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Index of Refraction Measurements at Syracuse

 

During the design phase of the BTeV RICH, we were informed that our vendor of C4F12 (3M) was going to stop its production because it was produced as a by-product of a gas, which was being phased out due to environmental considerations. This gas has been widely used in Cherenkov detectors because it has the highest index of refraction among the gases which are transparent to Cherenkov photons.

Other vendors carried the gas, except, it would cost ~5X as much!

We decided to embark on a mini-R&D effort to find a new suitable gas. Two other gases were considered: C4F8  and C4F8O, neither of which have been used in Cherenkov detectors up to this point.

The precise measurement of the index of refraction of a gas, and its dispersion (dependence on wavelength) requires measurement accuracy of ~1 part in 105. We designed and built a small Michelson interferometer with two small gas containment cells.

This is a cartoon of the setup. Laser light is incident on the Michelson interferometer from left to right. The beams are split by a beam splitter. Light in each arm of the interferometer passes through a cell, one is the control cell (held near P=0) and the other is filled with gas under test. No this is not LIGO! The cells are pumped down to ~0.01 PSIA. We then slowly leak gas into the sample cell, which increases the optical path length of the light in that arm of the spectrometer. The interference fringes alternate sinusoidally due to the changing phase between the beams in the two arms. The central fringe is focused onto a screen containing a photodiode and a Schmitt trigger, which puts out a 5V signal when the light intensity exceeds a minimum threshold. These pulses are then counted. The total number, N, of light-dark transitions is related to the index of refraction, n, as follows:

n - 1 = N l / DL

where l is the wavelength of the light and DL is the width of path length traversed in the gas cell.

 

 

 

A picture of the setup.

 
A closer look (laser beam's eye view) at the setup is shown here. The screen wheer the interference patterns is projected is to the left. The 0th order interference fringe is centers on a small hole in the screen, behind which is the photodiode and Schmitt trigger.

 

 

 Measurements at 633 nm (red), 543 nm (Green) and 412 (Blue) were taken for C4F8 , C4F8O and C4F10.

 

Here are the resulting measurements for each gas in units of (n-1)x106, along with predictions for C4F10 extrapolated from the UV. base on these measurements, and material compatibility studies, we decided to use C4F8O for the BTeV RICH.