The Charged ANTI (CHANTI) detector is required in order to reduce critical background induced by inelastic interactions of the beam with the collimator and the Gigatracker (GTK) stations as well as to tag beam halo muons in the region immediately close to the beam.
The general layout of the system is sketched in the figure below. It is composed of six stations placed inside the vacuum tube respectively at 27-77-177-377-777 mm distance from the GTK-3
A rectangular hole inside each station is 50 mm in y and 90 mm in x due to the rectangular shape of the beam. Outer square side length is 300 mm. For particles hitting the GTK-3 center, the CHANTI covers hermetically the angular region between 34 mrad and 1.38 rad wrt the beam axis, and for particles hitting one of the GTK-3 corners, the coverage is hermetic between 50 mrad and 1.16 rad. The CHANTI, by itself, is able to veto about 95% of all inelastic interactions of K in GTK-3, regardless of the final state. This vetoing efficiency reaches almost 99% if one restricts to potentially signal-like events, namely the ones where the kaon either did not survive the inelastic interaction or did not decay in the fiducial volume, and one track is reconstructed by the straw spectrometer.
Each station is made up of two layers, called layer x and y respectively. A y(x) layer is composed of 22(24) scintillators bars arranged parallel to the x(y) direction and individually shaped at the appropriate length. Each layer is, in the end, composed by two sublayers, made up by 10+12 1.1 CHANTI (10+14) bars, and staggered by half bar. Each bar is triangularly shaped, and two staggered bars face oppositely as in the schematic below: