ISHIHARA Toyoyuki Univ. of Tsukuba, Tandem Accelerator Center, Associate Professor, 物理学系(加速器センター), 助教授 (10013385)
SHIMA Kunihiro Univ. of Tsukuba, Tandem Accelerator Center, Lecturer, 物理工学系(加速器センター), 講師 (70087964)
|Budget Amount *help
¥2,900,000 (Direct Cost : ¥2,900,000)
Fiscal Year 1992 : ¥600,000 (Direct Cost : ¥600,000)
Fiscal Year 1991 : ¥2,300,000 (Direct Cost : ¥2,300,000)
When fast ions are incident in an axial direction of target crystal, the ions are deflected away from the aligned atoms as a result of ion-atom repulsive potential. Such shadowing processes can be well observed by using high-energy (keV) secondary electrons induced by MeV/u ions. If the ion has electrons in the inner-shell, the deflection angle from the aligned atoms should be smaller than for the fully-stripped case since the repulsive interaction potential is reduced. This causes a reduced shadowing effect i.e., an increase in the ratio of channeling to random electron yield, compared with that for fully stripped light ions of equal velocity. By using this effect, we have determined effective nuclear charges of fast heavy ions running in the crystal.
The experiments have been carried out using tandem accelerators at the University of Tsukuba, and at Japan Atomic Energy Research Institute (Takasaki). For Si crystals, we have collected data for 2.5-MeV/u ions of H, B, C, O, Si, S, and Cl, and for 3.5-MeV/u ions of H, B, C, and Si. We have also measured GaAs and MgO crystals for several ions of equal velocity.
We have found that even at a depth of about 100 A^^ﾟ most of MeV/u light ions maintain their initial charge states, while heavier ions quickly capture target electrons very close to the surface. For example, the charge states of 2.5-MeV/u B^<5+>, C^<4+>, C^<6+>, O^<5+>, and O^<8+> incident in the Si <110> or <100> directions remained almost unchanged at a depth of about 100 A^^ﾟ. However, heavier Si^<5+>, Si^<13+>, S^<5+>, S^<13+>, Cl^<8+>, or Cl^<15+> exhibit a screening effect equivalent to an electron capture of several electrons at the same depth.
Such knowledge on the charge states of fast ions in solid have been obtained for the first time by the Tsukuba-Takasaki joint research. Further experiments covering wide ranges of ion energy and ion species are still under way.