neutron diffraction
/ NOO-tron dih-FRAK-shun /
Think of two ways to map a forest: a camera that mostly catches the big leafy trees, and a sonar that pings every trunk equally, even the slender saplings the camera misses. X-rays are a bit like the camera, drawn to atoms with lots of electrons. Neutron diffraction is the sonar — it scatters off the tiny nuclei at the heart of atoms, so it 'hears' light atoms just as loudly as heavy ones.
Here is the idea. A neutron is a neutral particle, and quantum mechanics gives a moving neutron a wavelength. Slow it down to the right speed and that wavelength matches atomic spacings, so a crystal diffracts a neutron beam just as it does X-rays, into Bragg spots. But because neutrons bounce off nuclei rather than electron clouds, their scattering strength varies almost randomly from element to element — and it does not fade away at high angles the way electron-cloud scattering does.
This matters for two big reasons. First, neutrons pin down light atoms like hydrogen and tell apart neighbors on the periodic table that look identical to X-rays. Second, neutrons carry a tiny magnetic compass of their own, so they scatter off the magnetic arrangement of a material, revealing how the atomic magnets point — something X-rays barely see. The honest caveat: neutron beams are weak and come only from reactors or large accelerators, so the experiments need big facilities, big samples, and patience.
In ice, X-rays see the oxygen atoms clearly but nearly miss the hydrogens. Neutron diffraction lights the hydrogens up, so it was neutrons that nailed down exactly how the water molecules hydrogen-bond into ice's open hexagonal cage.
Neutrons reveal the hydrogen atoms in ice that X-rays almost overlook.
Neutron scattering strength does not climb steadily with atomic size the way X-ray scattering does — it hops up and down across the periodic table, and some isotopes of the same element scatter very differently. That quirk is exactly what lets neutrons distinguish atoms X-rays confuse.