Why neutrons and muons?
You can’t photograph an atom with light — its wavelength is thousands of times too big. To see how a material works you need a probe that’s the right size, gets inside, and doesn’t wreck what it’s measuring. ISIS makes two of the best ones in the world.
How small is an atom, really?
Materials are built from atoms spaced a few ångströms apart (1 Å = 0.1 nanometre = 10⁻¹⁰ m). To measure structure at that scale you need a probe whose wavelength is also a few ångströms — a ruler with markings as fine as the thing you’re measuring.
Moderated (“slowed-down”) neutrons land exactly there. That’s not luck — a moderator is a small tank of material the neutrons fly through to lose speed (see How ISIS works), and tuning the ruler is its whole job.
Drag the slider to shrink down the scale — past an amoeba, a human hair, a bacterium, a virus and a strand of DNA — from a coin to a single atom.
a £1 coin — about 2 cm across
Slow particles have long wavelengths
Here’s the only quantum fact this site needs: every moving particle of matter also behaves like a wave, and the slower it moves, the longer its wavelength. For neutrons the relationship is beautifully simple:
λ × v = 3,956wavelength (Å) times speed (m/s) is a constant
A neutron at 1,000 m/s — bullet-fast by everyday standards, yet tens of thousands of times slower than when it was born — has a 4 Å wavelength: atom-sized. This is why “slowing neutrons down” and “tuning the ruler” are the same job, and why, on a pulsed source (neutrons leave in short, timed bursts), clocking a neutron’s arrival over a known distance gives its speed — and therefore its wavelength.
Simplified, precisely: a particle’s wavelength is set by its momentum, not its speed. A neutron’s mass never changes and it travels far below light speed here, so momentum is just mass × speed and the two statements are interchangeable — which is why the constant above works. For light, which has no mass and never slows down, ‘slower’ has no meaning at all.
Neutrons see what X-rays miss
“Scatter” just means bounce: fire probes at a sample, record where and how strongly they deflect — there is no camera. X-rays scatter off electrons, so heavy atoms (lots of electrons) dominate and hydrogen (one electron) is nearly invisible. Neutrons scatter off the nucleus, and the strength varies almost randomly across the periodic table — hydrogen and lithium show up brightly even next to heavy metals.
That makes neutrons the probe of choice for water, polymers, proteins, and the lithium chemistry inside a working battery. Bonus: swapping an isotope (hydrogen → deuterium, which is hydrogen plus one extra neutron) changes the contrast without changing the chemistry — like highlighting one variable in a debugger.
And because neutrons are electrically neutral, they pass deep through metal — through an engine block, a weld, a pressure cell — without damaging anything.
X-rays: the heavy atom dominates, hydrogens almost vanish.
Neutrons feel magnetism
A neutron has no charge, but it is a tiny compass needle — it carries a magnetic moment. In many materials each atom is itself a tiny magnet too; as the neutron flies through, it responds to the field of every atom it passes.
So neutrons don’t just map where atoms are; they map how each atom’s magnetism points. Much of what we know about magnetic materials — hard drives, MRI magnets, superconductors — was measured with neutrons. ISIS work includes the structures of high-temperature superconductors (where “high” still means about −140 °C) and solid buckminsterfullerene — C₆₀, the football-shaped carbon molecule.
Muons: reporters embedded in the material
A muon is a heavy, short-lived cousin of the electron. ISIS implants them into a sample in short, intense bursts; the muons land far apart, so each acts as an isolated probe. Each muon parks at a well-defined spot between the atoms of the crystal, and — like the neutron — it’s a tiny compass needle. That needle, the muon’s spin, precesses — swings around like a gyroscope — at a rate set by the magnetic field right there, at that exact spot in the material.
After 2.2 µs on average the muon decays and fires out a positron (the electron’s antimatter twin, written e⁺), preferentially along its spin direction. Each muon dies at a different moment, so millions of positron directions sorted by time of death trace the swinging needle frame by frame — and how fast it swings reads out the local magnetic field inside the material. A probe that texts you its orientation as it dies.
Muons excel at faint magnetism, superconductivity, and tracking lithium-ion motion in battery materials. A separate trick: a negatively charged muon can fall into an atom like an extra electron, and the X-rays it emits as it settles fingerprint the element — elemental composition without cutting a sample, gentle enough for museum pieces.