By BBC News Online science editor Dr David Whitehouse

Physicists have made an atomic nucleus that contains a record eight more protons than neutrons.

This makes the new nucleus, nickel-48, unstable and it self-destructs in a minute fraction of a second. Researchers say, however, its very existence allows them to observe a form of radioactive decay that has long been predicted but not yet seen.

All but the simplest atoms consist of a nucleus of positively charged protons and neutral neutrons, surrounded by a cloud of orbiting electrons.

But the atomic nucleus can become unstable. Protons and neutrons stick to one another because of a nuclear force - the strong force.

Stable nucleus

However, the protons, being all of the same electrical charge, repel each other. And if there are too many protons when compared to neutrons, then the electric force of repulsion dominates and the nucleus splits apart. That is why almost every nucleus in nature has more neutrons than protons.

Another property of the atomic nucleus is that the protons and neutrons within it can stabilise the nucleus by arranging themselves into shells of different energy.

Nuclei with complete nuclear shells tend to be more stable. Scientists say that nuclei with a full shell of neutrons and protons are "doubly magic".

And nickel-48, with its 20 neutrons and 28 protons, is one of 10 or so nuclei that have a magic number of both protons and neutrons, which should give it enhanced stability - even though the protons outnumber the neutrons.

Atomic debris

The nickel-48 was created by a team of French physicists working at the Grand Accélérateur National d'Ions Lourds, in Caen, France.

To do it, the researchers smashed a beam of nickel-58 nuclei (28 protons and 30 neutrons) into a nickel-58 target. The collision produced two nickel-48 nuclei among millions of other pieces of atomic debris.

Writing in the Physical Review Letters, the scientists say that nickel-48 is a prime candidate to study so-called two-proton decay, a type of nuclear change that has never been observed.

The two nickel-48 nuclei lasted only fleetingly, but long enough for researchers to be able to do experiments on them and examine their properties in detail.