A crucial part of understanding relativistic heavy ion collisions is characterizing the geometry of the nucleus-nucleus interaction.  It is experimentally impossible to measure the impact parameter on an event by event basis and so typically results are classified in terms of ‘centrality’ defined to be proportional to the bulk energy observed in the collision.  A ‘centrality class’ of collision events with similar bulk energy characteristics may then be related, albeit in a model dependent manner, to a mean number of nucleons participating in the nucleus-nucleus interaction.  In order to avoid this model dependence and learn the number of nucleon participants on a more precise and event by event basis a new detector is proposed.  The detector will be composed of measurement stations downstream from the interaction point to measure the ‘spectator’ nucleons, those which do not participate in the nucleus-nucleus collision.  The different stations of the detector will cover specific mass range of spectator tp give us clear identification of types of spectator nucleons in heavy ion collisions.   A detailed simulation of the collisions and propagation of the spectator nucleons following the interaction showing expected detector performance parameters will be shown.