Research carried out at the microscopic scale - whether into the natural world or manmade materials - has been responsible for many of the major improvements in our quality of life.
Sample objects can now be studied with various kinds of radiation to reveal the locations of atoms and the forces between them at the atomic and molecular levels. An increasingly important approach is the use of beams of electrically neutral subatomic particles - neutrons - which can scatter off materials in a way that gives just such information. Neutron-scattering techniques are now applied across many scientific disciplines - physics, chemistry, engineering, materials science, environmental and geological sciences, and increasingly the life sciences.
The UK has a major facility, ISIS – based at the Rutherford Appleton Laboratory near Oxford and operated by the Science and Technology Facilities Council – which provides neutron beams for research. Academic and industry research groups from across the UK, and further afield, can apply to use ISIS.
ISIS experiments provide detailed insights into the arrangement and behaviour of atoms and molecules in a material, which are not obtainable by other methods.
What do neutrons tell us?
Neutrons are one of the two constituents, along with protons, of atomic nuclei. Fired into samples, they can tell us where atoms are and how they are moving deep inside materials.
This atom's-eye view helps us to explain why substances have the properties they do – for example, how they conduct electricity, why they have particular magnetic properties, or how tough they are – and so can help us make new substances that are useful in everyday applications.
When neutrons interact with the atoms in a solid or liquid, they scatter off the nuclei in a characteristic manner that depends on the atomic positions. The angles at which the neutrons emerge from the sample tell us the distances between the atoms. Any changes in the neutron energies as they pass through the samples also reveal the motions of atoms and molecules.
Making a neutron beam
Neutrons can be released by firing bursts of accelerated protons at a heavy metal target, knocking out, or spallating, neutrons from the target’s nuclei. ISIS has significantly developed this spallation neutron approach, which has been extremely successful. It is being taken up elsewhere in the world and will certainly be the basis of any future international neutron facility.
As well as producing neutrons, the proton pulses are also directed at a target designed to produce muons, which are another useful probe of the properties of materials.
The basic requirement for producing neutrons and muons is a powerful accelerator system to generate an energetic proton beam. ISIS has several accelerator stages, culminating in the circular synchrotron accelerator 50 metres across. This produces protons travelling at 84 per cent of the speed of light.
A wide range of experiments and instruments
Once the neutrons escape from the target, they are slowed down (a process called moderation) and directed along beamlines to some 20 instruments where experiments take place.
Each instrument is designed for a specific type of experiment. For example, large polymer networks are studied using an instrument which measures neutron scattering at small angles (small angle neutron scattering, SANS).
A bright future for ISIS
A second target for neutron production - Target Station 2 - with a suite of new beamlines and instruments, will increase capacity and widen research potential at ISIS. Further novel improvements are being made to increase the intensity of the neutron pulses, so ISIS users can look forward to the prospect of carrying out even more exciting science.
Having looked at the basic science underpinning ISIS, we now join a particular research group to see how they are making use of ISIS neutrons.
Next: The journey's aim