FASER's electromagnetic calorimeter is designed to stop high-energy electrons and photons, identify them, and measure their energies. On the other hand, energy deposits from muons and hadrons are minimal. Since most signal events consisting of e+e or photon pairs separated by less than a few millimeters, it is not feasible to measure the individual particle energies. As a result, the main calorimeter requirement is to measure the total electromagnetic energy with good accuracy for multi-TeV deposits in a compact detector.

The calorimeter uses four spare LHCb outer ECAL modules, shown in Fig. 1 and 2. These are Shashlik-type calorimeters with interleaved scintillator and lead plates. The modules (including the PMT) are 754 mm in length and have transverse dimensions of 121.2 mm by 121.2 mm. The full FASER acceptance is therefore covered with these four modules. The calorimeter contains 66 layers of 2 mm lead and 4 mm plastic scintillator, resulting in a total depth of 25 radiation lengths.

The energy resolution for TeV deposits in such a calorimeter is around 1%. However, this is degraded at the highest energies, since 25 radiation lengths does not fully contain all such showers.