Aerogel and PMT samples

Two types of aerogel has been tested as a Cerenkov radiator. One is hydrophobic aerogel tiles ($\rho$ $\simeq$ 70 mg/$cm^{3}$) of 11 $\times$ 11 $\times$ 1.1 $cm^{3}$ produced by Matsushita Electric Works Ltd. (Japan). The other is hydrofilic aerogel tiles ($\rho$ $\simeq$ 35 mg/$cm^{3}$) of 12 $\times$ 12 $\times$ 3.0 $cm^{3}$ produced at Novosibirsk (Russia). The refractive indices of the samples were measured using the minimum deflection method with a laser of $\lambda$ = 670 nm before the beam test. The measured values are 1.017 $\pm$ 0.001 for Matsushita aerogel, 1.007 $\pm$ 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 $10^7$.

Throughout the measurements, the average number of photoelectrons ($N_{pe}$) 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 $N_{pe}$ determination is about 10 %.