Mirror Type

Fig.34 shows the position dependence of $N_{pe}$ as a function of $x$ at $y = 0, 3 [{\rm cm}]$. The mirror is made of aluminized mylar with styrofoam backing. According to optics, all the direct Cerenkov lights emitted in the aerogel should be reflected on the photocathode of PMT. Although Mirror type has only one PMT on the side, a small dependence of the beam position was observed except for point at x = 5 cm, where the value is lower accidentally due to an air gap between the aerogel and the wall. The detected light yield should include not only the direct light but also scattered light since the interior of the box is covered with GORETEX. Ratios between direct light and scattered light is in interest.

To evaluate the ratios of direct and scattered light, we have carried out the following measurements; Firstly, the inner surface of Belle type was covered with Black paper to avoid light reflections on the wall. Rotating the Belle type by 90 deg. so that the beam goes through PMT, aerogel and PMT, photoelectron yields has been measured as 14 p.e. To subtract the photoelectron yields due to the cherenkov emission in the window glass of PMT, we repeat the measurement without the aerogel and obtained 4 p.e. This means that 10 = (14 - 4) pe were detected as direct light for a 12cm-thick aerogel radiator. In other words, photons equivalent to 10 photoelectons can exit from the aerogel in the forward direction. For a thickness of 12 cm, most photons are scattered since we detected 23 pe for Belle type. Photons produced at the last few centi-meter aerogel along the path may contribute to directed light detected, due to light scattering inside aerogel. In order to know the effect of the mirror, we have estimated the fraction of directed light detected by Mirror type. We used black papers for covering either the mirror or the wall. From the difference between overall covering and covering except for the mirror surface, the fraction was estimated to be $\sim$5 pe. Accordingly, the mirror option collect directed light about half. It is found that Mirror type collected scattered light rather than directed light.