Beam-induced resonant depolarization effects in a polarized atomic hydrogen gas target at HERA

In the HERMES experiment at HERA deep inelastic scattering of polarized positrons from internal polarized gas targets is used to study the spin structure of the nucleons. For the hydrogen target, a strong magnetic field parallel to the positron beam direction is used to maintain the polarization of the atoms inside the target cell. Depolarization of the atoms due to time dependent magnetic fields resulting from the time structure of the high energy positron beam have been studied using a Breit-Rabi type polarimeter. Resonant effects are observed as function of the magnetic holding field. They are in agreement with model expectations. At the working point at 335 mT in between two resonances depolarization is negligible.