The atomic nucleus: Nuclear stability

The 3000 or so known unstable nuclides have half-lives from fractions of a second to more than the age of the universe – and theory predicts thousands more

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Nature's processes provide us with a rich variety of elements, ranging from hydrogen with just one proton, to uranium with 92 protons. Almost 300 stable "nuclides" - combinations of different numbers of protons and neutrons - are known.

That is a small brood compared with the 3000 or so unstable nuclides known (the full list is represented in the chart). These nuclides decay by a variety of radioactive processes, with half-lives ranging from fractions of a second to more than the age of the universe. Another 4000 or so nuclides are predicted by theory, but are yet to be seen. Understanding this extended nuclear family is one of the great challenges of nuclear physics today

Alpha decay

Alpha decay involves the emission of a tightly bound helium nucleus (2 protons, 2 neutrons), generally with lots of energy. It is common among the heaviest nuclides, where charge repulsion between protons weakens the binding of the strong nuclear force.

The heaviest naturally occurring nuclide, uranium-238 (92 protons, 146 neutrons), decays to thorium-234 with a half-life of 4.5 billion years, which is about the age of the Earth. The amount of helium trapped in uranium ore was used by Ernest Rutherford in 1906 to estimate Earth's age - a precursor to today's radiometric dating techniques.

Beta decay

Beta decay occurs in nuclei towards the edges of the nuclide chart - that is, those with more neutrons or protons than a stable nucleus can accommodate. The most familiar example is beta-minus decay, in which a neutron decays to a proton, emitting an electron to keep the atom's overall charge in balance. An analogue process, beta-plus decay, involves a bound nuclear proton turning into a neutron, emitting an anti-electron, or positron. A similar transformation can occur through the process of electron capture (EC), in which a nuclear proton absorbs an orbiting atomic electron.

The observation that beta-decay electrons were emitted with a lower than expected energy led physicists Wolfgang Pauli and Enrico Fermi to conclude in the 1930s that a second particle must also be emitted - a ghostly, weakly interacting particle of the type now known as neutrinos.

A select band of 11 nuclides undergoes two beta decays simultaneously, with the emission of two neutrinos. Some theories beyond the current standard model of particle physics also predict the existence of a "neutrinoless" double beta decay - but claims to have spotted this process in action are controversial.

Gamma decay

In gamma decay, a nucleus's number of protons or neutrons does not change; what changes is how the nucleons in an excited nucleus orbit each other. When such a rearrangement reduces the overall energy, the excess can be carried away as a photon. This de-excitation is usually fast, lasting less than a billionth of a second. But where substantial orbit changes are involved, it can be slow, resulting in excited states known as nuclear "isomers" that can persist for years.

Proton decay

Where a nucleus has a considerable excess of protons - or equivalent shortage of neutrons - it can decay simply by emitting a proton. Single-proton decay was first observed in 1970 from an isomeric state of cobalt-53, which contains six fewer neutrons than the stable cobalt-59. In 2002, iron-45 (nine neutrons down on stable iron-54) was observed to undergo an even rarer two-proton decay.

Haloed nuclei

Oddly, an equivalent to proton decay for nuclei with a large excess of neutrons has never been observed. Some nuclides do however have weakly bound neutrons orbiting at a relatively large distance - a "neutron halo". A two-neutron halo around lithium-11 (four excess neutrons compared with stable lithium-7) makes it look as big as lead-208.

Stable, unstable

Bismuth-209 (83 protons, 126 neutrons) was once thought to be the heaviest of all stable nuclei, but in 2003 it was observed to decay by alpha-particle emission. Still, with a half-life of 2 1019 years, about a billion times the age of the universe, you are unlikely to notice.

Fission

Nuclear fission, discovered in 1938, is a comparatively rare decay mode. Only four naturally occurring nuclides - thorium-232 and uranium-234, -235 and -238 - undergo spontaneous fission, and even these are vastly more likely to decay via alpha emission. The key to fission as an energy source is to stimulate a chain reaction by bombarding one of these fissile nuclides with neutrons that are themselves produced during fission.

Island of stability

Calculations based on the nuclear shell model (see below") suggest comparatively stable nuclei exist beyond those known on Earth - for example around the "doubly magic" combination of 114 protons and 184 neutrons. The superheavy inhabitants of this "island of stability" would have been created in supernova explosions just as ordinary heavy elements were. Their apparent absence suggests they have short enough half-lives - less than 200 million years, perhaps - to have almost completely decayed away since Earth formed.

We can test the island idea by colliding lighter nuclei to make heavier ones. The current record-holder is a nucleus with 118 protons and 176 neutrons, first synthesised in Dubna, Russia, in 2002. The longest-lived superheavy nucleus survives for about half a minute - disappointingly short, perhaps, but far longer than the millisecond or shorter lifetimes typical at the upper extremities of the nuclide chart.

Landfall on the island of stability itself is no easy matter: nature does not provide the right stable nuclei to collide to reach the doubly magic 114, 184 combination. A new generation of accelerators aims to get there by first making the smaller nuclei and then colliding them before they disappear.

Keeping it together

Long after the nucleus was discovered (see "Firing shells at tissue paper", above), its basic structure remained a puzzle. By the early 1920s, Rutherford had isolated a positively charged constituent, the proton, while working at the University of Cambridge. Only in 1932, though, did his colleague James Chadwick isolate the other component of the nucleus: the chargeless neutron.

Neither protons nor neutrons, collectively called nucleons, are themselves elementary particles. They are made up of smaller constituents, quarks, plus gluons that hold them together. Slightly different compositions mean that the proton is lighter by a whisker. It weighs in at 938.3 megaelectronvolts (MeV) - still more than 1800 times the electron's mass.

The neutron, meanwhile, tips the scales at 939.6 MeV. While a proton left on its own is stable, or at least has never been observed to decay, a neutron changes into a proton through the process of beta decay, with a half-life of just 10 minutes.

Combine this with the fact that their common positive charge makes all protons repel each other, and it seems a miracle that nuclei stay together at all. That they do is down to the trumping effect of the strong nuclear force, which binds together protons and neutrons over very small distances, albeit in constellations of varying stability.

Shells and liquid drops

Two very different models have helped researchers visualise the atomic nucleus in the century since its discovery. The way neutrons and protons appear to stick together, rather like molecules in a liquid, gave rise in the 1930s to the "liquid-drop model", which accurately predicts the binding energies of nuclei and the amount of energy fission or fusion processes will release, once factors such as charge repulsion between protons are taken into account.

The quantum-mechanical Pauli exclusion principle, meanwhile, teaches us that nucleons - protons and neutrons together - cannot all occupy the same energy states. In this picture they orbit in concentric energy shells, much as electrons are ordered into shells around the nucleus to complete our picture of the atom.

Just as a full electron shell makes an element peculiarly unreactive - a noble gas - a nucleus with just the right "magic" number of neutrons or protons to fill a shell gets a stability boost. If both proton and neutron shells are full, then the nucleus is "doubly magic". Examples of these favoured nuclei are oxygen-16 (8 protons and 8 neutrons), lead-208 (82 and 126) and helium-4 (2, 2) - this last being better known as the alpha particle.

Phil Walker is a professor of nuclear physics at the University of Surrey in Guildford, UK. He has published more than 250 papers on nuclear structure, focusing particularly on nuclear isomers, and works in various collaborations with researchers from across the world. He is currently on a year-long secondment to the European particle physics laboratory CERN, near Geneva, Switzerland.