Photodetector studies and prototype simulation of a Cherenkov detector for an ILC polarimeter

At the International Linear Collider (ILC) the polarisation of the electron and positron beams needs to be measured with as yet unequaled precision of ΔP/P ≈ 0.25%. The key element of the polarimeter will be the precise detection of Cherenkov light from Compton scattered electrons. The poster 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 polarisation measurement at the ILC. To study the performance of the entire Cherenkov detector design, a three channel testbox is constructed during summer 2008 and will be tested first in the laboratory (using LED a/o laser light), then in the DESY-II testbeam and finally in spring 2009 at ELSA in Bonn (higher rates and multi-electron events). The accompanying optical simulation of the testbox is done using Geant4. The photodetector response is simulated separately. First results are shown. The simulation is expected to help understand and interpret the testbeam data.