Search for radiative double electron capture (RDEC) by fully-stripped ions in gas targets*

Radiative double electron capture (RDEC) by fully-stripped ions in collisions is a fundamental atomic process that can be used to study the electron-electron interaction. This is a one step process in which a single x-ray is emitted in association with the capture of two electrons to a bare projectile from the target. Only five attempts have been made since 1995 to observe RDEC and only one experiment (2.38 MeV/u O$^{8+}$ collision on a C foil) successfully reported positive results for the observation of the process.\footnote{A. Simon et al., PRL \textbf{104}, 123001 (2010).} We have started new measurements at WMU for gas targets (He, Ne and N$_{2})$ as well as a thin-foil C target with fully-stripped fluorine to compare the results with previous experiments and theoretical calculations. Gas targets are contaminant free compared to solid targets and the measurements have been started for gaseous N$_{2}$. This experiment is difficult and requires long counting times due to the relatively small RDEC cross compared to that for the single electron process of radiative electron capture.