Electron beams have been used extensively to produce neutron pulses. It has been proposed, for example by Abalin et al. [124], to use an electron accelera­tor to generate the neutron source of hybrid reactors.

Electrons with energies above a few tens of MeV are slowed down essentially by Bremsstrahlung photon emission. Above 10MeV the domi­nant process for energy loss of the photons is pair creation. In that manner an electromagnetic shower develops, consisting of a population of electrons, positrons and photons. Independently of their electromagnetic interactions these particles may also interact with nuclei. The photonuclear cross-section is dominated by the giant dipole resonance, with an energy which depends weakly on the nuclear mass and is around 10 MeV. Thus, if one wants to optimize the photonuclear rate of interactions within the electromagnetic shower, one should maximize the number of photons (real or virtual) with energies around 10 MeV. The optimum initial electron energy is then found to be between 100 and 200 MeV. For example, the Euratom Geel linear accelerator provides electrons with a maximum energy of 140 MeV, and operates, in practice, at 100 MeV. At this energy only 0.1 neutron is produced per incident electron. This corresponds to a neutron production yield of 1/GeV, to be compared with the typical 30 neutrons per GeV which can be obtained with protons. To overcome this small neutron production efficiency, Abalin et al. [124] suggest using multi­plying media with ks very close to one. We shall examine their proposal in more detail below.