Design

Three identical detector planes will each have a radiator length of 5 cm. This will be achieved with 5 layers of 10 mm thick tiles. Each plane will further be segmented into 7 optically separated diffusion boxes. In each segment, five layers of 4 by 2 tiles will cover an area of $460\times 230$ mm$^2$. The aerogel tiles will rest on one of the side walls and be held in place by thin nylon strings. All internal walls, including behind the aerogel, will be covered with one layer of Millipore filter paper as a diffuse reflector. Separating walls between two segments will be a sheet of tedlar film sandwiched between two layers of Millipore paper. Two 5 inch photomultipliers on each segment will collect the Cerenkov light. Detector entrance and exit windows will be made of thin light tight film (Tedlar). Each detector plane of 7 segments will form one gas tight box with an internal volume of $1620\times 460\times310$ mm$^3$ (223.6 liter or 7.9 ft$^3$). During operation (maybe even storage) the boxes will be continuously flushed with UHP/Zero air at roughly 1-2 ft$^3$/h. A schematic layout of the three detector planes is shown in figure 50.

As aerogel SP-50 from Matsushita Electric Works with an refraction index of $n=1.05$ will be used. This material is hydrophobic and therefore does not provide the problems that have been associated with absorption of water in older types. As photomultipliers, Hamamatsu R1250 and Photonics XP4572B/D1 will be used. The voltage dividers for the XP4572B/D1 (and maybe even those for the R1250) will be retrofitted with an internal amplifier. These amplifiers have been successfully employed in the Hall-A A2 detector, which also uses the Matsushita SP50 aerogel, and also the Hall C HMS aerogel detector.