Optical constants for the S20 photocathode, and their application to increasing photomultiplier quantum efficiency

The complex refractive index of an S20 multi-alkali photocathode has been d etermined for the first time over the whole visible and near infrared wavel ength range using reflectance measurements. The real part n is around 3 and shows little variation with wavelength. The imaginary part k falls from 1. 2 in the blue towards < 0.05 at 900 nm. Both n and k exhibit a marked drop around 470 nm. The data are used to model photocathode optical properties. It is shown that, by choosing an appropriate angle of incidence, the absorp tance, and thus photomultiplier quantum efficiency, can be increased severa l times compared to normal incidence. (C) 2000 Elsevier Science B.V. All ri ghts reserved.