Neutron Radiography

One can think of radiography as of photographing a semi-transparent object lit from behind. Most interesting objects are not semi-transparent for visible light though, but we're not limited to this small range of electromagnetic waves. X-rays pass through many objects rather easily, in particular soft tissue of human body, which is extensively used in medicine.

X-rays, being an electromagnetic wave, interact mostly with electrons when passing through matter. Attenuation of X-rays is a smooth, growing function of atomic number of atoms that build the matter (which corresponds to how dense electrons are packed in there). This yields two conclusions:

1. light elements (Hydrogen, Beryllium, Carbon) are relatively transparent for X-rays, while heavy ones (iron, lead, gold) block them
2. it's hard to distinguish on a radiogram done with X-rays between elements with similar atomic number (impossibility to distinguish isotopes being an extreme case)

X-rays are therefore perfect for imaging heavy, dense elements hidden inside soft and light ones - bones inside body, weapons beneath clothing.

But what if neutrons are used rather than X-rays?

Neutrons' pass through matter is dominated by their strong interaction with atomic nuclei as they, being electrically neutral, ignore electrons completely. This interaction is usually very complicated and involves nuclear reactions happening, which yields a very scattered pattern when looking at their attenuation in function of atomic number.

Attenuation in funcion of atomic number - comparison of thermal neutrons and X-rays.

In particular Hydrogen atom has one of the highest probabilities to capture a neutron, Beryllium not being much worse. On the other hand iron and lead don't interact strongly with passing neutrons. Attenuation also strongly differs between isotopes.

Thus neutron radiography is perfect for imaging light things hidden inside heavy metals.

Camera radiograms taken with X-rays (left) and neutrons (right).

Picture above highlights differences between radiography with X-rays and neutrons. While X-rays yield highest contrast on metal parts (eg. objective mounts) neutrons reveal mostly plastic parts (as plastics are build of hydrogen and carbon). Plastic film reel is clearly visible with neutrons and not visible at all with X-rays.

Images below show where neutron radiography can be advantageous over traditional methods. Left one is a lighter (note how nontransparent fuel is - hydrogen and carbon again) , right - a quick-disconnect fitting.

Being able to capture radiograms one can also perform tomography. After taking radiograms with object subsequently rotated by a small degree (until it's been rotated by 180°) data can be combined and Radon transform used to compute full 3D image of internal structure.

This technique is of particular interest for art historians since it can reveal what is inside sealed historical bronze sculptures giving insight into technique used by it's creator.

X-ray (left) and neutron (right) radiograms of Mars from Oberweningen

Tomography results. Left - clay core, right - bronze shell.

The most amazing thing that can be done with neutron radiography is visualizing insides of a combustion engine while it's running. And both fuel and lubricants (hydrocarbons) are clearly visible. Moreover, using stroboscope techniques it is possible to produce a video! (could not find one on the net though...)

Neutron radiography has many not mentioned applications in both science and industry: weld inspection, explosives imaging, aircraft parts investigation, radioactive reactor parts inspection, fuel-cells imaging, non-invasive wood dating, soil physics and more.

References:
http://www.nray.ca/
Eberhard H. Lehmann, E. Deschler-Erb, A. Ford Neutron Tomography As A Valuable Tool For the Non-destructive Analysis Of Historical Bronze Sculptures, Archaeometry 52, 2 (2010) 272–285
Eberhard H. Lehmann, Werner Wagner Neutron imaging at PSI: a promising tool in materials science and technology, Appl Phys A (2010) 99: 627–634