Prototype Counters

The major concern of the counter design is absolute light yield and its uniformity. Our Cerenkov counter is not Imaging-type, hence information on a ring radius of Cherekov light is not necessary. The light yield depends on design parameters, such as refractive index of aerogel, path length of particle trajectory, reflective material for the container, area of photocathodes, number of PMTs, and geometry of the counter box. The refractive index of aerogel has already chosen to be $\sim$1.01 from the desired particle separation range. The other design parameters will be optimized through various tests using test beam. For the basic design of the counter geometry, we gave attention to essential properties of silica aerogel and Cerenkov radiation, namely, light scattering and directionality, respectively. Three counter configurations were designed based on different design concepts.

Belle Type

Aerogel is known to be very diffusive material in particular at short wave length. The KEK Belle group has built an aerogel cherenkov counter in which the aerogel contained in a diffusion box viewed by PMT's from the side. The Cerenkov light scatters often in the aerogel loosing its characteristic directionality with respect to the particle trajectory. On the other hand, light absorption in the aerogel is known to be rather small. Having highly reflective diffusion box as a container of aerogel, photons are expected to reach photocathodes eventually. According to the sucessful aerogel counters by KEK BELLE group, this counter configuration is called as ``Belle type'' [1]. We take this prototype as a reference.

Fig.22(a) shows a schematic drawing of Belle Type prototype. The counter box, made of 2 mm thick aluminum plate, has a size of 12 $\times$ 12 $cm^{2}$ cross section and 12 cm depth. The area of cross section has been determined to keep the occupancy of the counter less than 10 % even in central Au+Au collisions. Tiles of aerogel, 1.1 cm thick each, were installed in stacks of eleven to form a 12-cm-thick radiator. Two PMTs are attached directly on the surface of the aerogel at the both sides. The inner surface of the counter box is covered with highly reflective sheet (GORETEX) for better light collections.

Integration Cube Type

In order to improve the uniformity of light yields, empty space behind the aerogel is introduced as shown in Fig.22(b). Scattered lights off the aerogel is integrated in the empty space, called integration cube, and detected by two PMT's. Having the integration cube behind the aerogel, directional lights as well as scattered lights have good chance of beeing detected. This modified configuration is called ``Integration Cube type''.

Mirror Type

The third prototype focus on the directional light; using 45 degree mirror behind the aerogel, direct Cerenkov light emitted in a narrow cone can be guided on the photocathode as shown in Fig.22(c). This configuration should be very effective when the aerogel is trasparent and less diffusion. As shown in the figure, this ``Mirror type'' requires single PMT. One PMT follows a flat mirror behind of the aerogel. The mirror was a aluminized mylar sheet backed by a styrofoam frame.

図 22: Schematic drawing of prototypes; (a)Belle type, (b)Integration Cube type, (c)Mirror type. Origin of the coordinate ((x,y) = (0,0)) locates at the center of front face of the counter. Coordinate axes are also shown.