The original purpose for the PV was to provide a fast signal indicating that a charged particle was entering the calorimeter (as opposed to an electrically neutral photon). A fast, accurate signal was required so that the calorimeter signals would not need to be delayed any longer than necessary. Delaying the signal lines from the calorimeter would require longer signal cables that would necessarily degrade the signal quality. Additionally, any extra time required to determine the trigger would mean an increase in the dead time of the detector. The BC-400 plastic scintillator manufactured by Bicron of which the PV was constructed provides the fast, accurate signals required.
The size of the PV detector was important for a few reasons. First, the material needed to be thin enough so as to not degrade the energy of those particles passing though it by much so that the CsI calorimeter would see (nearly) the full energy of the particle. Second, a thick/dense material would have a greater probability of converting one of the signature g 's from a p 0 decay before it reached the calorimeter. Finally, a thicker detector would require a larger inner radius for the calorimeter. A larger radius would require more CsI material in order to cover the same solid angle, which could drive up the cost considerably. Plastic scintillator is again a good choice here because it is light
and rigid enough to maintain its shape when formed as a 3 × 41 × 60 mm3 stave.
The PV was designed to cover the entire solid angle subtended by the calorimeter. Care was also taken to ensure that no clear path existed from the target to the calorimeter (i.e. that a particle could not slip through the seam between adjacent staves). The 18o angle on one side of the stave as indicated in Figure 4.1 provided the desired geometry.
The light guides were designed to flare out somewhat in order to allow more space between the PMTs and to provide better access to the central part of the detector.