Apparatus for generating thermal neutrons includes an electron accelerator for generating an electron beam and a converter for converting the electron beam into photons. A receiving device is provided for receiving the photons and includes a material which provides a photoneutron target for the photons, for producing high energy neutrons in a photonuclear reaction between the photons and the photoneutron target, and for moderating the high energy neutrons to generate the thermal neutrons. The electron beam has an energy level high enough to produce photons of sufficient energy to exceed the photodissociation threshold of the selected target material, but that is sufficiently low as to enable the material to moderate the high energy neutrons resulting from the photonuclear reaction.