  New highly transparent, hydrophobic silica aerogels with refractive indices
of 1.01 to 1.07 have been produced by Aspen Aerogels, Inc., and select tiles
have been tested using an electron beam at the DESY, Hamburg facility. A
diffusively-reflective threshold Cerenkov counter was designed and constructed
for the purpose of evaluating the aerogels, and can accommodate aerogel tile
areas of up to 20 cm by 20 cm. Measurements of the performance of the counter
using the new aerogels as Cerenkov radiators are given in terms of
photoelectron yields and a figure of merit.
