Page last updated at 00:26 GMT, Tuesday, 11 November 2008
'Super-microscope' opens at Isis



By James Morgan
Science reporter, BBC News

Neutron sources old and new
Isis 2 (right) is a gigantic leap from Chadwick's original neutron source (left)

When Sir James Chadwick discovered neutrons, he predicted they would make "no difference to humanity".

The beam of radiation he observed at Cambridge in 1932 was unusually penetrative. But the particles were devoid of any charge or character.

These "neutrons" would be curious to scientists, he admitted, but of absolutely no consequence to the likes of you and I.

Seventy-five years later, we know that Chadwick's "neutral" particles have underpinned some of the most potent and controversial inventions of the 20th Century, including nuclear weapons.

But it turns out that neutrons are also the ideal tools for mapping the inner architecture of molecules.

Secret agents

Their very blandness gives them the power to penetrate matter.

Structure
The inner structure of a material, as revealed by neutron scattering

Being neutral, they are not repelled by the protons in atomic nuclei, nor the electron clouds surrounding them.

Instead, they are able to ghost through "border control" unnoticed, and return with intelligence reports.

At Isis, neutrons have been sent in to explore the catacombs of molecules of every shape and size.

And they have re-emerged with dossiers which have helped scientists to create super-fast computers, make transport safer, and provide better medicines.

Not bad for boring couch potatoes.

Neutrons can be forced out of their comfort zone in the nucleus, and into action, by bombarding atoms with high energy particles.

Chadwick was able to do this on his lab bench at Cambridge with a simple vacuum chamber.

Isis, unsurprisingly, requires an awful lot more energy.

How neutrons are made at the new £200m Isis target station

The experiment begins by accelerating protons to 84% the speed of light.

Winding up

This takes place inside a circular synchrotron accelerator - a much smaller version of the Large Hadron Collider, at Cern, Geneva.

Each proton goes 10,000 times around the accelerator, a total distance of 1,655 kilometres - the same as a journey from London to Aberdeen and back.

This happens 50 times a second.

Pulses of these highly energetic protons are then fired at a heavy metal (tungsten) target - about the size of a packet of biscuits - buried inside a radiation proof wall.

They collide with the nuclei of the tungsten atoms, like a cue ball striking a pack of reds.

These collisions trigger an intranuclear cascade, which places individual tungsten nuclei in a highly excited state.

Isis 2
Neutrons scatter off the sample into surrounding detectors

Each high energy proton delivered to the target results in the production of approximately 15 neutrons - which scatter outwards from the target.

These neutrons generally have very high energies, and must be slowed down to be useful for condensed matter studies.

Fine tuning

This is achieved by passing them through an array of moderators around the target - which slow down the neutrons, by repeated collisions with hydrogen nuclei.

These moderators allow the operators of Isis to fine tune the energy, wavelength and the scattering geometry of the neutrons - to suit the properties of the materials they wish to survey.

The beams of neutrons are directed down pipes leading to the instrument stations where scientists carry out their experiments.

Sir James Chadwick
Sir James Chadwick had no idea the impact his discovery would make.

Slot your sample into the pathway of the beam, and the neutrons will penetrate, bounce around, and pop out on to banks of detectors.

The pattern they create on the detector helps reveal the inner structure of the sample material.

Similarly, subtle changes in the properties of the neutrons give clues to the properties of the atoms they encountered inside the sample material.

Different instruments are used to make different kinds of measurements.

There are 20 instruments already in use at Isis 1. Another seven have been installed in the new second target station building.

Scientists use these instruments to look at samples ranging from aeroplane wings to shampoo.

Sir James Chadwick would be very proud indeed.

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SEE ALSO
In defence of particle physics
10 Sep 08 |  Science & Environment
Diamond facility starts to shine
14 Jul 06 |  Science & Environment
Boost for British neutron lab
09 Apr 03 |  Science & Environment
Making 'ghosts' in a machine
06 Nov 02 |  Science/Nature

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