How the neutron was discovered (or, how small stuff hit the big time)
Without both the neutron and proton matter as we know it couldn’t exist, but it was the show-stealing positively-charged proton that initially stole the limelight… this is the story of how the humble neutron first made a name for itself on the scientific stage.
On the 7th of February 1932, the ‘neutron’ was described in an article in the journal Nature by its discoverer, the English physicist James Chadwick. As one of the only two particles to make up the atomic nucleus, along with the proton, the neutron is part of arguably the most important double act in history.
The neutron is the particle equivalent of Ernie Wise or Oliver Hardy — the the neutral straight-man to the proton’s positive personality. Like many of the great double acts, the neutron and proton had spent years plugging away in anonymity until, one day, they were discovered, plucked from obscurity, and thrust into centre stage.
The star power of the proton
The proton and neutron can be found at the heart of every atom (apart from hydrogen, which possesses just a solitary proton) and without them matter as we know it couldn’t exist. Yet, strangely, this most fundamental of double acts didn’t find fame together — and it was the proton that would enjoy the the first taste of international celebrity.
First discovered in 1919 by New Zealand physicist, Ernest Rutherford, the proton was initially encouraged to embark on a solo career as the only particle within the atomic nucleus.
Like all great celebrities, the proton was known to be accompanied by a crowd of groupies — known as the electrons. The electrons were employed to keep the atom well balanced and neutral (being negatively-charged, they balanced out the proton’s positive nature).
Early models of the structure of the atom reflected this — showing an atom with a nucleus made up of just protons surrounded by a buzzing cloud of orbiting electrons. And, for a while the arrangement seemed to work, but it soon became clear that something didn’t add up.
The need for a straight man
The trouble was that an atom’s atomic number didn’t always tally with its atomic mass — it was like there were more performers on stage than the billing had advertised.
To account for the mass discrepancy, Rutherford suggested that there might be an as yet unseen performer at work within the atom — another particle that had about the same mass as the proton but, rather than being electrically-charged, would possess no charge at all — a neutral particle that wouldn’t upset the balance between the positive proton and the negative electrons. The hunt for the neutron was on and the man to find it would Rutherford’s assistant, British physicist James Chadwick.
But being neutrally-charged the neutron was rather difficult to locate. Fortunately, discoveries in Europe would provide just the trail of bread crumbs that Chadwick needed to track the neutron down.
Hints of greatness
In 1930, researchers in Germany discovered that if you bombard the element berilium with alpha particles (a particle with two protons and two neutrons — like a helium atom but without the electrons), a strange neutral radiation was emitted that could penetrate matter.
The discoverers of this phenomenon thought it was just common-or-garden gamma radiation, but Chadwick wasn’t convinced and believed that it was actually a particle.
But his initial attempts to track down the particle in a cloud chamber (the usual method of tracing a particle) proved fruitless.
Then, in France, researchers discovered that if a lump of paraffin wax was placed in the path of the neutral radiation, protons were knocked out of it — to Chadwick, this was proof that a particle was at play.
Shooting for stardom
Anyone who has ever played (or watched) pool or snooker can understand why Chadwick came to this conclusion. Imagine the atoms within the paraffin are a bunch of snooker balls — if you blow on the snooker balls (our imaginary gamma radiation), you might succeed in moving a few of the balls but not much else. If you instead fire the cue ball at snooker balls, you will see that some of the balls are knocked out of the pack — just like the protons knocked from the paraffin atoms.
Chadwick replicated the paraffin experiment and he not only confirmed that the neutral radiation was indeed a particle but also, by tracing the paths and energies of the dislodged protons, was able to figure out that the particle must have about the same mass as the protons it had dislodged.
At last, the neutron had been discovered and, as well as sharing the limelight with the proton, it soon went on to become a star in its own right.
The discovery of the neutron made possible the nuclear age. Its ability to penetrate an atom’s nucleus meant that it could be used to tear atoms apart and release the energy within (nuclear fission).
Hidden talents
Neutrons also have more benign talents and are, as it turns out, an extremely useful tool for probing the atomic structure of matter. Their ability to penetrate matter means it can tell us exactly where the atoms and molecules are within a material and how they behave.
It is this ability to penetrate and investigate matter in a totally non-destructive fashion that make facilities like the Science and Technology Facilities Council’s (STFC) ISIS Neutron and Muon Source (ISIS) such a powerful performer on the stage of modern science.
At ISIS, beams of neutrons and muons (a sort of ‘heavy’ electron) allow scientists to study materials at the atomic level using a suite of instruments, often described as ‘super-microscopes’, each of which is individually optimised for the study of different types of matter.
Because neutron and muon experiments are non-destructive, valuable, fragile and irreplaceable materials — such as cultural artefacts thousands of years old — can be studied even beneath the surface without leaving so much as a scratch.
Story by: Ben Gilliland
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