The acceptance of the Aerogel counter is planned to cover 4m at radial distance of 4m from the beam line in the central rapidity (west arm). Each counter cell is about 14x14x10cm, which makes 13x13 cells in total. One of the current option (A) for the Aerogel counter is shown in Fig.15 with Belle type counter. Single cell of Belle prototype is shown in top-left panel in the figure. In order to fill up the full area of the acceptance, there will be 2 layers in radial direction and it's shifted by a half cell size in every second row in phi-z plane, as shown in the figure.
There are 2 other options (B) and (C) being under consideration. The option (B) shown in Fig.16. Both (B) and (C) option uses the same single cell with air gap space shown in the figure. It has been found out with an additional air gap space for the PMT mount, the position uniformity is much better with this type than the previous Belle type, although the total number of photo-electron seen in the single cell is smaller, but it's still close to 20 photo-electrons. The details of those comparison will be shown in the following chapters. The beauty of the option (B) is to have all the Aerogel segments at the same plane, with a possible drawback of every second segments having the incoming beam back from the air gap space into the aerogel, where they only sees scattered light, while the other segments would see some of direct light from the Aerogel as well. However it was also measured that the difference between them is still acceptable about 20 loss. The alternative way of stacking the cell with air gap space is shown in the figure.17, which is labeled as option (C), in order to have particles from the same direction for all the cells. If we would like to use mirror to collect direct light from the Aerogel with single PMT per cell, this would be the option.
The segmentation was originally derived by comparing the occupancy measured at the current time of flight counter. The current time of flight counter located at 5m in radial distance from the beam line and the single cell size is some 100cm. It sees all the charged particle above 0.18GeV/c plus backgrounds which includes low energy electrons and gammas from magnet yoke and EM calorimeter. It has about 10 occupancy for Au+Au collision. On the other hand, the aerogel counter is at 4m from the beam line and the cell size is 230cm, but detect only pions above 0.75GeV/c. The estimated occupancy is about 8 for this cell size with a transverse momentum distribution from RQMD as shown in Fig.18 together with all the numbers in the table.1.
PHENIX GEANT simulation (PISA) is performed with 12x12x12cm Belle prototype in the west arm as seen in Fig.19. The back ground source points are seen in Fig.20, where the many HIJING events are superimposed. The distribution of the number of fired cell per event is shown in Fig. 21. The estimated occupancy for this cell size was found to be 5 for central Au+Au collision with HIJING, which is consistent with the original estimation.