The ruins of the ancient necropolis of Neapolis lie 10 meters (about 33 feet) below modern-day Naples, Italy. But the site is in a densely populated urban district, making it difficult to conduct careful archaeological excavations of those ruins. So a group of scientists turned to cosmic rays for help—specifically an imaging technique called muography, or muon tomography—and discovered one previously hidden under the burial chamber, according to a recent paper published in the Scientific Reports journal.
As we have reported, there is a long history of using muons in image archaeological structures, a process facilitated because cosmic rays provide a steady supply of these particles. An engineer named EP George used them to make measurements of an Australian tunnel in the 1950s. But Nobel Prize-winning physicist Luis Alvarez put muon imaging on the map when he collaborated with Egyptian archaeologists to use the technique to search for hidden chambers in the Pyramid of Khafre at Giza. Although this works in principle, they haven’t found any hidden rooms.
Muons are used to it too hunting for illegal carry nuclear materials at border crossings and to monitor active volcanoes in hopes of determining when they might erupt. In 2008, scientists at the University of Texas, Austin, tried to follow in Alvarez’s footsteps, reusing old muon detectors to search for possible hidden Mayan ruins in Belize. And physicists at Los Alamos National Laboratory have developed portable versions of the muon imaging system to unlock the secrets of the construction of the soaring dome (Il Duomo) over the Cathedral of St. Mary of the Flower in Florence, Italy, designed by Filippo Brunelleschi in the early 15th century. The dome has been plagued by cracks for centuries, and muon imaging may help preservationists figure out how to repair it.
In 2016, scientists were using muon imaging signals are picked up indicating a hidden corridor behind the famous chevron blocks on the north face of Great Pyramid of Giza in Egypt. The following year, the same team found a mysterious void in another area of the pyramid, believing it to be a hidden chamber, which was then mapped out using two different muon imaging way.
There are many variations of muon imaging, but they all generally involve gas-filled chambers. As the muons zip through the gas, they collide with gas particles and emit a bright flash of light, which is recorded by the detector, allowing scientists to calculate the particle’s energy and trajectory. It is similar to X-ray imaging or ground-penetrating radar, except naturally occurring high-energy muons rather than X-rays or radio waves. That higher energy makes it possible to image thick and dense substances like the rocks used to build the pyramids. The denser the imaged object, the more muons are blocked, producing a bright shadow. Hidden rooms appear in the final image because they block fewer particles.
Neapolis was a Hellenistic city in a hilly area rich in volcanic tuff rock. That became soft enough to carve tombs, places of worship, or caves for habitation. The necropolis in what is now the Sanita district of Naples was one such creation, used for burials from the late fourth century BCE to the early first century CE. The site was buried in sediment over time by a series of natural disasters, most notably the flooding of lava dei virgini (“lava of virgins”). Unlike the volcanic lava that famously covered Pompeii, this “lava” was made up of mud and rocks that had been loosened from the hills during heavy rains.
The exact size of the necropolis is unknown, but it probably contained dozens of tombs, each holding multiple bodies. Four such tombs, now known as the Ipogeo dei Cristallini, were discovered in the late 19th century under the palace of the di Donato family, located on Cristallini Street. (The current owners the tombs were opened for public tours last year.) After the 1980 earthquake prompted a 3D structural analysis of the area, two more burial chambers were discovered: the Ipogeo dei Togati and the Ipogeo dei Melograni.
Those discoveries raised hopes of finding more hidden burial chambers. Researchers at the University of Naples Federico II, the National Institute of Nuclear Physics in Naples, and the University of Nagoya in Japan thought muography was an ideal way to do this. “Due to its non-invasive nature, this method is particularly suitable for urban environments where applications of active inspection methods such as seismic waves or bore holing are unthinkable,” wrote the authors. They base their detectors on nuclear emulsion technology; Emulsion detectors are simple, very compact, and do not require an external power supply.
Nuclear emulsion developed from radiation research in the early 20th century. Physicist Ernest Rutherford began using commercial photographic plates to capture the alpha rays emitted from various radioactive materials, which would cause the plates to darken. His colleague, Kinoshita Suekiti, modified the basic technique, preparing emulsion films of gelatin with a high concentration of very fine silver halide grains to detect alpha particles. Those particles act as sensors, triggered when a charged particle passes through the emulsion and loses its charge. Once recorded, the tracks can be seen under a microscope after the plates are formed, allowing scientists to measure their position and direction.
The nuclear emulsion is popular in the study of cosmic rays, leading to the discovery of pi-mesons and parity violation in K-mesons. When more powerful particle detectors and accelerators came on the scene, the nuclear emulsion fell out of use. However, it is still useful in certain physics experiments (eg OPERA and CERN FASER project) as well as medical, biological, and archaeological applications.
The authors of the necropolis study used two detector modules, each with a stack of four emulsion films sealed inside an envelope to control light and humidity, and left them in a underground cellar (formerly used for ham aging) from March 10 to April 7, 2018 Once the exposure period is over, the emulsions are made the next day. For comparison purposes, they relied on a 3D laser scanning spatial model of all accessible underground structures.
The muography revealed evidence (in the form of an excess of the expected number of muons) for a new underground structure, as well as enough information for the authors to estimate the structure’s size and position. that. The structure is consistent with a rectangular burial chamber, probably partially filled with stone and sediment. It measures 6.5 by 11.5 feet (2 by 3.5 meters) and is similar in structure to Ipogei dei Togati and Ipogeo dei Melograni. According to the authors, the burial chamber was built in the late fourth century/early third century BCE, and was probably the tomb of a wealthy individual or family.
DOI: Scientific Reports, 2023. 10.1038/s41598-023-32626-0 (About DOIs).