Asymmetry of bremsstrahlung from transversely polarized electrons

Abstract

By using an electron-photon coincidence method the photon emission asymmetry in the elementary process of bremsstrahlung from transversely polarized electrons was measured. For an electron beam of 300 keV incident on a gold target the asymmetry for spin up and down of the primary beam was measured for fixed directions of the outgoing electrons and photons as a function of the energy split. Asymmetries up to 30% were found. The measurements are a proper test for theories going beyond the first Born approximation. : 34.80.-i; 34.90. #q Bremsstrahlung emitted by a beam of transversely polarized electrons in the Coulomb field of an atomic nucleus shows a left-right asymmetry in the angular distribution when the spin direction is perpendicular to the photon emission plane defined by the momenta of the incoming electron and the emitted photon. PACSDisregarding the decelerated outgoing electrons such left-right photon emission asymmetries have been measured by several groups [1][2][3]. However, since in all these experiments only the emitted photons have been observed, the results are necessarily averaged over all electron scattering angles. Therefore the test of theoretical predictions is not as strong as it could be. To get detailed information on the elementary collision process [4] Mergl et al.[5] performed a coincidence measurement between outgoing electrons and photons emitted by a transversely polarized beam. The authors measured the photon emission asymmetry for spin up and down of the primary beam as a function of the photon emission angle for fixed directions of the outgoing electrons and for fixed energy split between photons and electrons.In the present paper we report on a measurement of the photon emission asymmetry for spin up and down of the primary beam as a function of the energy split between outgoing electrons and photons for fixed directions of the outgoing electrons and photons. That means the goal of the measurement is the same as in our first experiment [5], but now the theory is checked by varying different variables. For this a two-parameter energy-sharing coincidence experiment was built up.A sketch of the experimental arrangement is shown in Fig. 1. The source of the polarized electron beam (the same as in [5] and described in detail elsewhere [6]) used the photoemission of electrons from a GaAsP crystal irradiated by circularly polarized light of a helium-neon laser. After being deflected by a 90°cylindrical deflector, the extracted electrons are transversely polarized. The spin flip of the electron beam can be easily achieved by reversing the helicity of the laser light. The source is installed in a high voltage terminal of a 300-kV accelerator tube and produces a continuous transversely polarized beam with a polarization degree in the range of 35% to 40%. The degree of spin polarization was measured by a Mott analyzer put into the beam line in front of the entrance of the scattering chamber.In the Mott analyzer the electrons scattered through 120°by a gold foil were detected by a pair o...

Photon polarization and spin asymmetry in the elementary process of bremsstrahlung

“…Measurements of the spin asymmetry, C 200 , as a function of the kinetic energy of the outgoing electrons for fixed angles of the outgoing electrons and the emitted photons were performed by Geisenhofer and Nakel (1996).…”

Asymmetry of bremsstrahlung from polarized electrons

“…The accuracy of the present calculations can be tested by comparing them with the experimental values obtained by Nakel and coworkers [7,8] who determined the bremsstrahlung asymmetry by means of coincidence measurements. In Fig.…”

“…More comparisons of the present theory with experimental values are presented in the accompanying paper [8]. …”

“…However, it turns out that the results of Johnson and Rozics [2] are reproduced fairly well for Z"1 and, moreover, that the initially linear dependence on Z changes rapidly with increasing atomic numbers. The accuracy of the present results can be checked by comparison with recent experimental determinations of the bremsstrahlung asymmetry [7,8].…”

On the Born limit of relativistic electron and positron bremsstrahlung in the field of heavy nuclei

“…The presence of interference effects again demonstrates the enhanced sensitivity of the triply differential quantities as compared to the doubly differential ones. Actually, such cross section structures were already observed (but not interpreted) in early experiments on lighter target atoms [40], while for an Au target (at 0.3 MeV), only the onset of an increase of the cross section with frequency could be measured [41] and confirmed within the DW theory [19,42].…”