The two detectors have a common structure of two independent calorimeter towers, for photon and neutron measurements. The towers are made from tungsten absorber layers and scintillator layers, with a difference in the size of transverse sections for the two arms. The calorimeter towers are used to measure incoming particle energy and to identify families of particles. Each detector has a tracking system: the Arm 1 system consists of four double-layers os scintillating fibres; Arm 2 consists of microstrip silicon layers. The energy resolution for the detectors is over 3% for photons above 100 GeV and around 40% for neutrons. The position resolution for Arm 1 and Arm 2 is 200 μm and 40 μm for photons respectively, and is around 1 mm for neutrons for both the detectors.