ALICE is made up of several detectors, each of them designed for a different purpose. Here you can see a drawing of ALICE with the different detectors labeled:
In this post I introduced you to the Time Projection Chamber (TPC). The US is mostly involved in the electromagnetic calorimeter (EMCal), which is positioned outside of the TPC (labeled “EMCAL”). I work on the EMCal.
At some point in a chemistry or physics lab you probably did a lab with a calorimeter. The standard calorimeter experiment involves heating something up, submerging it in cool water, and measuring the change in the temperature of the water in order to determine the specific heat of the object. A calorimeter measures the energy deposited by an object.
In high energy physics calorimeters are used to measure the energy of particles. When a high energy particle hits our detector, it initiates an electromagnetic shower, emitting electron/positron pairs and photons. As the shower propagates through the calorimeter, it deposits more and more of its energy, until it either stops or comes out the other side of the calorimeter. An electronic calorimeter is most sensitive to photons and electrons.
Of course sometimes a high energy particle will hit one of the nuclei of one of the atoms in the calorimeter and deposit some of its energy this way. Hadrons – such as protons and pions – are more likely to do this than electrons because they interact strongly (through the strong force), unlike electrons and photons. This is how a hadronic calorimeter works. This would not happen in an ideal electromagnetic calorimeter, but no detector is ideal so some hadrons will leave a shower in our detector.
The TPC is used for measuring the tracks left by charged particles and can measure their momentum. The EMCal can measure particles’ energy. We can use the EMCal to measure photons, which the TPC cannot see. We can also use the EMCal to distinguish electrons from hadrons because electrons will leave most of their energy in the calorimeter while hadrons will not. The EMCal can be used to measure many neutral particles, whereas the TPC can only see charged particles. By combining the information from both of these detectors, we can characterize events better.
A sampling calorimeter is made up of layers of something which produces the shower and something which can collect the energy from the shower. In the ALICE EMCal we have 77 alternating layers of lead and scintillators. A high energy particle produces an electromagnetic shower in the lead. The photons released in the shower are collected by the layers of scintillators. The light from the scintillators is collected by fiber optic cables, converted into an electrical signal, and read out as digital signals.
The ALICE EMCal is made up of 12,288 towers, each of which is about 6 cm2. It covers about 107° around the beam pipe. This is what it looks like:
Each green square is one tower. For scale, in this post our piece of the EMCAL is 4×4 towers. We have to read out data from the EMCal towers. This takes a lot of cables and what we call Front End Electronics – electronic boards which manage the data until we record it. All of these pieces have to be carefully tested before they’re installed. Here you can see me testing one of the Front End Electronics boards at CERN:
Right now only about 36% of the EMCal is installed. We got great news recently – we’ll be able to install the rest of it in January! This is wonderful – but it means we have a lot of work to do. I’ll be spending November at CERN testing parts and helping prepare the EMCal for installation. Luckily there are plenty of places to get a Thanksgiving turkey in Geneva!
Tags: ALICE, detector, heavy ion physics
