 2-20 Hz. This is accomplished by an indigenously designed hardware system built around a Q-bus-based PDP 11/23 processor through a general-purpose 16-bit parallel I/O interface, viz. DRV-11. High-priority interrupt (INTA) is used for the event while lower-priority interrupt (INTB) is used by the monitor interrupt generator (MIG). A versatile software package called HEGROSOFT has been developed to control the above hardware acquisition system. It is designed to meet the following objectives with four-level task priorities: (a) to acquire the γ-ray event data with a recording dead time of ∼ 1 ms; (b) monitor the various counting rates and print them out at regular intervals; (c) attend to the operator interrupt service requests from the console; (d) provide services for a low-priority job queue to make use of the idle time of the processor. The complete acquisition system and its design philosophy are explained in detail. © 1990.