Precision Polarimetry at the International Linear Collider

At the ILC, the overall scheme for a precise measurement of the beam polarisation conists in the usage of two Compton polarimeters per beam (upand downstream of the e+e− interaction point) and a polarisation measurement from annihilation data [1, 2] to set the absolute scale. The two upand downstream polarimeters are complementary, add redundancy and are used to intercalibrate the systems and reduce systematics. The aim is to reach a precision of ∆P/P = 0.25% with the polarimeters. If positron polarisation is realized, this can be improved to ∆P/P = 0.1% by calibrating the absolute polarisation scale against annihilation data. The high energy polarimeters are planned to consist of a spectrometer chicane and a Cherenkov detector to make use of the polarization dependence of Compton scattering, where the energies of the recoil electrons depend on the relative spin orientation of the original electrons and the laser photons. On the order of 10 beam electrons are scattered per laser interaction. Their energy distribution is then transformed into a spatial distribution by the magnetic chicane and measured by a multi-channel Cherenkov detector. A precise determination of the beam polarisation is achieved by measuring the asymmetry of the spectra obtained by switching the circular laser polarisation between +1 and -1. The system is designed such that the statistical uncertainty is negligible when averaging over more than 1 s. Experience from previous polarimeters shows that the limiting systematic effect is the linearity of the detector response, especially the linearity of the photodetectors (PD) and readout electronics are of utmost importance [3, 4].