Neutrons are produced at ISIS by the spallation process. A heavy metal target is bombarded with pulses of highly energetic protons from a powerful accelerator, driving neutrons from the nuclei of the target atoms.
This results in an extremely intense neutron pulse, delivered with only modest heat production in the neutron target. In contrast, traditional reactor sources of neutrons suffer from intense heat production in the reactor core, pushing the current limits of materials technology. As a result of the low duty cycle of the ISIS accelerator, the time averaged heat production in the ISIS target is a modest 160 kW, but in the pulse, the neutron brightness exceeds that of the most advanced steady state sources.
The acceleration chain begins with a Penning H- ion source at a potential of -665 kV. The extracted H- beam is accelerated to ground and injected into the linac with an energy of 665 keV. The four tank Alvarez type linac accelerates the beam to 70 MeV providing a 200
ms long, 22 mA H- pulse.
On entry to the synchrotron, the H- ion beam is passed through a 0.3
mm thick aluminium oxide stripping foil, that removes both electrons from the H- ions in the beam converting them to protons. The proton beam is injected over approximately 130 turns of the synchrotron to minimise space charge effects and allow accumulation of 2.8x1013 protons.
Once injection is complete, the harmonic number two RF system traps the injected beam into two bunches and accelerates them to 800 MeV on the 10 ms rising edge of the sinusoidal main magnet field. There are six double-gap ferrite-tuned RF cavities which provide a peak accelerating voltage of 140 kV per beam revolution. Immediately prior to extraction the pulses are 100 ns long and are separated by 230 ns.
The proton beam will make approximately 10 000 revolutions of the synchrotron as it is accelerated before being deflected in a single turn into the extracted proton beam line (EPB). This is accomplished using three fast kicker magnets in which the current rises from 0 to 5 kA in 100 ns!
The entire accleration process is repeated 50 times a second.
The spallation target is made from the heavy metal tantalum. Protons hitting nuclei in the target material trigger an intranuclear cascade, placing individual nuclei into a highly excited state. The nuclei then release energy by evaporating nucleons (mainly neutrons), some of which will leave the target, while others go on to trigger further reactions. Each high energy proton delivered to the target results in the production of approximately 15 neutrons.
The neutrons produced in this process generally have very high energies and velocities and must be slowed down to be useful for condensed matter studies. This is achieved by an array of small hydrogenous moderators around the target. These exploit the large scattering cross-section of hydrogen to slow down the neutrons passing through, by repeated collisions with the hydrogen nuclei. The moderator temperature determines the spectral distributions of neutrons produced, and this can be tailored for different types of experiment. The moderators at ISIS are ambient temperature water (43°C), liquid methane (100 K) and liquid hydrogen (20 K).
The characteristics of the neutrons produced by the ISIS pulsed source are significantly different from those produced at continuous nuclear reactor sources. In particular, time-of-flight techniques are used on the polychromatic neutron beams, giving a direct determination of the energy and wavelength of each neutron, and allowing fixed scattering geometries to be used. Measurements can cover a wide spectral range in both energy and momentum transfer with good signal-to-noise levels, and the sharpness of the initial 0.4 µs neutron pulse is preserved by the small moderators, giving a rich, high energy component to the under-moderated spectrum, or allowing high instrument resolution to be achieved.
... and muons ...
Muons are produced in the collision between an energetic proton and the atomic nucleus of a light element. ISIS also produces intense pulsed muon beams by passing the 800 MeV protons from the synchrotron through a graphite intermediate target before reaching the main neutron production area. A small fraction of the protons suffer a close collision with a proton or neutron of a carbon atom to produce shortlived, charged pions with a half-life of 26 ns, decaying into muons. Beams of spin-polarised muons are produced from pions decaying at rest near the surface of the production target, known as
surface muon beams of positive muons, µ+, at the same 50 Hz repetition frequency as the neutron target.
... and even neutrinos!
In addition to neutron production in the main target, pion and muon decays give a rich flux of neutrinos, and the KARMEN experiment is designed to exploit the nature of the pulse structure in the investigation of problems in fundamental physics.