  The nucleus-nucleus impact parameter and collision geometry of a heavy ion
collision are typically characterized by assigning a collision "centrality". In
all present heavy ion experiments centrality is measured indirectly, by
detecting the number of particles or the energy of the particles produced in
the interactions, typically at high rapidity. Centrality parameters are
associated to the measured detector response using the Glauber model. This
approach suffers from systematic uncertainties related to the assumptions about
the particle production mechanism and limitations of the Glauber model. In the
collider based experiments there is a unique possibility to measure centrality
parameters by registering spectator fragments remaining from the collision.
This approach does not require model assumptions and relies on the fact that
spectators and participants are related via the total number of nucleons in the
colliding species. This article describes the concept of the centrality
detector for heavy ion experiment, which measures the total mass number of all
fragments by measuring their deflection in the magnetic field of the collider
elements.
