A precision Lamb-shift polarimeter for the polarized gas target at ANKE/COSY

The Lamb-shift polarimeter [1], described here, allows the polarization measurement of a beam of hydrogen (deuterium) atoms, or of a slow proton (deuteron) beam (500 - 2000 eV) with an absolute precision better than 1% within a few s. For a hydrogen beam of 3.10(16) atoms/s, 3.10(6) photons/s are registered in a photomultiplier. The signal-to-background ratio in the Lyman-alpha spectrum is excellent, thus beam intensities of one to two orders of magnitude less would still be sufficient to carry out a precise measurement. Therefore, it will be possible to extract a few percent of the atoms from a storage cell to measure the polarization in the cell. In order to obtain absolute nuclear polarization values the polarization derived from the Lyman-alpha spectrum has to be corrected by a number of correction factors calculated with high precision from the behaviour of different components of the polarimeter.