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SCIENCE AND SOCIETY Only skin deep?

New research shows how alpha particles form in the ‘skin’ of an atom’s nucleus, write ROX MIDDLETON, JOEL HELLEWELL and LIAM SHAW

IMAGINE an atom: like a miniature solar system with the nucleus in the centre and electrons orbiting around like planets. 

Although the picture is familiar, analogies like this can mislead. New research published in Science last week by a team largely based in Darmstadt, Germany, emphasises that the atom is far stranger than our simple images. We still have much to learn about its inner citadel: the nucleus. 

The discovery of the atomic nucleus is a little over a century old. Earlier ideas about the atoms imagined them like tiny billiard balls. 

Then, in the early 20th century, the atom was proposed to be a sort of positively charged spongy material, with negatively charged electrons dotted throughout. 

In a stodgy Edwardian analogy, this was referred to as the “plum pudding” model (the electrons being the plums).

It was an experiment that proved this idea wrong. The physicist Ernest Rutherford had been looking at radioactive materials. He had found they could give off three sorts of radiation, naming the slowest and shortest form “alpha.” 

He then found that the particles of alpha radiation were the same as helium nuclei stripped of their electrons. These “alpha particles” could be accelerated due to their electric charge, perfect for firing at high speed into other materials. Thus an important experimental strategy for particle physics was born.

In 1912, Hans Geiger, co-inventor of the Geiger counter, was firing some alpha particles at a thin sheet of gold foil. 

He passed on the news to Rutherford that some alpha particles bounced back. Rutherford called this discovery “quite the most incredible event that has ever happened to me in my life.” He said that “it was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” 

To generate this rebound, Rutherford realised the gold atom’s positive charge was concentrated at a tiny centre: its nucleus. So, the atom was largely a void. A tiny nucleus at the centre, with electrons whizzing around the outside. 

The nucleus contained positively charged particles (protons). But it wasn’t until 1932 that James Chadwick discovered the existence of the neutron: a particle similar to the proton but with neutral charge, which lurked in the nucleus. 

Unlike alpha particles, which are repulsed by the positive charge of a nucleus unless they are given a lot of energy, neutrons can be fired straight into the nucleus itself and just slip inside it with relatively little energy. 

Especially for very heavy, very highly charged nuclei, neutrons opened up a whole new world of exploration. 

Physicists such as Enrico Fermi began to use neutrons experimentally, adding them to nuclei to see what happened. 

Fermi believed that by adding neutrons to uranium, the most massive element known (atomic number 92), he would at first produce an unstable variation on the nucleus. 

This destabilised nucleus would then undergo radioactive decay (a neutron would “split” into a proton and an electron) and so increase its number of protons by one, making an entirely new “transuranic” element with a higher atomic number: nuclear alchemy.

Fermi won a Nobel Prize in physics in 1938 for discovering “new radioactive elements produced by neutron irradiation.” The presentation speech noted his success in “producing two new elements, 93 and 94.” 

In fact, as suggested by chemist Ida Noddack, Fermi was totally wrong on this. The very next year it was shown that after destabilisation, the uranium nucleus had in fact split into two lighter elements, a process called fission. 

This staggering revelation on the brink of WWII changed the world forever. Within six years, it had led to the development of the first fission bombs dropped on Hiroshima and Nagasaki. 

Since then, nuclear physicists have established that, like the atom, the nucleus itself is not uniform. The distribution of neutrons extends out further than the distribution of protons, making an external neutron-rich region called a “neutron skin.” This has its own properties, just as the peel of an orange is very different to its flesh. 

Understanding more about the neutron skin can tell us more about the nucleus, by providing information about the interplay of forces used in mathematical models. 

In the new research published last week, researchers fired a 392 mega-electron-volt (MeV) proton beam from a particle accelerator at a target made of the element tin. 

For comparison, the most powerful cyclotron at the end of the 1930s was 16 MeV. 

These energetic protons knocked out alpha particles from the “neutron skin,” in order to discover how different parameters changed with density. 

There is a proposed relationship between the thickness of the neutron skin and how a “symmetry energy” relating protons and neutrons should change with nuclear density. Their work suggests that this relationship should be revised.

As well as telling us more about the atoms all around us, these results can be applied to imagining the properties of “neutron stars.” These exotic objects are the remnants of star explosions, when star-matter collapses to a bright dense lump, like an extraordinarily dense gigantic nucleus.

These were suggested just two years after the discovery of the neutron, although it was only in the 1960s that they were observed by astronomers including Jocelyn Bell Burnell and Samuel Okoye. 

We can’t visit a neutron star. But we can use our experiments on their tiny terrestrial relatives — nuclei — as models to understand them. 

The discovery of clustering inside the atom has profound implications for the inhomogeneity that might occur inside a neutron star heavier than the sun, and therefore what internal forces are within it. 

These new experiments suggest that alpha particles form in the neutron skin of the nucleus, rather than deep inside. 

Over a century since alpha particles revealed the secretive existence of the nucleus, we can start to imagine the tumult of forces inside them, and imagine alpha particles forming in the nucleus’s boiling surface — like clots in the skin of hot custard. 

But this is just an analogy, of course.  

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