A large electromagnetic shower detector with high rejection power against pions

Abstract

A large electromagnetic shower detector for identification and energy measurements of γ-rmrays (between 150 and 1600 MeV) and electrons (between 400 and 1100 MeV), in the presence of high pion background, is described. The detector is based on the principle of simultaneous measurement of the spatial development of the electromagnetic cascade and of its energy release. It consists of 1) two six-gap thin-plate spark chambers for the reconstruction of the incoming particle trajectories; and 2) nine elements, each made of a lead foil, a spark chamber, and a plastic scintillator, all sandwiched together; here the shower development is studied. When used for γ-detection, a 0.5 cm Pb foil is placed in front of the thin-plate spark chambers, in order to allow the detection of the γ-conversion process and the identification of the γ-direction. The dimensions of the detector are 60 x 120 cm 2 front face, and 50 cm depth along the electromagnetic shower development. A pion rejection power of the order 5 x 10 −4 between 400 and 1100 MeV, for electron efficiencies varying from 70% to 80%, is obtained. The pion rejection efficiency in the γ-case is highly improved by the anticoincidence efficiency factor, while the γ-detection efficiency depends on the precision required in the reconstruction of the γ-ray direction. The γ-ray and electron energy resolution is about ±15%.