RBS Theory: Straggling

After passing through a target with a finite thickness the probing He atoms will not only lose energy, but they will also no longer be monoenergetic. Instead, they will have distribution about the energy which is predicted by energy loss calculations. The process through which a probing He atoms loses energy involves a large number of interactions with individual atoms along its trajectory through the sample. This causes statistical fluctuations in the energy loss process which, along with the inherent limits of energy resolution in the RBS detection system, limit the energy resolution which can be achieved for atoms backscattered from larger sample depths. Since the accurate determination of both depth and mass dependent on the energy of the backscattered particle, straggling limit depth and mass resolution for buried features.

As noted in the Stopping Power section, the majority of energy loss occurs through interactions with electrons. As a result, energy straggling increases with the atomic number of the target element since atomic number also reflects the number of electrons present. Energy straggling causes the low energy edge of peaks to slope. For thick, high Z materials this effect can be quite pronounced. If energy straggling is not taken into account, then sloping edges of peaks can be misinterpreted as intermixing between two layers. The accuracy of depth resolution depends on how accurately the contribution of straggling can be calculated.

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