If irradiated with select projectile nuclei at kinetic energiesEkin, target solid thin-film chemical elements can undergo a nuclear reaction under resonance conditions for a sharply defined resonance energy. The reaction product is usually a nucleus in an excited state which immediately decays, emitting ionizing radiation. To obtain depth information the initial kinetic energy of the projectile nucleus and its stopping power in the sample has to be known. To contribute to the nuclear reaction the projectile nuclei have to slow down in the sample to reach the resonance energy. Thus each initial kinetic energy corresponds to a depth in the sample where the reaction occurs.
For example, a commonly used reaction to profile hydrogen with an energetic 15Nion beam is with a sharp resonance in the reaction cross section at 6.385 MeV of only 1.8 keV. Since the incident15N 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 1H12C. It is inelastic because the Q-value is not zero. Rutherford backscattering reactions are elastic, and the interaction cross-section σ given by the famous formula derived by Lord Rutherford in 1911. But non-Rutherford cross-sections can also be resonant: for example, the 16O16O reaction has a strong and very useful resonance at 3038.1 ± 1.3 keV. In the 1H12C 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 15N incident beam energy. Hydrogen is an element inaccessible to Rutherford backscattering spectrometry since nothing can backscatter from H. But it is often analysed by elastic recoil detection.
Non-resonant NRA
NRA can also be used non-resonantly. For example, deuterium can easily be profiled with a 3He beam without changing the incident energy by using the reaction, usually written 2Hα. The energy of the fast proton detected depends on the depth of the deuterium atom in the sample.
