Abstract
Within the scope of this work, the magnetic spectrograph Q3D at the
ISL (Hahn-Meitner- Institute, Berlin) was used for ERD (Elastic Recoil
Detection) analysis. The very high energy resolution of the spectrograph
(dE/E = 3 . 10-4 ) leeds to an excellent depth
resolution of the measured depth profiles of elements with Z = 1 - 16.
With this system we are able to analyse very thin layers (1 - 100 nm),
i.e. to estimate the stoichiometry and the depth profiles with a depth
resolution of approx. 1 nm (near the surface of an flat sample) of all
measured isotopes.
As a result of the high energy resolution, the ERD method is sensitive
to smallest variations of the stopping power, for example to the stopping
power variation of the projectile in the very first layer near the surface,
where the charge equilibrium was not yet been reached.
A novel method was developed to investigate this region, which makes
it possible to measure charge changing processes and energy losses in dependence
of the incoming (qi) and outgoing (qf) charge
state of the projectile. A first experiment with Ne ions at an energy of
2 MeV/u penetrating through thin carbon foils has been carried out successfully.
Analysing the measured charge state distributions f(qf) with
the Q3D for various incident qi (6+ - 10+),
aseperated by a high voltage at the target, we were able to extract the
cross sections for all relevant charge changing processes (electron capture,
ionisation, excitation and decay). Further we determined the energy loss
of the Ne ions in four carbon foils of various thickness depending on qi
and qf. Using a Monte-Carlo Simulation and the measured
energy losses for the case qi = qf, we succeeded
to eliminate the influence of those ions, which have suffered some charge
changing fluctuations before they left the foil with qf = qi.
In this way we deduced the stopping power of the pure `frozen charge states´
S(q). With these S(q)-values and using the analysis of the charge state
distributions as a function of the target thickness, the energy loss of
the projectile can be described correctly even in the non-equilibrium region. |
