A large area high efficiency γ-ray detector with good time and space resolution

We describe the construction of a large area high efficiency γ detector for use in the 30 to 1000 MeV range. Its basic principle is the combination of lead-scintillator sandwich counters with optical spark chambers to achieve high γ efficiency along with good space and time resolution. The scintillator is arranged in the form of a hodoscope so that the spark chambers could be triggered on a selected number of simultaneous γ-rays. The performance in terms of γ efficiency, energy and direction resolution, and shower development are discussed.