Evaluation of different photodetector types for an ILC polarimeter

At the International Linear Collider (ILC) (ILC Reference Design Report, 2007 〈 http://www.linearcollider.org/rdr 〉 [2]) the polarization of the electron and positron beams needs to be measured with as yet unequaled precision of Δ P / P ≈ 0.25 % . The key element of the polarimeter will be the precise detection of Cherenkov light from Compton scattered electrons. The poster 〈 http://ndip.in2p3.fr/ndip08/Poster/P3/66-Helebrant.pdf 〉 [1]) deals with the choice of a suitable photodetector (PD). In a recently assembled test facility various types of PDs have been checked. Results are presented with a special focus on the linearity of the device, since this is expected to be the limiting factor on the precision of the polarization measurement at the ILC.