A form of radiography
-- radiation imaging -- which uses a neutron flux as its "illumination", rather than the more typical gamma particles
or x rays
To a first-order approximation, gamma particle are absorbed by materials in proportion to their Z number. In other words, gamma radiography (and similarly, x-ray radiography) yields pictures in which bones, metals, and so forth are clearly imaged and delineated from things like tissue or plastics, which are pretty transparent.
Neutrons, by contrast, are absorbed according to the neutron capture cross-section of the material being imaged. In practical terms for ordinary objects, this means that absorption is proportional to the amount of hydrogen in things, because hydrogen is the only really common element with a fairly high neutron capture cross-section.
The upshot of this is that neutron radiography yields images in which water, plastics, hydrocarbons and such are clearly imaged and delineated from metals and nearly anything else, which are pretty much transparent.
For example, if you were to attempt to x-ray a car, you would be able to see and inspect the welds in the steel, but anything plastic would be invisible; but if you took a neutron radiograph of the car, you would be able to inspect the plastic gas tank and the gasoline it contains, but the frame would be invisible.
It's a pretty neat trick.