In RBS method, the cross section for elastic scattering of the ion on an target atom rises with Z1 of the target atom and Z2 of the ion. Subsequently, the heavier ions you use, the better sensivity (accuracy) you get. Why is then helium in most common use? Wouldn't it better with N or C ions?
It can be done with heavier ions, but (logically) you lose any information on target atoms that are lighter than the projectile. Also, ion sources get more complicated as the projectile ions can have more and more different charge states.
There are several reasons for the use of 1.0-2.0 MeV He ions:
a) Quantitative analysis relies on knowledge of the scattering cross-section which is close to the Rutherford value for He ions in the energy range 1.0-2.0 MeV. Heavier ions lead to screening effects that modify the scattering cross-section.
b) Depth scales rely on ion energy loss. This is reasonably well established for H and He but less so for heavier ions.
c) Lighter ions can be used to analyse greater depths than heavier ions of the same energy.
d) The energy resolution and life-time of solid-state (surface barrier) detectors is better for lighter ions than heavier ions.
e) Lighter ions create less radiation damage in the sample under investigation
In general, 1.0-2.0 MeV He ions represents the best compromise of the above factors, and of mass-resolution for intermediate mass elements.
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