image: . Schematic illustration of cosmic-ray muons penetrating the overburden from various angles. Muons continually bombard the ground at a known rate and angular distribution. As muons lose energy when passing through matter, their flux is attenuated depending on the integrated density along their path. A higher-than-expected flux from a specific direction indicates reduced attenuation, suggesting the presence of a void or low-density region (marked here as yellow lines). The muon telescope detects these angular flux variations to create maps of subsurface structures.
Credit: Illustration from the article
A technological breakthrough at Tel Aviv University revolutionizes the world of archaeology: a proof of feasibility for using cosmic radiation detectors to discover underground spaces. The detectors identify muons – particles created when cosmic radiation collides with Earth’s atmosphere, which penetrate the ground before losing their energy and coming to a stop. Thus, by detecting muons, archaeologists can map hidden voids such as tunnels and channels. The research team demonstrated the technology's effectiveness at the City of David archaeological site in Jerusalem, showing how the system successfully maps underground spaces based on changes in the soil's absorptivity to cosmic radiation particles.
The study was led by Prof. Erez Etzion from TAU's Raymond and Beverly Sackler School of Physics and Astronomy, and Prof. Oded Lipschits from TAU's Jacob M. Alkow Department of Archaeology and Ancient Near Eastern Cultures. Other participants included: Prof. Yuval Gadot from the Department of Archaeology and Ancient Near Eastern Cultures; Prof. Yan Benhammou, Dr. Igor Zolkin, and doctoral student Gilad Mizrachi from the School of Physics and Astronomy; Dr. Yiftah Silver and Dr. Amir Weissbein of Rafael Advanced Defense Systems; and Dr. Yiftah Shalev of the Israel Antiquities Authority. The study's results were published in the Journal of Applied Physics.
“From the pyramids in Egypt, through the Maya cities in South America, to ancient sites in Israel, archaeologists struggle to discover underground spaces,” says Prof. Lipschits. “Above-ground structures are relatively easy to excavate, and there are also various methods for identifying walls and structures below the surface. However, there are no effective methods for conducting comprehensive surveys of subterranean spaces beneath the rock on which the ancient site is situated. In the Judean Foothills, for example, the top layer of hard limestone overlies soft chalk, in which the ancients easily carved out vast spaces for water reservoirs, agricultural uses, storage, or even dwellings. Clearly, in such regions, most above-ground archaeological sites resemble Swiss cheese beneath the rock, but we have no way of knowing this. If by chance we excavate above ground, reach the rock, and identify an entrance to a cavity, we could excavate it, but we have no way of locating the subterranean spaces in advance. In the current study, we propose for the first time an innovative method that has been proven very effective in detecting underground spaces with detectors of cosmic radiation, specifically muons.”
The researchers explain that a muon is an elementary particle similar to an electron but 207 times more massive. Muons are created in the atmosphere when energetic particles, mainly protons, collide with the nuclei of molecules in the air. This collision generates unstable particles called pions, which decay very quickly into muons. Muons also have a very short lifetime, decaying after 2.2 microseconds, but they move at speeds close to the speed of light, and thanks to Einstein’s special relativity theory, many of them manage to reach and penetrate the ground.
“The muon shower hits the ground at a fixed and known rate,” explains Prof. Etzion. “Unlike electrons, which are stopped by the ground at just a few centimeters deep, muons lose energy slowly as they pass through the ground, and some can penetrate much deeper – even up to 100 meters for highly energetic particles. Therefore, by placing muon detectors underground and monitoring the environment, we can identify empty cavities where energy loss is minimal. This process is similar to X-ray imaging: the X-ray beam is stopped by bones but passes through soft tissue like flesh or fat, and a camera on the other side captures the resulting image. In our case, the muons act as the X-ray beam, our detector is the camera, and the underground features are the human body.”
As noted, the researchers conducted an impressive demonstration in a rock-hewn installation known as Jeremiah’s Cistern at the archaeological site of the City of David. Combining a high-resolution LiDAR scan of the interior cavity with simulations of the muon flux, they were able to map structural anomalies. Detecting changes in soil penetrability to muons, the system demonstrated the feasibility of using muon tomography for archaeological imaging.
“This article is a first milestone,” says Prof. Lipschits. “We ask physicists to respond to the archaeological need and develop smaller, simpler, cheaper, more durable, more accurate, and more power-efficient detectors. In the next stage, we intend to combine physics and archaeology with AI to produce a 3D image of the subsurface from the vast data generated by the detectors. Our test site will be Tel Azekah in the heart of the Judean Foothills, overlooking the Elah Valley.”
“This is not our invention,” adds Prof. Etzion. “Already in the 1960s, muons were used to search for hidden chambers in the pyramids in Egypt, and recently the technology was revived. Our innovation lies in developing small and mobile detectors and learning how to operate them at archaeological sites. After all, there is a difference between a detector in laboratory conditions and a detector that must be taken to a cave or excavation, where practical problems of electricity, temperature, and humidity inevitably arise. Detection ranges depend on measuring time; the farther the detector's location, the fewer particles reach it, but realistically, it is possible to analyze images from a distance of up to 30 meters within a reasonable timespan. Therefore, our goal is to place several detectors or move one detector from place to place to produce a 3D image of the entire site eventually. And we have just begun. The next stage involves sophisticated analysis, which will allow us to map everything beneath our feet – even before the excavation begins.”
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