Reflectors

The inner surface of the counter box is covered with highly reflective material, since most photons may undergo numerous reflections prior to reaching the photocathode. In order to select the best reflective material for Cerenkov light collection, we tested 7 samples. Using the exponential behavior of Belle type, we compared the light collection efficiencies of the samples. The measurement results are summarized in Table 3. $N_{pe}$ and attenuation length are contributing to light collection. Goretex sheet is found to have the highest light yield than the other reflectors tested. The light yield at the center was improved by a factor of 6 (from ``Black paper'' to ``Goretex''). For more precise consideratoin, the reflectances as a function of wavelength are needed. According to the data sheet from the manufacturer(W.L.Gore & Associates,Inc.), Goretex sheet has high reflectivity in the short-wavelength range(300-400 nm). Moreover, the Goretex sheet is radiation hard up to 8.6 Mrad of an equivalent dose in the visible range beween 425 - 675 nm [2]. We plan to use the Goretex sheet as a real reflector. For all the configurations, reflector was the key component of light collection system.

表 3: Comparison of the tested reflectors.

Reflective material	Thickness	$N_{pe}$ (pe)	Attenuation
	($\mu$m)	for light velocity particles	length (cm)
Goretex	430	28	6.6
Lumirror	190	24	6.8
Tyvek (double layers)	260	23	6.2
Tetratex	200	22	6.1
Millipore	160	20	-
Tyvek (single)	130	17	5.2
Al mylar	100	14	4.8
Black paper	210	5	2.0