Liquid Argon Calorimeter

Calorimeters (calorie meter) in physics and chemistry experiments measure the total heat of a reaction or process, which of course is an energy measurement. Heat is just one form of energy; particle physicists use the term calorimeter for any device designed to measure energy.

The three inner sections of the detector have given us information on the charged particles in the event, but the neutral particles, which make up a third or more of the total, are so far invisible. These finally make their mark in the Liquid Argon Calorimeter, a hollow aluminum spool filled with pure liquid argon and stacks of lead plates.

This calorimeter ("calorie meter") module has alternating layers of lead sheets and lead tiles, immersed in liquid argon when in use. A high-energy particle makes a shower of other particles in the lead. Electronics attached to the module records the shower size (a measure of the particles energy) and its shape (a way to distinguish between certain types of particles).

Note the angled towers of tiles: they slope outwards from the collision point. The SLC Large Detector (SLD) contains 288 modules.

Electromagnetic Shower

When electrons, photons, and the heavier particles like protons, neutrons, and pions travel through the calorimeter modules they interact in the metal plates, producing more particles of lower energy. These in turn produce yet more, even lower-energy particles in a cascade which we call a shower. Eventually, the shower consists of many particles, many of which are electrically charged.

As charged particles travel through the argon they cause ionization. The ionization collects on the metal plates, giving a electrical signal that can be recorded. The amount of charge deposited in the calorimeter in a single shower and the depth to which the shower develops are proportional to the total energy of the particle that initiated the shower. Thus this device can measure particle energies for both the charged particles and the neutral ones.

The showers from charged particles are identified by the relationship between the location of the shower and the direction of a track seen in the inner layers. A shower with no associated track tells us about a neutral particle that was produced in the event.