Excavating Jerusalem with Cosmic Rays

Team maps ancient Jerusalem with muon detectors

Mapping Jerusalem with Cosmic Rays

Mapping Jerusalem with Cosmic Rays: Aerial view of the City of David. Courtesy AVRAM GRAICER, CC BY-SA 3.0, via Wikimedia Commons.

One of the most difficult ancient sites to excavate, Jerusalem is rife with archaeological mysteries. With excavation limited to specific and often narrow parts of the age-old city, archaeologists are often forced to think outside the box to answer their questions. Enter a team of archaeologists and physicists from Tel Aviv University (TAU) who are using cosmic rays, in the form of muons, to map the ancient city without ever even picking up a trowel.

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When Archaeology Meets Sci-Fi

Every second, cosmic rays from interstellar space smash into the earth’s atmosphere, slamming into various materials and showering the ground below with a rain of subatomic particles created by these collisions. Among these particles are muons, one of the basic building blocks of the universe. Traveling from the upper atmosphere to the ground at nearly the speed of light, these high-energy particles could be the key to unlocking many of Jerusalem’s archaeological mysteries. A group of TAU archaeologists and physicists are now using these subatomic particles to map subterranean spaces in areas that they would never be able to excavate, including tunnels and caverns underneath Jerusalem,

While the muons that hit the ground have a wide spectrum of energy, when they penetrate the surface, they lose energy at a known rate. Thus, by setting up a muon detector below the surface, the team can measure just how much energy the muons lose. Cross-referencing that with the distance from the detector to the surface, they can detect subterranean voids, such as ancient tunnels. Triangulating this data across multiple muon detectors, scientists can then build a map of these subterranean spaces. In the early 2020s, archaeologists in Egypt used a similar method to identify a previously unknown corridor hidden in the pyramid of Khufu. What could this technique find in Jerusalem?

One hope is to identify possible passageways leading to the area of the Gihon Spring, the city’s ancient water source. “All ancient cities needed a water supply and fortified, secret ways to get there in case of emergency or war,” said a project coordinator and TAU archaeology professor Oded Lipschits in an interview with the Times of Israel. The muon detector that the team set up near the Gihon Spring is one of a handful of detectors installed around the City of David. That particular detector is pointing towards the ancient Temple Mount in hopes of finding possible passageways.


A look at a new muon detector in development. Courtesy Nathan Steinmeyer, BAS.

The project, however, is currently in its infancy and operates in many ways as a pilot program and a field test for the technology. The brainchild of Lipschits and TAU physicist Erez Etzion, the project was originally planned for Tel Azekah, one of the sites where Lipschits excavates. It was eventually decided, however, that Jerusalem would be a better pilot site. Since the muon detectors capture particles falling from the atmosphere, they need to be located below the thing they want to study. While it would have been difficult to find a suitable location at a site like Azekah, Jerusalem, with its many caves and tunnels, offered lots of places to install the detectors.


Etzion holding the muon detector that his team sent to the International Space Station. Courtesy Nathan Steinmeyer, BAS.

Using their experience building similar detectors for experiments at CERN and even the International Space Station, the TAU physicists, in collaboration with the Faculty of Engineering and Rafael LTD, built the muon detectors in-house. After all, it is not the sort of thing that can be picked up at the local hardware store. Two of the Jerusalem detectors are what Etzion refers to as third-generation detectors, building on an older concept that was first used in archaeology in the 1960s. In an interview with Bible History Daily, Etzion explained that these detectors, roughly 2-foot-tall cubes, can take up to a year to build, as each new detector is its own new experiment and field test. While the detectors used at CERN and the space station operate in tightly controlled environments, these need to function in the damp and dirty conditions of subterranean Jerusalem.

The first detector the team set up in Jerusalem could not even be assembled in the lab. As the entryway to the cave was too small to accommodate the device, they needed to carry it piece by piece and assemble it on-site. After having the detector break because of the cave’s high humidity, which was only resolved through a special containment tent and hoses that piped out the moisture, the team finally got the machine working. Despite this, it will still take months to get any usable data. Although there is a constant barrage of muons onto the surface of the earth, only a few make it to the required depth, and the size of the detector has a large impact on how many of those limited particles are recorded.

In the future, the team hopes to have ten or more detectors working around the clock in the City of David, but this is easier said than done. On top of taking so much time to make, each gen-three detector costs more than 100,000 dollars. Luckily, the TAU team is already on their way to putting out a new fourth-generation device, which drastically simplifies the detector’s hardware without reducing its effectiveness. But even with these new detectors, the project could still end up costing millions of dollars to carry out, and the team hopes that their early advancements on both the archaeological and technological sides will encourage outside investment in the project.

Read more in Bible History Daily:

Hidden Corridor Found Inside the Pyramid of Khufu

Cold War Spy Satellites Aid Archaeologists

All-Access members, read more in the BAS Library:

Hi-Tech Archaeology: Ground-Penetrating Radar—New Technology Won’t Make the Pick and Trowel Obsolete

Jerusalem and the Holy Land(fill)

Not a BAS Library or All-Access Member yet? Join today.

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