The core principle behind this technology lies in the unique properties of muons, which are elementary particles similar to electrons but approximately 207 times heavier. They originate in the upper atmosphere when high-energy cosmic protons collide with molecular nuclei, producing pions that decay rapidly into muons. Despite their fleeting intrinsic lifetime of just 2.2 microseconds, muons travel at velocities nearing the speed of light, enabling many to reach and penetrate the Earth’s surface. This phenomenon is further explained by Einstein’s theory of special relativity, where time dilation allows muons to survive long enough to be detected underground.

Unlike electrons, muons feature a remarkable capacity to traverse dense materials. While electrons are typically impeded and halted after penetrating only a few centimeters of soil or rock, muons gradually lose energy and can pass through several meters, even reaching depths of up to 100 meters depending on their initial energy levels. This slow attenuation is crucial for archaeological applications, as variations in muon flux correspond directly to differences in the density and composition of subsurface structures. Areas containing voids or less dense materials cause less muon absorption, resulting in a higher measured flux that points to the presence of underground cavities.

The analogy often employed to explain this process is reminiscent of medical X-ray imaging, where dense bone tissue blocks X-rays, casting shadows on the image, while softer tissues allow passage of the rays. Similarly, in muon tomography, the muons serve as a natural, cosmic X-ray beam, the detector functions as the capturing camera, and the underground geological formations act as the absorptive tissues. This framework enables comprehensive three-dimensional imaging of hidden features below the surface, without disturbing or excavating the site.

In an impressive demonstration of the technology’s potential, the research team conducted their experiments at the City of David archaeological site in Jerusalem, utilizing a rock-hewn chamber known as Jeremiah’s Cistern. By integrating high-resolution LiDAR scanning data of the cistern’s interior with sophisticated computer simulations of muon flux attenuation, the scientists successfully mapped structural anomalies indicative of hidden subsurface features. This interdisciplinary methodology validates the feasibility of muon tomography for archaeological imaging and presents a new avenue for non-invasive exploration.

The study was spearheaded by Professor Erez Etzion from Tel Aviv University’s Raymond and Beverly Sackler School of Physics and Astronomy and Professor Oded Lipschits from the university’s Jacob M. Alkow Department of Archaeology and Ancient Near Eastern Cultures. The diverse team also included experts from Israel’s Rafael Advanced Defense Systems and the Israel Antiquities Authority, reflecting the highly collaborative nature of this research. Their findings were published in the prestigious Journal of Applied Physics, marking a milestone in applied archaeological science.

One critical challenge addressed by the research was the adaptation of muon detectors for fieldwork in archaeological contexts. Unlike controlled laboratory environments, excavation sites present logistical difficulties including limited power supply, variable temperature and humidity conditions, and cumbersome terrain. To overcome these hurdles, the team engineered compact, mobile, and power-efficient muon detectors capable of operating robustly in situ. These advances enable prolonged monitoring and data collection essential for creating detailed subsurface maps.

The muon detection technique holds promise especially in regions like the Judean Foothills, where archaeological sites are layered with hard limestone overlying softer chalk. Historically, ancient inhabitants exploited this geology by carving out extensive networks of reservoirs and other subterranean structures, which remain largely invisible to conventional survey methods. Now, the ability to identify these “hidden cavities” could revolutionize the manner in which archaeological excavation and preservation are planned, prioritizing sites with known subsurface features.

Looking ahead, the research team envisions integrating artificial intelligence and machine learning algorithms to interpret the vast quantities of muon detection data collected across multiple detectors. This computational approach aims to reconstruct intricate three-dimensional images of entire archaeological sites, synthesizing raw muon flux readings with geological and structural information. The next phase of application is planned for Tel Azekah, a strategic and historically significant site situated in the heart of the Judean Hills overlooking the Elah Valley.

While the concept of muon radiography is not entirely new—it dates back to the 1960s when scientists sought hidden chambers in the Egyptian pyramids—this study represents a transformative advancement by tailoring the technology specifically for archaeology with enhanced mobility, durability, and sensitivity. The team’s innovation lies in practical deployment within challenging excavation settings and real-time data analysis, expanding the scope and precision of underground exploration beyond previous limits.

Moreover, the muon detection method offers enormous potential beyond archaeology. Its sensitivity to density variations can aid geological surveys, civil engineering projects, and even homeland security by detecting hidden tunnels or voids beneath critical infrastructures. By refining detector design and improving data processing techniques, the range and resolution of muon imaging are expected to grow, propelling this cosmic particle-based technology into wider scientific and practical arenas.

To summarize, this pioneering research fosters a new era in archaeological methodology by exploiting the naturally occurring muon radiation shower that the Earth continually receives. Through detecting and analyzing changes in muon absorption by subsurface materials, archaeologists now have a powerful, non-invasive tool to visualize hidden spaces beneath ancient sites. This innovation not only preserves the integrity of invaluable historical locations but also paves the way for unprecedented discoveries waiting beneath the ground.

Subject of Research: Muon tomography for archaeological subsurface imaging

Article Title: First Demonstration of Underground Muon Imaging at the City of David Archaeological Site

News Publication Date: Information not provided in the source content

Web References: https://pubs.aip.org/aip/jap/article/138/8/084504/3361099/First-demonstration-of-underground-muon-imaging-at

References: Journal of Applied Physics, DOI: 10.1063/5.0273376

Image Credits: Illustration from the original article linked on EurekAlert.org

Keywords: Archaeology, Cosmic rays, Muon tomography, Subsurface imaging, Particle physics, Muon detectors, LiDAR integration, Non-invasive surveying, Judean Foothills, Tel Aviv University

The importance of muon veto systems cannot be overstated in the realm of neutrino observation, especially those experiments conducted close to ground level or in environments with low intrinsic background noise. Cosmic muons, being highly penetrating particles, generate secondary neutrons and radioactive isotopes when interacting with detector materials, complicating the extraction of authentic neutrino signals. At the forefront of innovation, the TAO experiment addresses these concerns with its new top veto tracker system, a crucial component that promises to markedly improve the fidelity of neutrino measurements.

