Two types of aerogel has been tested as a Cerenkov radiator. One is hydrophobic aerogel tiles ( 70 mg/) of 11 11 1.1 produced by Matsushita Electric Works Ltd. (Japan). The other is hydrofilic aerogel tiles ( 35 mg/) of 12 12 3.0 produced at Novosibirsk (Russia). The refractive indices of the samples were measured using the minimum deflection method with a laser of = 670 nm before the beam test. The measured values are 1.017 0.001 for Matsushita aerogel, 1.007 0.001 for Novosibirsk aerogel.
Photomultiplier tubes (PMTs) from Hamamatsu Photonics, R5155(2 inch) and R6233(3 inch), have been used in the tests. These PMT have a bialkali photocathode and a high quantum efficiency (30 % maximum at 420 nm accodring to Hamamatsu). The diameter of effective photocathode is 46 mm and 70 mm for 2 and 3 inches tubes, respectively. While R6233 has borosilica glass window, R5155 has quartz window and transparency is extended to UV region of 185 nm. We modified the standard voltage-divider of R6233 to obtain a gain as high as .
Throughout the measurements, the average number of photoelectrons () have been determined from the total charges of each PMT output divided by the gain factor. The gain factors of individual PMT's were calibrated separetely by measuring the response to a pulsed light source (PLP-02, Hamamatsu) at the wavelength of 415 nm. Typical calibration curve is shown in Fig.23. Systematic error of determination is about 10 %.