Performance of Belle type counter with n=1.017 Matsushita aerogel and Hamamatsu R6233 (3 inch) PMT's is investigated. Fig.27 shows the position dependence of photoelectron yield. Box is scanned horizontally ( in Fig.22) at . Beam is injected perpendicular to the box. In the figure, photoelectron yield from each PMT and sum of two PMT's are plotted as a function of [cm]. Photoelectron yield of each PMT decreases as an exponential with the distance to the PMT; . Sum of two PMT's gives dip at the center where total of 23 p.e. are obtained. With larger size of the box, the dip at the center becomes deeper. As is seen, it is found that Belle type shows clear position dependence in horizontal direction().
Similarly, position dependence of photoelectron yield is studied in vertical direction. In Fig.28, box is scanned vertically at cm. Photoelectron yield of the left PMT, which is closer to the beam, shows broad peak at around , while that of the right PMT stays constant. This effect might be due to the effect of solid angle; solid angle of photocathode have clear peak when the beam is closer to the PMT.
Fig.29 shows the position dependence of photoelectron yield in logo plot. Photoelectron yield at (x,y) position is shown as a height of logo. Position dependence in horizontal direction is clealy seen. When the size of box is larger, the dip at the center becomes significant.
In the real experiment, particles may enter the box with shallow angles. Incident angle dependences are investigated by rotating the box With respect to the test beam. The center of rotation locates at the center of the aerogel. In Fig.30, photoelectron yields are plotted as a function of incident angle of the beam. At 90 degree, when the beam is perpendicular to the aerogel box, the yield shows minimum and it increases with larger angle. The curve shown in the figure is , which is proportional to the pathe length of the particle trajectory in the aerogel. Successful description with the suggests that photoelectron yield is simply proportional to the path length of the track, implying that directionality of cherenkov emission is lost quickly in the aerogel.