Another word on Dow Corning DC-200. If you have an opportunity to score some with the intent of using it as a PMT-scintillator index matcher, beware that it is available in a wide range of viscosities. For this application, you want the most viscous variety Dow makes (100,000 centistokes). It's the consistency of thick honey and water-clear. A small quantity is sufficient, as only a few drops are really needed to do the job. An acceptable substitute might be turntable tonearm damping fluid, which is very viscous clear silicone fluid. This may possibly still be obtainable from electronic repair supply houses or from places that cater to audiophiles. One might also approach the local audio repair shop with hat in hand. If they can't sell or spare you a few drops, they may at least be able to tell you where you can get it. Why do any index matching at all? It does make a difference. With a scintillator-PMT system, you potentially have 2 air - dielectric interfaces, which means two places where photons can be reflected back away from the PMT. If you have a perfectly flat mirror finish on the scintillator and the PMT face,this may not be much of a problem, but most of the plastic parts I have seen are not well polished, so there will be air spaces. The best situation with regards to transmission is normal incidence, where the photons approach the interface straight on. Other angles result in more reflection. The formula for reflection at normal incidence is: R=((n2-n1)/(n2+n1))^2 R = reflectivity, n2 and n1 are the refractive indices of the 2 materials. 1-R equals transmission. For two interfaces, the two transmissions are multiplied together. Refractive index for BC-400 (typ scint) is 1.58, for Schott glass PMT window, n = 1.52, air has n=1. This combination of scint-air-PMT gives a transmission of 0.949 X 0.957 = 0.908. If you substitute DC-200 (n=1.4) for air, transmission goes up to 0.996, quite an improvement.....
Created on Thursday, April 19, 2001 1:45 AM EDT by Richard L. Hester