Accelerator technology
ISIS produces intense beams of neutrons and muons using a high-energy particle accelerator. At the heart of ISIS is an 800 MeV proton accelerator that produces intense pulses of protons 50 times a second. Travelling at around 84% the speed of light, these proton pulses are fired at two tungsten targets to produce neutrons through a process called spallation. Muons are produced upstream of one of the neutron targets when the proton beam passes through a carbon plate producing pions, which rapidly decay into muons.
You can find out more about the technology behind ISIS below, or in our Practical Guide to ISIS (pdf).
Ion source
The first stage of the ISIS machine is the ion source, which is a pulsed source of H– ions. Hydrogen gas is ionised by an electric discharge taking place between an anode and a cathode inside a stream of hydrogen gas. In operation, a stream of hydrogen gas with a little caesium vapour is delivered to the ion source.
When the H– ions emerge from the ion source assembly, they have an energy of 35 keV. They then travel through the Low Energy Beam Transport line (LEBT), which focuses and steers the beam into the radio-frequency quadrupole (RFQ), which uses intense radio frequency electric fields to focus, bunch and accelerate the H– beam.
In the video on the right, Scott Lawrie gives an introduction to the ISIS ion source.
Linear accelerator
The ISIS linear accelerator (linac) consists of four accelerating tanks in which high intensity radiofrequency (RF) fields accelerate the beam to 70 MeV.
The ISIS linac was originally built as a new injector linac delivering 75 mA proton pulses at ~1 pps to the old Nimrod 7 GeV proton synchrotron. Tanks 2 and 3 of the ISIS linac were the second and third tanks of the Proton Linear Accelerator (PLA) that began running at RAL in the 1950s.
The stream of H– ions from the linac is injected into the synchrotron over ~150 turns. As the beam goes into the synchrotron, the H– ions are stripped of their electrons to become protons, and the proton intensity in the synchrotron builds turn-by-turn. By injecting H– ions rather than protons it is possible to overcome electrostatic repulsive forces during the multi-turn process and as a result get to about 10 times higher proton intensity in the synchrotron.
Synchrotron
The ISIS proton synchrotron is a machine with a circular orbit of radius 26 m (and a circumference of 163 m), designed to take the 70 MeV beam from the linac and accelerate it to 800 MeV.
The ISIS synchrotron is made up of ten sections or ‘superperiods’, numbered 0-9, in each of which the beam is bent through an angle of 36°. The essential elements of each superperiod are a dipole magnet and three quadrupole magnets. The dipoles (the first element in each superperiod) provide the 36° bend, and the quadrupoles provide focusing to prevent beam expansion.
When the synchrotron has accelerated them to 800 MeV the protons are moving at 84% of the speed of light and taking 0.65 μs to travel once around the 163 m circumference.
Producing neutrons and muons
To extract the protons, a high-speed magnet must suddenly be switched on within 0.1 μs to kick the two proton bunches out of the synchrotron. The two extracted proton beam lines (EPB1 and EPB2) deliver beam from the synchrotron to the corresponding target stations.
The neutron targets are tantalum-clad tungsten targets and produce neutrons through a process called spallation. A high-energy proton hitting a heavy-metal nucleus can knock a few particles or clusters of particles out of the nucleus and can also split the rest of the nucleus into two excited fragments. The fragments then de-excite by emitting neutrons with energies of the order of 1 MeV, and the knocked-out particles can go on to induce further spallation reactions. The overall result at ISIS is some ~10-15 neutrons produced per incident proton.
The ISIS intermediate target, in EPB1 is a one cm thick piece of graphite that is placed directly in the beam to generate pions that then decay into muons — unstable particles with a mean life time of 2.2 μs. Muons, produced by pions decaying at rest, are selected by the muon beam line that transports these muons to several instruments
