Detector Design

The configuration of the silica aerogel Cerenkov counter system, ACC, in the central part of the Belle detector is shown in Fig.  [42,43]. ACC consists of 960 counter modules segmented into 60 cells in the $\phi$ direction for the barrel part and 228 modules arranged in 5 concentric layers for the forward end-cap part of the detector. All the counters are arranged in a semi-tower geometry, pointing to the interaction point. In order to obtain good pion/kaon separation for the whole kinematical range, the refractive indices of aerogels are selected to be between 1.01 and 1.03, depending on their polar angle region. A typical single ACC module is shown in Figs. (a) and (b) for the barrel and the end-cap ACC, respectively. Five aerogel tiles are stacked in a thin (0.2 mm thick) aluminum box of approximate dimensions $12 \times 12 \times 12 ~cm^3$. In order to detect Cerenkov lights effectively, one or two fine mesh-type photomultiplier tubes (FM-PMTs), which are operated in a magnetic field of 1.5 T [44], are attached directly to the aerogels at the sides of the box. We use PMTs of three different diameters: 3 in. (R6683), 2.5 in. (R6682), and 2 in. (R6681) of Hamamatsu Photonics, depending on refractive indices, in order to get uniform response for light velocity particles.

Figure: The arrangement of ACC at the central part of the Belle detector.

Figure: Schematic drawing of a typical ACC counter module: (a) barrel and (b) end-cap ACC.