Detecting some regulated or prohibited commodities through the use of traditional non-intrusive detection equipment remains a challenge today. Take cocaine, for example. It is very often concealed in shipments of bananas and other fruits with a very similar density to the drug. Since x-ray is a density-based discriminating technology, it is rather difficult to differentiate cocaine from the fruit on an image generated by an x-ray scanner.

A technology called muon tomography could solve this issue. It uses information about the absorption of cosmic ray muons to measure the thickness of the materials crossed by the muons, to generate three-dimensional images. Muons are naturally occurring sub-atomic particles which are 100% safe to humans, food, medicines and animals. They have been around for billions of years and are created when inter-galactic high-energy cosmic rays collide with molecules in the Earth’s atmosphere. These muons travel at nearly the speed of light and are highly penetrating. Most of them are able to travel tens or hundreds of metres into the Earth’s crust.

The first known use of simple muon detectors was documented in 1970 when Luis W. Alvarez investigated the interior of the Chephren’s pyramid at Giza, Egypt, to find hidden and unknown chambers in the stonework[1]. Although the technology has existed for some decades now, the high cost of its components has severely limited its spread. Muon flux activity is expected at around 10,000 particles per square metre per minute, and in order to account for all muon trajectories, speed and spin, some serious computations must be carried out. Moreover, in an environment such as trade or for security purposes, where time is of the essence, this operation must be performed over a short period of time.

Recent developments, especially with silicon chips and machine-learning technology becoming more affordable, have changed the game. A scan operation that would have taken several hours twenty years ago, now requires only a few minutes. Muon tomography today finds applications in domains such as geology (study of volcanoes), archeology (study of pyramids and tombs), construction (to assess the safety and integrity of structures) and, since 2003[2], cargo non-intrusive inspection (NII).

For NII purposes, muons are tracked through detectors placed at the top and bottom of the object to be scanned. As particles travel through the top detector, their trajectory is calculated. As they pass through the object, there will be a degree of scattering which is dependent on the atomic structure of the object. The denser an object is at the atomic level, the more scattering of the muons will occur. The exiting angle of the muon is measured at the bottom detector, and the scattering angle data is computed against a database using an image reconstruction algorithm. A 3D image is then generated. But, more interestingly, the atomic structure of the object is also estimated and can be matched with a commodity if it is listed in the machine’s database. If a target commodity is found, the system sends an alert with information on the coordinates of the item of interest. The machine’s database can be updated according to the client’s needs. A weight estimation is also provided; its accuracy depends on the scanning time – for example, the scanning time usually used for a sea container gives an accuracy level of tens of kilograms.

The natural energy level of muons is on average 1,000 times that of the largest x-ray machines, so muons penetrate even the densest of cargo including lead, steel, cement, rocks, uranium and iron ore, which x-rays cannot. The ionizing nature of the x-rays also means they can change cell structures in biological material and their use is thus subject to stringent legislation to prevent individuals from being dangerously exposed to them. The danger of X-ray exposure is mitigated by reducing the exposure time to a minimum, setting a certain distance from the ray source, shielding with lead or other heavy material, or a combination of all three. Since the muon flux is not ionizing and occurs naturally, it is not harmful to humans and calls for no shielding whatsoever. In fact, since you started reading this article around 20,000 muons have passed through your body.

This means that the detectors can be installed anywhere, including at border control points where only documentary controls are currently conducted, as well as at ports, airports and mail centers. Muon tomography can be used as a primary means of non-intrusive inspection of all people, vehicle and cargo movements, and can be integrated into existing IT systems.

The capability of muon flux technology (MFT) to provide material identification opens up new possibilities in terms of efficiency and the experience of the person being controlled. Let’s take air travel, for example. Nowadays we queue up to go through airport security checkpoints, to have both our baggage and ourselves scanned, one at a time. This is mainly due to the limitations of x-ray scanning technology, which is energy-consuming. What the scanner operators are looking for are clues to the presence of explosive devices such as detonators, wires and timers, or manual weapons such as knives and guns. Although some machines have automatic threat recognition (ATR) software installed, they still require qualified operators and enough time to process all the passengers.

MFT is set to detect actual explosive material, not just the devices. It also makes it possible to scan liquids, which current technologies cannot do. The MFT detector panels may be built into the walls of the building, thereby eliminating the need for queueing altogether. The whole room then becomes a passive scanner, with passengers and their baggage being scanned without intruding on people’s privacy or jeopardizing their health.

The other difference between muon flux technology and x-ray is that MFT scanners have a much lower carbon footprint and use less electricity. As there are no moving parts, and the detectors are made primarily from plastic, they are also expected to have a significantly longer lifespan.

The selling price of the scanners will be about the same as that of x-ray CT systems. The maintenance costs are lower, as there are no depreciable radiation sources or moving parts in the scanner. Some of the scanners will be designed to tolerate hot climates and seaside conditions.

Software enabling data matching of cargo contents to electronic manifest data, also known as eFTi^[3], is under development. So far the focus has been on containerized cargo, but the detectors and software can be used in all sorts of configurations, from small baggage scanners to large container or truck scanners, and even entire buildings. Solutions are scalable as the sensor modules can be arranged in various ways, like LEGO® blocks.

The first MFT scanner system to be used for Customs inspection will be piloted in Summer 2023, and more deployments are planned in the years to come. We believe that 50 years after its first trials, muon tomography is ready to revolutionize border controls and that the power of the cosmos can now help make the world a safer and fairer place.

More information
https://www.gscan.eu

[1] Alvarez, L. W., Anderson, J. A., Bedwei, F. E., Burkhard, J., Fakhry, A., Girgis, A., … & Yazolino, L. (1970). Search for Hidden Chambers in the Pyramids: The structure of the Second Pyramid of Giza is determined by cosmic-ray absorption. Science, 167(3919), 832-839.

[2] Borozdin, K. N., Hogan, G. E., Morris, C., Priedhorsky, W. C., Saunders, A., Schultz, L. J., & Teasdale, M. E. (2003). Radiographic imaging with cosmic-ray muons. Nature, 422(6929), 277-277.

^[3] Electronic freight transport information (eFTI) developments – ESC (europeanshippers.eu)