One of the great conundrums of modern archaeology is balancing the need to understand what an ancient object is made from while simultaneously wanting to preserve it and cause as little damage as possible. In some cases, it is possible to take a small sample of the object for analysis (usually achieved by scraping away surface material or, for deeper samples, drilling a core), but this is a destructive process and is unthinkable for delicate or rare objects.
Whenever scientists, archaeologists, art historians or restorers come across a relic of historical or artistic interest, they want to understand its place or origin, date of manufacture, how it was produced and what it was made from. They need this information not just to place it into historical context, but also in order to best understand how to preserve or restore the object. While much of this information can come from the item's location and context during an excavation or from previous historical records, that is not always the case.
Sometimes the most interesting insights to an artefact's past are literally hidden beneath the surface. In the case of the Roman Empire, historians speculated that when the Roman economy was under stress, silver coins would be debased by the addition of copper. However, there was no way of testing this theory without damaging the coins. ISIS offers users access to the technique of muonic atom X-ray spectroscopy, which has the ability to look at the different layers of an artefact non-destructively, providing much more detail into not just what the artefact is made of but how it was made, adapted, reused or corroded over time. Crucially, the technique can do this without damaging the artefact being studied.
The use of muons to study cultural heritage artefacts is a growing area, driven by both the need to discover the internal composition of the objects, and the development of new techniques in muon spectroscopy. Muonic atom X-ray Spectroscopy has the potential to allow scientists to identify the chemical make-up of any material non-destructively, vital for conservation and preservation.
While these techniques have been used before, the latest experiments at ISIS aim to take them to new levels and apply them to actually build a 3D model of the artefact that shows its full chemical composition, without so much as scratching the surface. These recent experiments were carried out at the RIKEN-RAL facility at ISIS, where muons are produced by accelerating protons and smashing them in to a carbon target.
The muons created by these collisions are implanted into the sample under study. Muons are the short-lived, heavy-weight cousins of the humble electron – weighing in at 207 times the mass of the electron. Muons are ideal for implanting to samples because, although they undergo numerous interactions with atoms when passing through matter, they lose very little energy in each collision. As they are so much more massive than electrons that they just brush them aside and carry on going until they captured by an atom deep within the sample.
When a negative muon is captured by an atom (creating a muonic atom), it displaces an electron and causes the atom to emit energy in the form of X-rays. Because they have a mass more than 200 times greater than an electron, muons can generate powerful X-rays that can pass back out of the sample and be detected by our instruments. The X-rays are unique to the atom that emitted them – making it possible to identify the elements inside the sample. In this way, the ISIS team are able to create chemical profiles of the objects and identify their chemical composition.
Users of ISIS have previously used the technique to study the metallurgical make-up of Roman coins, of 15th Century cannons and of Japanese swords but, in their latest experiments, they turned the metal-probing power of muons on to bronze fragments from Sardinia. The objects under study were fragments of votive lamps (burned during prayer) in the shape of small ships dating from the 8th to 9th century BCE.
The tests have revealed that the technique is sensitive to all chemical elements and that it is possible to target the muons to specific depths with the object – tunable from between just a few micrometres to several centimetres. This means that it is possible to build a complete chemical profile at a variety of depths – making it possible to produce three-dimensional maps of the sample's elemental composition… all without so much as scratching the object under study.
The work, published in the Journal of Radioanalytical and Nuclear Chemistry, will pave the way for future studies on other archaeological artefacts, engineering samples, biological systems, and battery materials and all without damaging the object being studied.
The full paper can be found online at: https://doi.org/10.1007/s10967-019-06506-9.