Work Package 2 (WP2): Atomic Physics

Projectile Electron Spectroscopy (APAPES)

Objectives and intended results

Use the technique of high-resolution projectile Auger electron spectroscopy for performing double differential cross section measurements in fast ion-atom collisions. By exploiting the existing ZAPS experimental setup (Zero-degree Auger Projectile Spectroscopy), that had been developed during the research programme “Thales” (NSRF 2007-2014, http://apapes.physics.uoc.gr/), a series of measurements utilizing helium-like ion beams, pre-excited in the 1s2s ³S state will be performed. More specifically, the helium-like ion beams of Li⁺, B³⁺, C⁴⁺, N⁵⁺, O⁶⁺ and F⁷⁺ at collision energies of 0.2-1.5 ΜeV/u will be used. These beams will be delivered in variable mixtures of the 1s² ¹S ground and 1s2s ³S pre-excited states by the new upgraded post-strippers installed at the high-energy sector of the Tandem accelerator Thus, projectile K-Auger electron spectra, obtained in collisions of the above ion beams with gas targets, will be measured. In this way, the process of electron transfer to the 1s2s ³S pre-excited ion state, resulting in the production of 1s2l2l’ lithium-like ion states, such as the 1s2s2p ⁴P and 1s2s2p ²P, can be studied. The population mechanisms of the 1s2s2p ⁴P and 1s2s2p ²P states are of particular interest for the determination of the ratio of their cross-section production, R_m=σ(⁴P)/σ(²P). The proposed isoelectronic study and determination of the ratio R_m is expected to decide upon the role of the two suggested population mechanisms of the 1s2s2p ⁴P and 1s2s2p ²P states and to clarify the existing significant deviations from the theory.
Upgrade the experimental setup by integrating an existing larger collision chamber. The collision chamber that is currently in use is small and suitable only for gas targets confined in a cell. The new chamber has a much larger volume (600 mm diameter) that allows for a convenient integration of components that we plan to install. These include: (a) Gas-cell targets differentially pumped more efficiently with a larger turbomolecular pump. (b) Gas-jet targets differentially pumped that will allow us to extend our future studies to electron-atom collisions (e-2e measurements) as well as to collisions of negative ions with atoms. (γ) Crystalline targets for studying the process of channeling after installing a specialized goniometer.
Studies on the production of metastable ion beams as a function of the terminal gas stripper pressure, the foil stripper, the collision energy and the ion in use. These metastable beams offer access, through collisions with gas targets, to excited atomic states which are not accessible to ion beams at the ground state or they are accessible but through different mechanisms. Our interest if focused on: (a) Helium-like metastable beams whose content is of significant importance for the study of the process of single electron transfer in collisions of helium-like ions with gas targets. (b) Beryllium-like metastable ion beams which provide access to long-lived excited states that we intend to include in our study.
Investigation of the RCE process (Resonant Coherent Excitation) in channeling conditions in thin crystalline targets. The use of crystalline targets is a realistic application ZAPS technique. Its realization is aligned to the upgrade of the experimental setup with the use of the larger volume collision chamber and the integration of the goniometer. The RCE process has so far been studied only via x-ray spectroscopy. The ZAPS technique is expected to highlight details of the mechanism through the Auger decay of the excited ion states not allowed to decay via x-rays.
Development of lifetime measurement techniques applied to long-lived ion states. Our team has developed codes with the SIMION ion-optics code proposing a new technique for the measurement of the J-levels lifetimes of long-lived ion states. Our interest is focused on the 1s2s2p ⁴P_J state for which the accurate lifetime values of the J-levels are indispensable for the studies of single electron transfer to helium-like metastable ion beams. This development is aligned to (a) the upgrade of the collision chamber, where the proposed lifetime measurement setup will be developed and (b) the production of beryllium-like metastable ion beams.

Tasks to implement

Theoretical Calculations: The experimental results need to be supported by theoretical calculations on atomic structure, electron transfer and cascade feeding by coworkers, specialists in the field of atomic physics.
More precise determination of the metastable fraction 1s2s ³S in helium-like beams using the already installed gas and foil terminal-strippers as well as the post-stripper. Methods of data analysis developed by WP2 team inherently include the quantitative determination of the metastable fraction. This need to be determined in relation to the stripper in use and the stripping energy, and be compared with corresponding studies in the literature.
Support of the experimental studies by simulations within the SIMION ion-optics package. The individual studies to be examined are: (a) Optimization of the operation of the spectrometer cylindrical entry lens by using additional electrodes. (b) Determination of the effective detection solid angle for the parallel plate spectrometer and its correction factor. The results will be compared with existing calculations in the literature. (c) Further studies in determining the lifetimes of J-levels of the 1s2s2p ⁴P_J
Development of an electron gun to be integrated into the new collision chamber and to be used in the future for: (a) calibration measurements for the spectrometer; (b) checking the smooth operation of the whole experimental setup; and (c) the study of collisions with gas-jet targets, which is a substantial upgrade of the entire experimental setup using the new larger collision chamber.

Outputs (Deliverables)