Crystal-GRID: Study of orientation-dependent slowing-down in single-crystalline ZnS

Crystal-GRID is a direct method to study orientation-dependent slowing down of recoil atoms in mono-crystals at intermediate recoil velocities of up to 1 Angstrom/fs. The method is based on the excitation of nuclei by neutron capture and their deexcitation by successive photon emissions. A first photon emission leads to a recoil of a few 100 eV. Special deexcitation channels are considered where the recoiling nucleus is not yet at rest when the second photon is emitted. The Doppler shifted energy of this second gamma quantum is measured. When observing many deexcitations a Doppler broadened gamma line shape is obtained.

The slowing down of recoiling atoms is anisotropic due to the regular and discrete positions of the collision partners in a lattice. The use of single-crystalline targets allows to observe this anisotropy via a pronounced structure of the Doppler broadened gamma line. As the Doppler shift only depends on the projection of recoil velocities on the axis of observation, different line shapes can be obtained when performing various measurements with crystals aligned differently.

Measurements have been carried out with the high-precision gamma spectrometers GAMS 4 and GAMS 5 at the ILL. Doppler broadened line shapes could be obtained using ZnS single crystals in three orientations. They are compared to predictions from molecular dynamics (MD) simulations. Thereby, parameters of classical interatomic potentials describing the slowing down have been extracted.

Detailed results will be presented for ZnS, showing that the universal screened Coulomb potentials (ZBL, KrC) have to be modified in the examined energy region. Using the modified potentials, effects such as channelling and blocking have been studied by computer simulations. It will be discussed to which extent the potential corrections are important for simulations of ion beam techniques.