Electromagnetic Calorimeter
It's not just a brick, of course - it is a fine "lasagna" of lead and plastic scintillator. The lead sheets contain the heavy nuclei and electrons that help to stop particles; the plastic (hundreds of thousands of thin transparent "scintillator" rods) is the active detecting medium. Like the time-of-flight scintillator, the plastic rods are specially formulated to emit light when a particle passes through. The light bounces down the rods and is detected by photomultiplier tubes at the edge. Since both the scintillators and the phototubes are fairly small, we can see exactly where and how much light is emitted by the passing particles.
The location of the light emissions tells you a lot about the particle's type. For example, electrons start emitting energy as soon as they enter the ECAL. Protons and nuclei will penetrate some distance (depositing a little energy) and then perhaps start "showering" suddenly (depositing a lot). This is called a "shower shape" analysis. We can figure out the particle type and charge, mistaking a proton for an electron less than one time in 100,000.
By looking at the total light emission in a particular shower, we can often figure out the energy very accurately.
The ECAL is heavy, and requires a sturdy aluminum structure to hold it up.