Another important property of the gas is its optical
transparency, especially in the short wavelength region.
The reason is that the number of Cerenkov photons
behaves like
, where
is the photon wavelength.
The transparency of various gases is shown in Fig. 4. Carbofluoride data are extracted from [9], Freon data from [10].
Obviously, the transparency of 
is much better
than that of Freon-12. The main gain is at
.
At shorter wavelengths, the mirror reflectivity is low.
This wavelength region therefore adds little to the
photon detection efficiency.
gas also has good transparency
and a large index of refraction, however, its
boiling point is as high as
,
which makes it difficult to use in CLAS.
Figure 4:
Wavelength dependence of gas transparencies.
Black circles show the estimated mirror reflectivity
for aluminized mylar coated with
.