The QQDS magnetic spectrometer “Little John” for High Resolution Depth Profiling

The aim of the modified magnetic spectrometer “Little John” [1] is to measure concentration profiles of light elements in thin layers with sub-nanometer depth resolution by Elastic Recoil Detection Analysis (ERDA). For these measurements heavy ions from the Rossendorf 5-MV-Tandem accelerator are directed to the sample. The ejected recoil atoms are detected and energy analysed under forward angles. The depth resolution depends directly on the energy resolution of the spectrometer. High energy resolutions can be obtained using magnetic particle spectrometers, where the energy measurement is transformed into a position measurement at the focal plane.
The depth scale is provided by the stopping power of energetic heavy ions moving in matter, the available data of which assume a dynamic charge state equilibrium due to electron loss and capture along the ion trajectory. In the case of ultrathin layers the path length of the particles are too short to achieve this equilibrium. Since magnetic spectrometers separate particles with identical energy but different charge states it is necessary to consider charge state dependent stopping cross sections for quantitative data analysis. Here only very few data are available in the literature.
In this work we introduce an experimental setup at “Little John” for charge state distribution measurements of light heavy ions and present first results.

References: [1] H.J. Gils, J. Buschmann, S. Zagromski, J. Krisch and H. Rebel, Nucl. Instr. and Meth. A276 (1989), p. 151.
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