= Solution
One suitable implementation is a dense homogeneous scintillating-crystal <electromagnetic calorimeter>, with crystals directed approximately towards the collision region and read out by magnetic-field-compatible photodetectors. An incident <electron> emits <bremsstrahlung> <photons>; these produce <electron>-<Positron> pairs, which radiate again. This <electromagnetic shower> develops over successive <radiation lengths> until the daughter <energies> approach the <critical energy of an electromagnetic shower>, below which <ionization> dominates. The deposited <energy> creates scintillation light, and calibrated integrated detector signals estimate the incident <energy>.
A depth of roughly $25$ <radiation lengths> contains most of a high-<energy> <electromagnetic shower>. Small transverse cells resolve its lateral profile, distinguish nearby deposits and permit position reconstruction. Crystal material, optical transmission, gain and electronics must remain stable under the LHC radiation exposure. Fast shaping, timing, segmentation and radiation-tolerant readout permit operation at high collision rates; <collision pileup> and <particle showers> from nearby particles require reconstruction and calibration. A dense crystal array offers good intrinsic <energy resolution> because it measures almost all the shower deposition rather than only the sampled fraction.
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