Studying precious pearls using novel neutron imaging technique

30 January 2018

Pearl in a shell

STFC scientists have used neutron imaging to study pearls
(Credit: Pixabay)

A new, non-destructive neutron technique has been used to study the inner form of one of mankind’s most precious biological objects – the pearl.

This technique could be used to help differentiate between the highly valuable naturally-formed pearls from the less desirable farm-cultured variety.

This imaging study looked at the aptly-named soufflé pearl, which is so-called because of its empty core.

Using the IMAT instrument at the Science and Technology Facilities Council’s ISIS Neutron and Muon Source, every square centimetre of pearl sample was bombarded with 5.9 million neutrons per second. A powerful camera was then used to map the neutrons and build a detailed tomographic reconstruction of the pearl.

The findings from this research could be used by gemologists to non-destructively infer the pearl-growing process of the sample – a result which is of importance to this market.

IMAT scientist Dr Winfried Kockelmann said: “By utilizing the unique capabilities of the new neutron imaging instrument we are able to non-invasively study pearls, and other precious objects, and build microstructure maps of their interiors, in addition to the usual neutron CT images produced on IMAT.

“We hope that by studying the differences between cultured pearls, like the soufflé, and its naturally-occurring rivals, we will be able to come up with a quick and easy test to establish whether a pearl has been cultured or not.”

Any mollusc with a shell, such as an oyster, can produce a pearl. Natural pearls are the result of an accidental event, possibly the invasion of a tiny organism into the living molluscs’ tissue, which activates the same biogenic mechanisms responsible for the formation of the shell of the mollusc.  It takes years for a fully formed pearl to grow, and less than one pearl oyster in 10,000 will produce this rare gem, which is what makes natural pearls so highly valuable.

The 1900s marked a turning point in the pearl industry as lower cost cultured pearls began to flood the market. Pearl farmers were able to produce large quantities of pearls under controlled conditions and as culturing techniques advanced it became harder to distinguish between natural and cultured pearls – but this is a vital requirement for the jewellery market.

The global production for cultured saltwater pearls is now worth over $400 million. However, in that marketplace ‘natural pearls’ are considered superior, partly as the nacre – the substance that makes up the outer layers of a natural pearl – will not wear out for generations whereas cultured pearls can wear out if you use them heavily.

The international research collaboration, which includes scientists from the UK, Italy, Japan and the USA, will be publishing its findings in Microchemical Journal, not only providing detailed information on the pearl but also highlighting the potential of neutron imaging on biological samples.

Giuseppe Vitucci from the Italian team at the University of Milano-Bicocca said: “Despite their well-known tiny aspect, each pearl hides tons of peculiarities in its morphology, as well as in its atomic composition, some of which cannot be detected using the standard x-ray technology. By contrast, the cold neutron pulses available at the IMAT beamline, together with the new energy-resolving detection system, may reveal such ‘cool’ secrets. No pun intended.”

During this investigation, an X-ray image was also taken of this cultured pearl for comparison. While the vacuum core and irregular morphology were distinguishable, further details of this sample were not easily recognisable using X-ray alone.

Neutron imaging cannot compete with the resolution of X-ray imaging but it does have several other advantages. As neutrons are electrically neutral they interact weakly with most materials and therefore penetrate them deeply. Additionally, neutrons are highly sensitive to certain elements that X-rays aren’t, such as hydrogen, due to their large neutron scattering cross sections. Neutrons are also able to distinguish between isotopes of the same element as, unlike X-rays, they interact with the nucleus of the atom.

IMAT scientist Dr Triestino Minniti added: “In this study the key point is the connection of the specimen morphology, obtained non-destructively by means of a neutron CT scan, with its mineralogy and microstructure properties. For the latter, a precise definition of the materials lattice parameters are required that could only be obtained with sufficient resolution at a neutron imaging instrument like IMAT at ISIS, rather than one located at a steady-state neutron source.”

Further investigations using neutrons of different energy ranges may allow a more complete description of similar biological samples and form the basis for upcoming work at the facility.

Despite the fact that IMAT is still undergoing commissioning, this instrument is already showcasing the potential of neutron imaging as an innovative tool to study biological samples. The future applications of this young imaging and diffraction instrument are expected to span a variety of industries from aerospace to earth science.

Find out more about the IMAT instrument at ISIS Neutron and Muon Source.

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Becky Parker-Ellis
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