There are only two practical methods of generating neutrons in large enough quantities to be useful for experiments: nuclear reactors or big accelerators.
ISIS uses the most efficient accelerator method of neutron production, which is to impact high-energy protons on a heavy metal target – a process known as spallation.
At ISIS, around 2x1016 neutrons are created each second using a 200 µA pulsed beam´ of 800 MeV protons colliding with a tungsten target.
Accelerating particles becomes progressively more difficult with increasing energy. A sequence of four different accelerators is required to produce protons with 800 MeV of energy.