Central to the tracker’s performance is its unique configuration of 160 plastic scintillator modules, each composed of elongated PS strips intricately embedded with WLS-fibers. These fibers are not arranged arbitrarily; instead, they follow an optimized bending pattern within the scintillator matrix that ensures maximal light collection and transmission. The fibers channel scintillation photons uniformly towards fiber focusing readouts, where SiPMs convert the captured light into electrical signals with exceptional sensitivity. This design synergy elevates light yield significantly beyond conventional standards, addressing a critical parameter for effective muon detection.

According to Prof. Wei Wang, the corresponding author spearheading the research, the innovation lies in the intelligent spatial arrangement of the WLS fibers combined with cutting-edge readout methodologies. “This unique design is a significant step forward in muon veto detection,” Prof. Wang notes. The improvements not only yield higher photon counts per muon event but also offer sharper differentiation between true muon signals and background noise, enhancing the accuracy and reliability essential for high-stakes neutrino experiments.

The experimental evaluations reveal intriguing spatial dependencies in light output along the length of each PS strip. When muons intersect near the extremities of the 2000-mm scintillator strips, the system records an elevated total light yield, albeit accompanied by a degree of asymmetry in signal strength from each fiber end. Quantitatively, a single end of these long modules consistently registers photoelectron yields exceeding 40.8 p.e., while slightly shorter 1500-mm modules achieve yields beyond 51.5 p.e. Such high yields are a testament to the meticulous optimization of fiber placement and the use of optical coupling techniques.

Enhancing the coupling efficiency between the WLS fibers and the SiPMs, the research team applied optical grease, a strategy that proved beneficial by boosting the effective light yield by an impressive 12.5%. This incremental improvement is crucial given the finely balanced conditions under which the veto system must operate. High light yields contribute directly to the system’s ability to distinguish genuine muon-induced signals from spurious background events, a capability that significantly suppresses false positives and ensures the integrity of neutrino event selection.

Detection efficiency, arguably the most critical metric for any veto system, was rigorously tested under multiple trigger modes. The “module” mode, which sums signals from both ends of a scintillator module, demonstrated a stellar efficiency exceeding 99.67% at a 30-photoelectron threshold. Even more impressively, in the “AND” mode, requiring concurrent threshold surpasses at both ends, efficiency remained above 99.60% at a lower 15-photoelectron threshold. Such robust performance at varying thresholds underscores the reliability of the design under diverse operational conditions.

This exceptional detection efficiency achieved even at elevated thresholds ensures that the TAO top veto tracker maintains unparalleled performance stability. It confirms that the detector can consistently and accurately flag muon events while minimizing dead time and false triggers. This level of operational precision is indispensable for the TAO experiment’s stringent requirements, which demand muon identification efficiency surpassing 99.5% to effectively counter cosmic-induced backgrounds.

Beyond TAO, the scalable nature of the plastic scintillator modules and their innovative design promises broad applicability for next-generation neutrino observatories and other particle physics experiments with stringent background suppression needs. The methodology and results set a benchmark for the deployment of cost-effective, high-efficiency muon veto systems across multi-ton volume detectors, potentially influencing the standard paradigms of low-background experimental design.

Moreover, the study’s findings provide valuable insights into fiber optics integration, photon detection efficiencies, and module scalability, all of which are critically relevant for the design of large-scale neutrino telescopes and underground physics experiments. These technological contributions not only validate the TAO top veto tracker’s capabilities but also serve as a roadmap for enhancing detector technologies where particle identification and background discrimination are vital.

The successful combination of high light yield, distinct signal-background differentiation, and sustained efficiency underlines the TAO veto system’s role as a breakthrough auxiliary technology in the neutrino research community. As cosmic ray muons remain an omnipresent challenge in particle physics, developments like this chart a pathway toward cleaner signals and more precise measurements, accelerating discoveries regarding neutrino properties and fundamental particle interactions.

This work emerges at a timely juncture, where global collaborations in neutrino science seek increasingly sensitive and reliable detection methods. The proof-of-concept demonstrated by SYSU and IHEP ensures that future experiments can adopt or adapt these technologies, enhancing the hunt for rare neutrino interactions buried beneath layers of cosmic-induced noise.

For those interested in delving deeper into this remarkable achievement, the comprehensive study detailing the performance metrics, design architecture, and experimental validation of the plastic scintillator modules for the TAO top veto tracker is published in the journal Nuclear Science and Techniques. The article, titled “Performance of plastic scintillator modules for top veto tracker at Taishan Antineutrino Observatory,” became publicly available on April 11, 2025, and can be accessed via DOI: 10.1007/s41365-025-01696-2.

Subject of Research: Not applicable

Article Title: Performance of plastic scintillator modules for top veto tracker at Taishan Antineutrino Observatory

News Publication Date: 11-Apr-2025

Web References: http://dx.doi.org/10.1007/s41365-025-01696-2

Image Credits: Feng-Peng An

Keywords

Muons, Neutron detectors, Cosmic neutrinos, Light signaling, Neutrons