To make a neutron beam, you need to first have some particles travelling REALLY fast.
To make this happen, ISIS has two particle accelerators. There is a linear accelerator called the Linac, and a circular accelerator, called the synchrotron.
Particles are first accelerated in the linear accelerator and separated into bunches like sausages. By the end of the accelerator, the particles are travelling at 37% of the speed of light!
They are then passed into the synchrotron, where they are accelerated even more. By the time they have finished 10,000 laps of the synchrotron they are travelling at around 84% of the speed of light!
The particles then travel towards a tungsten target, no bigger than a packet of biscuits that is situated in the experimental hall. When the particles smash into the target, neutrons are dislodged from the atoms. These neutrons are then detected by the neutron instruments.