First Measurement of the Linear Polarization of Radiative Electron Capture Transitions

Abstract

For radiative electron capture into the K shell of bare uranium ions, a study of the polarization properties has been performed. For this purpose a position sensitive germanium detector has been used as an efficient Compton polarimeter. This enabled us to measure the degree of linear polarization by analyzing Compton scattering inside the detector and to determine the orientation of the polarization plane. Depending on the observation angle and the beam energy used, the radiation is found to be linearly polarized by up to 80%. In all cases studied, the plane of polarization coincides with the collision plane. The results will be discussed in the context of rigorous relativistic calculations, showing that relativistic effects tend to lead to a depolarization of the radiation emitted. DOI: 10.1103/PhysRevLett.97.223202 PACS numbers: 34.80.Lx, 32.30.Rj, 32.80.Cy, 32.80.Fb Radiative capture of free or quasifree electrons (REC) into high-Z ions has been found to provide a wealth of information on the electron-photon interaction in the presence of strong fields [1,2]. Most important, today, REC is an established tool for studying the time-reversed process, the elementary photoeffect, for one-and few-electron ions at high Z which can otherwise not be addressed in experiments [2]. As a consequence, most properties of the REC were the subject of detailed experimental and theoretical studies (see [2 -5] and references therein). However, a very important aspect of linear polarization so far has not been the subject of experimental investigations although the results of the angular-differential studies already indicate that REC may exhibit distinctive polarization features. Even for the time-reversed process and neutral heavy elements only very few experimental data are available [6,7]. There, the results are strongly affected by electron scattering in the target and except at very high photon energies (above 1 MeV) an unambiguous interpretation of the data is hampered by distortion effects caused by the solid targets used [7]. In contrast, these effects are completely absent in REC studies. Moreover, the polarization features of K-REC radiation have recently attracted particular attention because of its predicted sensitivity to a possible spin polarization of the particles involved in the collision (electrons or ions) [8]. As a consequence, the detection of the linear polarization of K-REC might be applied as an important tool for the control and diagnostics of the degree of the spin polarization of heavy ion beams confined in storage rings. Currently, such a technique is much needed for experiments aiming on a detection of parity nonconservation effects in highly charged ions and for the search of the electric dipole moment of heavy nuclei as proposed for a test of the standard model [9][10][11]. In this context, we like to emphasize that REC into the K shell is the most important charge exchange channel for heavy ions in collisions with light target atoms and, therefore, represents an intense source of radiation.In ...

Level sequence and splitting identification of closely spaced energy levels by angle-resolved analysis of fluorescence light

“…Alternatively, we may consider and analyze also the linear polarization of this fluorescence light, which can be measured nowadays with quite high accuracy either by means of solid-state Compton polarimeters [45,46] or, even more precisely, with Bragg crystal polarimeters [47][48][49].…”

Fine-structure effects on the polarization of Kα1 radiation from heavy, highly-charged ions

“…At the GSI storage ring in Darmstadt, for example, such a radiative electron capture (REC) has been studied for relativistic collisions of high-Z projectile ions with atomic targets [4,5]. For these collisions, detailed measurements have been performed for the REC total and angle-differential cross sections as well as for the polarization properties of the recombination radiation [6]. In addition, most recent studies have also focused on the subsequent radiative decay which follows the REC into excited ionic states.…”

Photon polarization in the two-photon decay of heavy hydrogen-like ions

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