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PREDICTING how cracks will form in critical welds on aircraft or spacecraft
is difficult, and making detailed images of the stresses that cause them is even
tougher. But help will soon be at hand with a new lens that focuses neutrons
more sharply than ever before.

To picture the structure of a metal, including any stresses it contains,
researchers look at the scattering pattern made by bombarding the material with
neutrons. This pattern depends on the gaps between atoms, which in turn depend
on stresses in the material.

But there鈥檚 a limit to how small the stress patterns can be and still be
visible, and this depends on the intensity of the neutron beam. The current
limit is a few millimetres. To do better, physicists must build more intense
neutron sources鈥攐r find a way to focus the beam onto the sample more
sharply.

Mike Johnson and Mark Daymond at the Rutherford Appleton Laboratory in
Oxfordshire have come up with a solution. They have designed a lens that can
intensify the beam for all wavelengths of neutrons, using 鈥渟uper-mirror鈥�
coatings made from extremely thin layers of nickel and titanium. Normally,
neutrons simply pass through the material, but if they meet the flat surfaces at
a shallow angle, they are reflected.

By curling the super-mirror coatings into concentric cylindrical
arrangements, any neutrons that graze the walls are brought to a focus
regardless of their wavelength. And by using a large number of concentric
cylinders, researchers can make lenses that capture neutrons over a wide angle.
鈥淪imilar lenses are used to focus X-rays,鈥� says Johnson. The focus can be
further improved by curving the cylinders so they form part of the surface of an
ellipse (see Diagram).

The concentric cylinders have to be as little as 30 micrometres apart. 鈥淭his
makes them very difficult to build with air as the gap between the layers,鈥� says
Johnson. He and Daymond realised that silicon is practically transparent to
neutrons and wondered whether it might provide structural support for the
super-mirror coating while keeping the layers separated by the required gap.

They tried out their idea by placing the super-mirror coating on a number of
wafers of silicon, then sandwiching the wafers together and curling them into a
cylinder. This prototype lens is being tested at the ISIS facility in
Oxfordshire, the world鈥檚 most intense source of pulsed neutrons.

Kurt Clausen, an expert on neutron beams at Ris National Laboratory in
Denmark, says the lens will allow much greater resolution of stresses in
materials. 鈥淚f we want to build aircraft with new materials such as composites,
we must understand the stresses inside them,鈥� he says. By focusing the beam to a
spot and moving it back and forth across the material, researchers will be able
to build up a high resolution picture of the stresses in a relatively high
volume. 鈥淧hysicists have been looking at this problem for a long time so it鈥檚
neat that they鈥檝e been able to do this,鈥� Clausen says.