Question: What is a ISIS neutron and muon source

Silvia Imberti answered on 4 Mar 2019:

ISIS is a facility in Oxfordshire (“The Isis” is an alternative name for the River Thames, used from its source in the Cotswolds until it is joined by the Thame at Dorchester in Oxfordshire) for the production of a “stream” of neutrons (and let’s not forget about the muons). It used to be a big accelerator for particle physics. In 1985 it became obsolete (it is not big enough, think of CERN) and it has been re-deployed for studying the property of materials. Because we do not want to split the atom, we do not need as much energy as it is needed for particle physics. In fact, we slow down out neutron beam, so we can look at atoms and molecules, where they are and how they move.

The process in brief: we accelerate protons up to 84% light speed with a synchrotron (the usual donut shaped accelerator) and then it fires them off towards a target made by a foil of heavy metal. This shakes off neutrons in all directions (we then need to screen them, because neutrons can do a lot of damage to humans). We slow down the neutrons until they reach the wavelength of 0.1-1 nm i.e. the order of magnitude of the spacing between atoms in a crystal for example. This means that when they pass through a crystal, they become diffracted, i.e. change direction, a bit like splitting white light into different colours using a crystal. From the position of the neutrons before/after the crystal, we can reconstruct the position of the atoms in the crystal.

You may have heard of something similar with a beam of x-rays, which indeed they do at out fellow x-rays source Diamond Light source.

BUT, with neutrons at 0.1-1 nm, not only the spacing is similar to the atomic one, but also the energy! This means that neutrons can lose energy by “bouncing off” an atom (you know atoms are always moving about, and shaking and flipping and wriggling). So if you measure the energy of the neutron before/after meeting the sample, you can tell how the atoms where moving.

Finally because neutrons are the same size as hydrogen, we can see hydrogen very well, and all soft matter in general. (Think how well you can bounce a snooker ball against another of the same size/weight… Essentially, we play atomic snooker!)

This turns out very neat if you are studying hydrogen storage, or organic chemistry, or biological molecules or… you guessed it: WATER!