Schematic of the CIM-AAMU hydrogen depth profiling detector: A N 15 ion beam from the Pelletron penetrates a target material near the center of a Bismuth Germanate scintillation detector ( BGO ). If hydrogen is in the target, the N 15 (p, ag )C 12 nuclear reaction takes place and a 4.44 MeV gamma is produced. That gamma ray most probably produces an electron-positron pair. The BGO is dimensioned such that one of the two oppositely directed 0.511 MeV positron annihilation gamma rays escape to the Sodium Iodide ( NaI ) scintillation detector. An exact coincidence of that 0.511 MeV signal in the NaI detector with the signal from the remaining 3.93 MeV in the BGO detector proves the existence of hydrogen in the target material. The counting rate is proportional to the hydrogen concentration in the target. Moreover, the nuclear reaction occurs only for a specific N 15 ion energy; thus the hydrogen concentration is determined at the precise depth where the ion beam has lost enough of its initial energy to produce the nuclear reaction.