Neutron beams used to authenticate historic coins

By imaging antique coins with beams of low-energy neutrons, a research team led by the US National Institute of Standards and Technology (NIST) has demonstrated a way to distinguish century-old coins from fakes. Authenticating coins is critical because scientists rely on them to chronicle the economic, political and scientific developments of nations.

The US–Korean research team used neutrons to examine two Korean coins — one minted in the 1800s, the other a replica — because these subatomic particles penetrate heavy metals, such as copper, iron and lead, and interact strongly with hydrogen-bearing compounds that form as a by-product of corrosion. The location and pattern of corrosion within the two coins, both composed of copper alloys, provided hallmarks for verifying their age.

The team employed two complementary imaging techniques. One of the methods, known as neutron tomography, uses a beam of neutrons to take a series of two-dimensional images of an object from different perspectives as the object rotates; these two-dimensional snapshots are then combined to reveal the three-dimensional structure of the coins. The other method, known as neutron grating interferometry, records neutrons scattered at small angles and homes in on microstructures, such as pitting and pores within the coins, that are signs of corrosion.

In the authentic coin, the researchers found that corrosion had penetrated deep within the body, indicating that the degradation was a gradual process that occurred over many decades. In contrast, corrosion in the recently minted replica was mainly confined to the surface, consistent with rapid corrosion over a short time period.

The team also used neutron grating interferometry to examine the size of pores within the coins, which provided another method to distinguish between the historic coin and the fake. Pores are created when coins buried in soil or exposed to moisture chemically interact with their environment. The interaction causes metallic compounds to leach out of the coins, leaving behind millimetre-sized holes.

As time goes on, however, compounds in the environment begin to penetrate the coins. These compounds, which includes corrosion by-products such as copper carbonates, sulfates and chlorides, pack together and fill the pores, diminishing their size. Indeed, the imaging revealed that the historic coin, which was exposed to its environment for a much longer period of time, had much smaller pores — only micrometres to nanometres in size — compared to the millimetre-sized pores in the replica.

With their initial study now published in the journal Scientific Reports, the scientists plan to continue their work with a larger supply of Korean coins; it could also be applied to a broader range of metallic artefacts from a diversity of cultures. Neutron-imaging methods can also assist conservation efforts by determining the amount and locations of corrosion in authentic coins, suggesting areas that would benefit from a protective coating.

Image credit: iStock.com/songE