RICH index of refraction
This photo shows the interferometer, and the display of the photodiode readout.
The interferometer consists of a laser source (He-Ne laser with a wave length
of 632.5 nm). Two pin holes 1/4 of an inch apart were made on a thin cooper
foil. The two beams are then sent through parallel paths in the aluminum block with the gas connections seen before the big lens. One of these paths is kept at at good vacuum (less than a milliTorr). The second path is initially at vacuum, but to perform the measurement of the index of refraction, gas is brought
from the detector vessel slowly till it reaches the same pressure as the detector.
The interferometer measures the difference in optical paths ( ln ), it counts
fringes that sweep the photodiode as gas is brought into one of the paths.
After the aluminum block with the two paths mentioned above, the two beams of light are focused back into one point by the big lens. In order to reduce the total length of the system, two flat mirrors bend the light towards the photodiode. A lens of short focal length is used to form the interference pattern onto the
active region of a photodiode. The voltage output of the diode was read with the Keithley KNM-DCV31 and two of the fringes can be seen displayed on the monitor.
A preliminary measurement with air produced 43 fringes. And using the gas from the RICH the system counted 321 fringes.
The relation between index of refraction and number of fringes is:
L * (n-1) = Lambda * N
where L is the length of the aluminum block with the two paths, Lambda is the wave length of the light, and N the number of fringes.
The measurement for air translates into n-1 = 271.9 X 10-6
And the measurement for our mixture of perfluorocarbons at 18.5 psia translates into:
n - 1 = 2029 X 10-6
The temperature during these measurements was 18.5 degrees centigrade.
(Created 9-May-2000)