The multiplicity trigger is designed to select central events. This is possible because of the direct correlation between particle multiplicity and centrality in heavy ion collisions. HIJET/GEANT monte carlo simulations have been used to help design this trigger.
For the monte carlo studies a 1 mm thick radially segmented scintillation counter was
placed 10 cm from the gold target. The scintillation counter is a disk which covers
the angular range from 
to 
( 
= pseudorapidity = 0.88 - 2.44).
The region between the target and scintillator is vacuum, and
lead or a magnetic field can be used to eliminate the cloud of delta
rays produced by the gold ion traversing the gold target. (Note that the lead in the study
presented below is cylindrical with the same inner and outer radius as the scintillation
counter. Future designs will include lead that is tapered so that the particles which
hit the scintillator will traverse equal amounts of lead.)
The energy deposited in each segment of the scintillator is then recorded and analysed.
An event is simulated with GEANT by sending a 11.71 GeV/nucleon gold ion through a 10% gold target. This ion produces delta rays until it interacts at a random depth in the gold target. This collision is simulated by the HIJET event generator. The interaction products and delta rays proceed through the target and in the lead, magnetic field, or vacuum region. Finally, particles deposit energy in the scintillator. All of GEANT's physics processes were turned on for the simulation so that all decays and secondary interactions in the target, lead and scintillator are included.