RADIATIVE ELECTRON CAPTURE BY FAST HIGHLY STRIPPED HEAVY IONS CHANNELED IN A THIN CRYSTAL

Interaction of moving ions with single crystals is known to be very sensitive to the orientation of the incident beam with respect to the crystalline directions of the target. We have shown that channeling conditions strongly modify the slowing down and the charge exchange processes of high energy heavy ions. The reason is that channeled particles are prevented from approaching the target atoms, and then can interact only with loosely bound target electrons. This results not only in drastically reducing the electron density experienced by them, which lowers energy loss and electron Ioss, but also in inhibiting the radiationless capture of bound electrons, since most of the available electrons are quasi-free valence electrons. In an experiment where high energy Xeq+ ions, with q = 52, 53, 54, are directed onto a thin Si single crystal we observe that, whereas unchanneled ions reach charge equilibrium very rapidly, most channeled ions keep their incident charge state unchanged all along their passage through the crystal. On one hand the loss of their electron(s) (q = 52, 53) is impossible. On the other hand the only way that is left to them to capture quasi-free electrons is the radiative electron capture (REC). This process does occurs also for unchanneled ions but is quite difficult to observe, particularly because of the overwhelming non radiative Mechanical Electron Capture(MEC). REC photons have been observed in channeling conditions and correspond to electron capture into the K, L and M shells of the projectiles. The shape of the REC photon lines reflects the momentum distribution of the electron encountered by channeled ions. Channeling offers a unique opportunity to study the radiative electron capture since it allows ions of well defined charge state to travel through a dense quasi-free electron target. This opportunity could be extended to other processes involving high energy heavy ions