Quality of the aerogels

All the aerogel tiles thus produced have been checked for optical transparency, transmittance of unscattered light, refractive index, dimension, etc. Fig. shows typical transmittance curves obtained by a photo-spectrometer for aerogels of four different refractive indices. The $n$ = 1.028 aerogels have better transmittance than the others. Their average transmission length ($\Lambda$) at 400 nm is 46 mm, while the others are around 25 mm. Here $\Lambda$ is defined by the function: $T = T_0 ~exp(- d/\Lambda)$, where $T_0$ and $T$ are the incident and transmitted light intensities, respectively, and d is the thickness of an aerogel tile. These aerogels were produced from the alcogel which was prepared by using methyl alcohol as a solution.

Figure: Light transmittance spectra for silica aerogels (thickness = 23.3 mm) of $n$ = 1.01, 1.015, 1.02 and 1.028. The silica aerogels of $n$ = 1.028 prepared by using methanol as the preparation solvent shows the best transmittance data.

The refractive indices are well controlled as $\Delta n/(n - 1) \sim$ 3% for all the produced aerogel tiles, which is essentially the same as the measurement error of the refractive index determined by measuring a deflection angle of laser light (He-Ne: 543.5 nm) at a corner of each aerogel tile.
We carried out a test to ensure the radiation hardness of aerogels by placing aerogel samples ($n$ = 1.012, 1.018 and 1.028) in high-intensity $\gamma$-rays from a $^{60}$Co source [48]. Transparencies and refractive indices of aerogels were measured up to 9.8 Mrad, which corresponds to more than 10 years of running at KEKB. Neither deterioration on the transparency nor change in the refractive indices were observed within the errors of measurements after the irradiation. Measurement accuracies were 0.8 % for the transparency and $<$0.0006 for the refractive index. Deterioration in transparency and changes of refractive index after 9.8 Mrad $\gamma$-ray irradiation were observed to be less than 1.3% and 0.001 at 90% confidence level, respectively.