text: For example, a commonly used reaction to profile hydrogen with an energetic N ion beam is
  :N + H → C + α + γ (4.43 MeV) 
  with a sharp resonance in the reaction cross section at 6.385 MeV of only 1.8 keV.  Since the incident N ion loses energy along its trajectory in the material it must have an energy higher than the resonance energy to induce the nuclear reaction with hydrogen nuclei deeper in the target.  
  This reaction is usually written H(N,αγ)C.  It is inelastic because the Q-value is not zero (in this case it is 4.965 MeV).  Rutherford backscattering (RBS) reactions are elastic (Q = 0),  and the interaction (scattering) cross-section σ given by the famous formula derived by Lord Rutherford in 1911.  But non-Rutherford cross-sections (so-called EBS,  elastic backscattering spectrometry) can also be resonant:  for example,  the O(α,α)O reaction has a strong and very useful resonance at 3038.1 ± 1.3 keV.
  In the H(N,αγ)C reaction (or indeed the N(p,αγ)C inverse reaction),  the energetic emitted γ ray is characteristic of the reaction and the number that are detected at any incident energy is proportional to the hydrogen concentration at the respective depth in the sample. Due to the narrow peak in the reaction cross section primarily ions of the resonance energy undergo a nuclear reaction. Thus, information on the hydrogen distribution can be straight forward obtained by varying the N incident beam energy.
  Hydrogen is an element inaccessible to Rutherford backscattering spectrometry since nothing can backscatter from H (since all atoms are heavier than hydrogen!).  But it is often analysed by elastic recoil detection.