Real time data acquisition system for very high energy gamma ray astronomy experiment

Abstract

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.