Thickness dependence

We measured the light yield as a function of the thickness of aerogel for Belle and Mirror types. Light yield versus the aerogel thickness is shown in Fig.35. The PMTs were located at the center of aerogel in the beam direction for Belle type. The distance between the surface of the aerogel and the PMT was fixed at 6cm for Mirror type. The light yield of Belle type was propotional to the thickness until 26 cm, but on the other hand the yield of Mirror type is inclined to saturate due to light scattering and absorption. When Rayleigh scattering occurs inside the aerogel, the directionality of Cerenkov light is completely lost. Photons produced in front-side aerogel have much more probability to be absorbed by aerogel or reflectors before reaching a photocathode. These behaviors are consistent with the diffusive property of the aerogel as discussed.

図 35: $N_{pe}$ per PMT as a function of the aerogel thickness. Circles are for Belle type, and triangles are for for Mirror type. The solid curves are guide for eyes. Note that in this measurement Tyvek sheets were used as reflector.