Electromagnetic Presamplers (PB & PE)

Most electromagnetic showers are initiated before reaching ECAL as there are approximately two radiation lengths between the interaction region and the calorimeter, because of this there is a presampler [21] incorporated into the ECAL. The presamplers help in energy resolution of the shower and discrimination between particles.

The barrel presampler is a 6.623 m long cylinder of radius 2.388 m situated between the time-of-flight barrel and the barrel electromagnetic calorimeter. It consists of 16 chambers containing two layers of streamer mode drift tubes with sense wires parallel to the beam axis. The barrel presampler covers a polar angle range of $\vert\cos\theta\vert < 0.81$. The two endcap presamplers (PE) are situated between the time-of-flight endcap (TE) and the endcap electromagnetic calorimeters. Each PE is divided into 16 overlapping wedges and gives full azimuthal angle coverage and a polar angle coverage of $0.86 < \vert\cos\theta\vert < 0.95$.