Heavy Ion Elastic Recoil Detection Analyses at low Energies

M. Mallepell, C. Kottler, M. Döbeli

In the course of a Ph.D thesis a new spectrometer for Heavy Ion Elastic Recoil Detection Analysis (HI-ERDA) was developed. The potential of this technique at very low energies was explored and the new instrument has been applied successfully to solve a number of analytical questions.

Background and motivation:

ERDA is a method that allows for quantitative determination of surface depth profiles. The idea of the method is to detect and identify the recoils elastically scattered from the sample surface by an incident heavy ion beam. Even though ERDA has become a well established method in the last few decades, its availability is restricted to a small number of accelerator facilities, because detector systems often require relatively high energetic projectiles (E/M ≥ 1 MeV/amu. The motivation of this project was to design a detector set-up that provides the possibility to perform ERDA measurements with low-energy projectiles (E/M ≤ 0.1 MeV/amu).

The new set-up consists of a time-of-flight (ToF) spectrometer in front of a gas ionization chamber (GIC). Combined measurement of energy and ToF allows for recoil mass separation and thus element identification (ToF-ERDA).

One of the key improvements consists in the novel type of entrance window used for the ionization chamber. The detector is equipped with a very thin silicon nitride membrane (130 nm thick) instead of a conventionally used plastic foil (thickness ≈ 600 nm). Recoils entering the detector volume through the membrane suffer much lower energy loss and energy loss straggling compared to plastic windows. This dramatically enhances the detector performance. The second important feature idea is related to the ToF spectrometer. The flight time of scattered ions is usually measured between two thin carbon foils. The energy loss straggling of the recoils in the first detector foil however limits the resolution of the spectrometer. Therefore, a very thin diamond-like carbon foil (DLC) (thickness: 0.5 μg/cm2) is used.

It was demonstrated that all light elements from H to K can be discriminated and identified and thus, quantitative near surface depth profiling is achieved even with very low energetic projectiles (e.g. 12 MeV 127-I). A depth resolution of 2 nm and sensitivities for trace element analysis down to 10^-3 can be obtained. As one of the examples for the applicability of the method, the role of oxygen in the growth process of Perovskite-structures was studied using isotopically enriched 18-O gas during growth. Individual depth profiles of both isotopes 16-O and 18-O could easily be obtained.

Contact:
M. Döbeli

Ph.D thesis of Christian Kottler