Gamma photons can initiate nuclear reactions if their energies are higher than the binding energy of the target nucleus. Therefore, there are relatively few nuclear reactions with gamma photons. The reaction

2H 1 y ! n 1 p1 (6.25)

was already discussed in Section 5.5.2. This reaction can be initiated by the gamma photons of 24Na isotopes since the energy of the gamma photons is 2.76 MeV, while the binding energy of the deuterium nucleus is 2.2 MeV. So, when a salt con­taining 24Na isotopes is dissolved in heavy water (D2O), a mobile neutron source can be produced.