Angular distribution and linear polarization of Bremsstrahlung arising from ion-atom collisions

Bremsstrahlung is one of the basic photon-matter processes occuring in collisions of electrons with atoms and ions. Ion-atom collisions in ion storage rings provide an unique tool to study this process under single-collision conditions in the domain of strong, unpertubated Coulomb fields, such as it prevails in few-electron, high-Z projectiles. Of special interest is the high energy limit of the Bremsstrahlung distribution as it is closely related by means of continuity over the ionization threshold to the radiative electron capture (REC) process [1, 2]. The latter can be seen as the inverse process of photoionization and has been the object of extensive experimental and theoretical studies during recent years [3]. In this work we aim to extend our well-established experimental technique for investigations of the REC process to precise measurements of the angular distribution and linear polarization of the Bremsstrahlung high-energy limit. The experiment was performed at the internal gasjet target of the ESR by interacting bare uranium and xenon ions with a H2 target at energies of 96.6 MeV/u and 150.25 MeV/u, respectively. The emitted x-ray photons were detected by an array of standard solid state X-ray detectors at 35◦, 60◦, 90◦, 120◦ and 150◦ and a novel-type Si(Li) Compton polarimeter at 90◦ with respect to the ion beam axis. The down-charged ions were separated from the main beam by a dipole magnet and afterwards detected by a particle detector located at the outer site of the ring. In Fig. 1 the 90◦ X-ray spectrum stemming from the collision of U with H2 is displayed. A coincidence technique between the down-charged particles and the detected X-rays allowed to discriminate the photons connected to REC events from the non-coincident Bremsstrahlung contribution. In Fig. 2 the preliminary experimental Bremsstrahlung distribution for 90◦ and 150◦ is compared to predictions by Jakubassa [4]. While the total intensity differs by about 40 % the overall shape of the experimental distribution is reproduced by theoretical data. In addition, the degree of linear polarization of the Xe → H2 Bremsstrahlung high energy limit was obtained at an observation angle of 90◦ by means of Compton polarimetry. The preliminary value of 75± 5 % is in agreement with the theory value of 77.5 %. Summarizing, in the presented experiment we studied the angular distribution as well as the linear polarization of the Bremsstrahlung high energy limit arising from colli-