The ALICE Zero Degree Calorimeters (ZDC)
When two accelerated heavy nuclei collide head-on (central collisions), very high energy densities and temperatures are reached. As shown in the figure below, collisions can be central or peripheral; it is crucial for physicists to distinguish between them. In peripheral collisions, only few protons and neutrons are involved and most of the nucleons fly along the beam line without participate (spectator nucleons) to the collision.
The ZDCs are calorimeters which detect the energy of the spectator nucleons in order to determine the overlap region of the two colliding nuclei. It is composed of four calorimeters, two to detect protons (ZP) and two to detect neutrons (ZN). They are located 115 meters away from the interaction point on both sides, exactly along the beam line. That is why we call them Zero Degree Calorimeters (ZDC).
The ZDCs are “spaghetti calorimeters”, made by a stack of heavy metal plates grooved to allocate a matrix of quartz fibres.
The metal plates are made of a special material namely a tungsten alloy for neutrons and brass for protons.We call the material of the metal plates “passive material”, while the quartz fibres “active material”. High energy protons and neutrons hitting the passive material create a cascade of particle, called “shower”.
When one of these shower particles crosses a fibre, if its speed is high enough, it can produce light (Cherenkov effect). This light propagates in the fibre by total reflection up to its end, where a photomultiplier converts the light into an electric signal.
The amplitude of the electric signal is proportional to the energy of the incoming protons and neutrons allowing to measure the energy carried away by the spectator nucleons and therefore indirectly the size of the overlap region of the two colliding nuclei.