At the Reactor Institute Delft, researchers uncover the internal structure of centuries old museum artefacts by scanning them with neutrons. “From the radioactivity that the objects briefly absorb during this process, we can determine what materials were used hundreds of years ago,” says PhD student Yueer Li.
PhD student Yueer Li with a copy of the figurine she examined with neutrons at the RID. (Photo: Lambert van Eijck)
Every time physicist Yueer Li wants to take measurements, she first has to enter the nuclear reactor at the Reactor Institute Delft (RID) through a heavy yellow airlock. There, behind the metal doors of a grey cabinet, a small Indonesian bronze figurine from the ninth century AD – called Kuvera – was waiting for her last June.
Li revealed Kuvera’s internal structure by scanning the figurine with neutrons from the reactor while it sat on a pedestal next to the RID’s nuclear reactor. This made the figurine briefly radioactive. To allow it to ‘cool down’, Li placed it near a gamma spectrometer positioned nearby – a step most researchers consider an inconvenient wait, but one that was essential to Li’s work.
Gamma radiation
Scanning historical metal objects with neutrons is not new to the RID. Li is conducting her research under the supervision of neutron expert Dr Lambert van Eijck from the Faculty of Applied Sciences. His group previously scanned a Van Leeuwenhoek microscope and a 3,500 year old ceremonial sword using neutrons. Because metal objects absorb some of the particles, scanning from various angles allows researchers to build a picture of the object’s interior.
“You also get gamma radiation in the process, which we used to ignore,” Li explains. “So far, that radiation hadn’t been factored into analyses. We’ve now shown that gamma radiation can reveal the materials that museum artefacts are made of without damaging them.” The research was published in Nature late last year.
Hidden foil
The study of the Kuvera figurine was done in collaboration with the Rijksmuseum Amsterdam. Sara Creange, a restaurator at the museum, had contacted Van Eijck about a small piece of foil sticking out from the base of the hand-sized statue. “People often used to hide things in figurines like this,” Van Eijck explains. “According to legend, it would bring the deity in the statue to life so it could be worshipped in a temple. Our technique allowed us to examine what was hidden in the base of the Kuvera.”
The base of the Kuvera did indeed contain a piece of foil
Although the project originally began in 2019, Li picked it up again in 2022 after a three-year wait for reactor renovations. The neutron scan revealed that the base of the Kuvera did indeed contain a piece of foil. Li and Van Eijck decided to analyse it further using gamma spectroscopy – a technique that examines gamma radiation. Since every element emits its own characteristic gamma wavelength after neutron absorption, this method allowed them to identify the materials in both the figurine and the foil.
“Initial analyses suggested the foil was mostly gold as there was a strong signal peak from that element,” Li says. “But after running simulations, we found it was mostly silver with a small amount of gold.” For Van Eijck, these simulations became a key part of the work. “Much of what we know in art research comes from surface-level analysis. Thanks to simulations like these, we can now pinpoint an object’s internal composition at the time it was made just by measuring gamma radiation.”
Simulations
To simulate the signals, Li used the neutron scans to recreate the figurine in a computer model. She gave the virtual Kuvera various material compositions and masses, and calculated what kind of signals the gamma spectroscopy should emit. “By comparing the predicted signals with the actual measurements and tweaking the simulation until they matched, we could determine what materials were inside the figurine,” she explains.
Surprised by how accurately Li’s results matched the predictions
Li used Geant4, special software developed by CERN, the particle physics laboratory in Switzerland. It includes a database of elements and their associated gamma emissions.
When Li calibrated her model using a piece of copper in the neutron beam, Van Eijck was surprised by how accurately her results matched the Geant4 predictions. “All the predicted particle interactions were spot on,” he says. “No complex calibration was needed. We’re now trying to explain this to colleagues so they’ll start using gamma spectroscopy more often too.”
Risks and awards
Before the figurine could be taken to the reactor, curator Sara Creange checked the humidity and temperature at the RID and assessed the impact of the neutron beam. “The Rijksmuseum has a long tradition of working with scientists to study our objects,” she says. “The risks involved in transporting the object are acceptable if you consider the information you gain in return.”
Creange is highly enthusiastic about the results. “We have an X-ray machine at the Rijksmuseum, but it can’t penetrate very far into the material. I’m thrilled that the neutrons and gamma radiation enabled us to see the distribution of elements inside the figurine.”
Travel trough stack of slices of neutron tomography of Indonesian figurine. (Data & processing: Yueer Li, RID)
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