Synchrotron-driven spallation sources

The use of synchrotrons for pulsed neutron spallation sources is an example of spin-off from the accelerator development prompted by particle physics. The first proposal for an accelerator-driven source with a thermalised neutron facility was in the 1960s (Intense Neutron Generator, ING), but this project was never built. There was earlier work on 'electrical breeders' and the direct bombardment of a heavy-metal target for spallation had already been foreseen by Lewis (1952), but the first demonstration of the spallation source concept was at ANL in 1972 (ZGS Intense Neutron Generator Prototype, ZING-P). A spallation source uses a medium-energy accelerator to excite the nuclei of a heavy-metal target (often Ta or W alloy), from which neutrons 'evaporate'. These fast neutrons are slowed in moderators set around the target. Neutrons make excellent probes for condensed matter as their neutrality ensures deep penetration, their magnetic moment reacts to magnetic structures and their weak interaction minimises radiation damage allowing in vivo experiments. Neutron sources are widely used in biology, chemistry, materials science and basic physics, as well as for technological applications such as radiography and materials testing. Spallation sources are environmentally friendly compared to reactors. The time structure of accelerator beams offers some experimental advantages and peak neutron intensities can exceed those of reactors. Although, the present demand for neutrons can be met by reactors, this situation is unlikely to continue due to the increasing severity of safety regulations and the policies of many countries to close down their reactors within the next decade or so. At the same time, the demand for neutrons is expected to grow and, consequently, there has been an increasing interest in accelerator-driven sources.