It is half of the ultimate double act – the particle equivalent of Ernie Wise or Oliver Hardy – the neutron is the particle straight-man to the proton’s charged-personality.
Like all great double acts, the neutron and proton spent years plugging away in anonymity until, one day, they were discovered, plucked from obscurity and thrust into centre stage.
The proton and neutron can be found at the heart of every atom (apart from hydrogen, which possesses just a lonely proton) and without them matter as we know it couldn’t exist.
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.
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).
For a while the arrangement seemed to work, but it soon became clear that something didn’t add up. 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 this 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.
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 – to Chadwick, this was proof that a particle was at play.
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 has 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). Without Chadwick’s discovery, there would have been no nuclear bomb (ok, so it’s not all good) and there would have been no nuclear power stations (not really selling this are we?!).
Aside from helping to blow up Pacific islands, the neutron also has more benign talents and is an extremely useful tool for probing the atomic structure of matter.
Its ability to penetrate matter means it can tell us exactly where the atoms and molecules are within a material and how they behave.
If you think particle science is limited to the esoteric (such as what caused the Big Bang) you would be mistaken. At facilities like the Institut Laue-Langevin (ILL) in Genoble (coincidentally, is celebrating its 40th anniversary this year) neutrons are used like supercharged X-rays to understand the world at the atomic level.
At ILL, the neutron has been used to develop magnetic soap for mopping up oil spills, targeted cancer treatments and new ways to combat viral and bacterial infections. It has even helped make aircraft safer by finding structural defects hidden well beyond the reach of the human eye.
And how does ILL create the neutrons it uses? They have their very own nuclear reactor that feeds high-intensity beams of neutrons to an array of 40 instruments, which are used by some 1,200 researchers from over 40 countries every year.
The ILL is just one of the stages on which the neutron has performed in the 80 years that have allowed its meteoric rise from obscurity to be one of the premier particle A-listers.