No abstract

Fully relativistic treatment of the electron-atom and positron-atom bremsstrahlung is reported. The calculation is based on the partial-wave expansion of the Dirac scattering states in an external atomic field. A comparison of the electron and positron bremsstrahlung is presented for the single and double differential cross sections and the Stokes parameters of the emitted photon. It is demonstrated that the electron-positron symmetry of the bremsstrahlung spectra, which is nearly exact in the nonrelativistic regime, is to a large extent removed by the relativistic effects. 78.70.En When a charged particle traverses an atomic field, a part of its energy may be converted into radiation. This is the atomic bremsstrahlung, one of the fundamental collision processes. It is one of the important mechanisms of the energy loss in hot plasmas and in particle beams traversing thick targets. The process of the electron-atom bremsstrahlung has been extensively studied in the literature. The single and double differential cross sections of this process were tabulated [1, 2] and the polarization properties of the emitted radiation were calculated [3][4][5][6].Much less is known on the positron-atom bremsstrahlung.Several reported investigations of this process [7-9] were dealing primarily with the bremsstrahlung energy loss. It was shown that, for high energies of the incident and final projectile, the cross sections of the electron and positron bremsstrahlung are very much similar. However, when the energy of the final projectile decreases, the cross section of the positron bremsstrahlung becomes increasingly suppressed as compared to the electron one, because of the strong Coulomb repulsion between the positron and nucleus at short distances. The ratio of the positron-to-electron bremsstrahlung stopping power was the main subject of those early works.To the best of our knowledge, the angular dependence of the cross section and the polarization properties of the relativistic positron-atom bremsstrahlung have never been studied. They are becoming subjects of experimental interest today, with the advent of techniques for the production of highly polarized positrons beams [10] and the detection of polarization correlations in the bremsstrahlung radiation [11,12].In the present investigation, we make a comparative study of the positron-and electron-atom bremsstrahlung, by analyzing the double differential cross section and the polarization correlations between the incident projectile and the emitted photon. The calculation is performed within the fully relativistic approach based on the partialwave representation of the Dirac continuum states with a fixed value of the asymptotic momentum. This work extends our previous calculations of the electron-atom bremsstrahlung [5,13].

We report the observation of an interference between the electric dipole (E1) and the magnetic quadrupole (M2) amplitudes for the linear polarization of the Ly-α1 (2p3/2→1s1/2) radiation of hydrogenlike uranium. This multipole mixing arises from the coupling of the ion to different multipole components of the radiation field. Our observation indicates a significant depolarization of the Ly-α1 radiation due to the E1-M2 amplitude mixing. It proves that a combined measurement of the linear polarization and of the angular distribution enables a very precise determination of the ratio of the E1 and the M2 transition amplitudes and the corresponding transition rates without any assumptions concerning the population mechanism for the 2p3/2 state.