The reaction product is usually a nucleus in an excited state which immediately decays, emitting ionizing radiation.
To contribute to the nuclear reaction the projectile nuclei have to slow down in the sample to reach the resonance energy.
Thus, information on the hydrogen distribution can be straight forward obtained by varying the 15N incident beam energy.
For example, deuterium can easily be profiled with a 3He beam without changing the incident energy by using the reaction, usually written 2H(3He,p)α.
The energy of the fast proton detected depends on the depth of the deuterium atom in the sample.