$K{\alpha }_1$ radiation from heavy, heliumlike ions produced in relativistic collisions

Bound-state transitions in few-electron, heavy ions following radiative electron capture are studied within the framework of the density matrix theory and the multiconfiguration Dirac-Fock approach. Special attention is paid to the $K{\alpha }_1$ $(1s_{1∕2}2p_{3∕2}{}^{1,3}P_{J=1,2}\to 1s_{1∕2}^2{}^{1}S_{J=0})$ radiative decay of heliumlike uranium ${\mathrm{U}}^{90+}$ projectiles. This decay has recently been observed at the GSI facility in Darmstadt, giving rise to a surprisingly isotropic angular distribution, which is inconsistent with previous experiments and calculations based on a “one-particle” model. We show that the unexpected isotropy essentially results from the mutual cancellation of the angular distributions of the ${}^{1}P_1\to {}^{1}S_0$ electric dipole and ${}^{3}P_2\to {}^{1}S_0$ magnetic quadrupole transitions, both of which contribute to the $K{\alpha }_1$ radiation. Detailed computations on the anisotropy of the $K{\alpha }_1$ radiation have been carried out for a wide range of projectile energies and are compared to available experimental data.

Figure 1

Lyman-${\alpha }_1$ $(2p_{3∕2}\to 1s_{1∕2})$ and $K{\alpha }_1$ $({}^{1,3}P_{1,2}\to 1{}^{1}S_0)$ decay in the hydrogen- and helium-like uranium ions.

Figure 2

Anisotropy parameters of the Lyman-${\alpha }_1$ (dashed line) and $K{\alpha }_1$ (solid line) radiation following REC into excited states of (initially) bare and hydrogenlike uranium ions, correspondingly. In addition to the theoretical calculations for the $K{\alpha }_1$ decay, we also present the anisotropy parameters (5, 6) of its fine-structure ${}^{1}P_1\to {}^{1}S_0$ (dash-dotted line) and ${}^{3}P_2\to {}^{1}S_0$ (dotted line) components. All results include the feeding transitions from the higher excited states.

Figure 3

(Color online) Angular distributions of the Lyman-${\alpha }_1$ (left panel) and $K{\alpha }_1$ (right panel) radiation following REC into excited states of (initially) bare and hydrogenlike uranium ions with energy $T_p=220\mathrm{MeV}∕u$ [12, 13]. Apart from the experimental (solid points) and theoretical (solid line) results for the $K{\alpha }_1$ emission we also present the angular distributions of its individual ${}^{1}P_1\to {}^{1}S_0$ (dashed line) and ${}^{3}P_2\to {}^{1}S_0$ (dotted line) components, multiplied by a factor 2. All data are normalized with respect to the intensity of the isotropic Lyman-${\alpha }_2$ radiation following the capture into bare ${\mathrm{U}}^{92+}$ ions.

Figure 4

Anisotropy parameters of the $K{\alpha }_1$ radiation following REC into the $1s_{1∕2}2p_{3∕2}{}^{1,3}P_{J=1,2}$ levels of the (finally) heliumlike xenon ${\mathrm{Xe}}^{52+}$ (dotted line), gold ${\mathrm{Au}}^{77+}$ (dashed line), and uranium ${\mathrm{U}}^{90+}$ (solid line) ions. All results include the feeding transitions from the higher excited states.