Nuclear Reaction Analysis

Nuclear reaction analysis (NRA) is a nuclear method in materials science to obtain concentration vs. depth distributions for certain target chemical elements in a solid thin film.

If irradiated with select projectile nuclei at kinetic energies E_kin these target 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 (which has to exceed the resonance energy) and its stopping power (energy loss per distance traveled) 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 (the higher the energy, the deeper the reaction).

For example, a commonly used reaction to profile hydrogen is

15N + 1H → 12C + α + γ (4.965MeV)

with a resonance at 6.385 MeV. The energetic emitted γ ray is characteristic of the reaction and the number that are detected at any incident energy is proportional to the concentration at the respective depth of hydrogen in the sample. The H concentration profile is then obtained by scanning the 15N incident beam energy. Hydrogen is an element inaccessible to RBS due to its low mass, although it is often analysed by elastic recoil detection.

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

3He + D = α + p + 18.353 MeV

reaction. The energy of the fast proton detected depends on the depth of the deuterium atom in the sample.