A mountain may appear solid, but its interior can contain fractures, cavities and variations in rock density that are difficult to detect through conventional surveys. Examining these hidden features without drilling or disturbing the surrounding structure has long been a challenge for geologists and engineers. An emerging technique called muography offers a way to investigate them using particles that constantly arrive from outer space.
An international research team involving scientists from India and Belgium has developed a portable detector capable of tracking cosmic-ray muons with an efficiency of more than 95 per cent. The technology could make muography more practical for studying geological formations, volcanoes, tunnels and historical monuments.
The Indian institutions involved include the National Institute of Science Education and Research (NISER), Bhubaneswar, and the Homi Bhabha National Institute. Their collaborators include researchers from Université catholique de Louvain and the Inter-University Institute for High Energies in Belgium, according to Research Matters.
Reading rock formations with cosmic rays
Cosmic rays are high-energy particles originating in space. When they collide with atoms in the Earth’s upper atmosphere, they produce secondary particles, including muons, which travel continuously towards the Earth’s surface.
Muons are heavier relatives of electrons. Their high energy allows them to penetrate substantial thicknesses of rock, concrete and other dense materials. However, some are absorbed or lose energy while passing through matter. Consequently, fewer muons pass through denser material than through less dense sections.
Muography uses this difference to reconstruct the internal structure of an object. Detectors placed at suitable positions measure the number and direction of incoming muons. Scientists analyse the resulting data to identify variations in density. The principle is similar to X-ray imaging, but instead of using an artificial radiation source, it relies on naturally occurring particles.
Making the detector portable
The research team has developed a detector using glass-based Resistive Plate Chambers (RPCs). These consist of two parallel glass electrodes separated by a narrow, gas-filled gap.
When a muon passes through the chamber, it ionises the gas, producing an electrical signal. Copper strips record its position, allowing researchers to reconstruct its trajectory.
The scientists built and tested three prototypes, designated A, B and C. During a proof-of-principle experiment, they placed a lead block between the detector layers to measure how a dense material affected the passage of muons.
An initial open-sky measurement recorded nearly 55,000 muon tracks. With the lead absorber in place, the count fell to 30,894, demonstrating the detector’s ability to register changes in muon transmission through dense material.
The system achieved a detection efficiency of at least 95 per cent and a timing resolution of approximately 1.8 nanoseconds. Its sealed construction is also intended to reduce gas consumption and make the equipment easier to operate in remote locations.
What could it reveal?
The technology has several potential applications. In volcanology, muography could help scientists examine density variations inside volcanoes, including structures associated with magma movement. Such observations could complement seismic measurements and other monitoring methods.
Civil engineers could use the technique to investigate tunnels, ageing bridges and dams for internal irregularities. Archaeologists could examine monuments without damaging their structures, while geologists could explore underground formations and mineral deposits.
Muon imaging has already been used experimentally to investigate volcanoes in Japan, Italy and France. It has also been employed in archaeological research, including studies of the Egyptian pyramids. The latest development seeks to overcome a practical challenge by making detectors more compact, robust and suitable for deployment in the field.
The limitations
The technology is not yet ready for routine mountain surveys. The experiment demonstrated the detection of muon absorption using a lead block; it did not demonstrate the successful imaging of an entire mountain or establish the ability to predict volcanic eruptions.
Further improvements, including lighter casings, wider directional coverage and more sophisticated readout systems, are needed. Field trials will also be essential to establish how reliably the detectors perform under actual geological conditions.
Nevertheless, the research points to a promising application of particle physics: using naturally occurring particles from space to investigate what lies beneath the Earth’s surface, without drilling, excavation or physical intrusion.
