Affiliation: | aInstitute of Physics, Graduate School of Arts and Sciences, University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan bDepartment of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan cAtomic Physics Laboratory, RIKEN, Wako, Saitama 351-0198, Japan dNational Institute of Radiological Sciences, Inage, Chiba 263-8555, Japan |
Abstract: | X-rays emitted from Ar17+, Fe24+ and Kr35+ ions of about 400 MeV/u transmitting through a thin Si crystal of about 20 μm thickness have been measured in a planar channeling condition and compared with those in a random incident condition. We have found that the X-ray yield from Ar17+ ions is larger for the channeling condition than for the random incidence, while those from Fe24+ and Kr35+ ions are rather smaller. Such tendencies are explained by considering the projectile dependences of excitation and ionization probabilities together with X-ray emission rates. A crude simulation has qualitatively reproduced these experimental results. When the crystal thickness is small, the X-ray yield is smaller in the channeling condition than in the random incident condition, because excitation is depressed. However, for thicker crystals, the X-ray yield is larger, since the survived population of projectile-bound electrons is larger due to small ionization probabilities under the channeling condition. This inversion occurs at a specific crystal thickness depending on projectile species. Whether the thickness of the used crystal is smaller or larger than the inversion thickness determines enhancement or depression of the X-ray yield in the channeling condition. |