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Muons are subatomic particles that behave a lot like electrons but are around 200 times heavier. As the US Department of Energy explains, “Muons created in the atmosphere constantly hit every inch of the Earth’s surface and pass through almost any substance.”
Many scientists have noted that measuring the passage of muons offers the chance to measure the interior of whatever object the particles pass through during the 2.2 microseconds or so they exist before decaying into an electron and neutrinos. Muon radiography works by tracking the charged subatomic particles muons, generated when cosmic rays interact with Earth’s atmosphere. These muons travel down at high speeds through the atmosphere and can pass through solid objects.
Muography is an imaging technique based on the measurement of absorption profiles for muons as they pass through rocks and earth. As they pass through the space, nuclear emulsion plates are used as detectors to ‘catch’ the particles and develop an image of where the muons passed through, and where they were absorbed or deflected. This same method has been used on pyramids in Egypt, using the constant rain of atmospheric muons that bathes the Earth every day, and techniques that aren’t vastly different to those used with gamma rays, X-rays, neutrons, protons, and electrons in imaging applications.
This technology has developed rapidly over the last 15 years, and it is currently branching out into many different applications and moving from academic research to commercial application. The technological developments in the detection of elementary particles have opened the way to its application in various fields, such as archaeology, studies of geological structures, civil engineering and security issues.
Russian Researchers used a nuclear physics technology known as muon radiography to reveal details of Hidden underneath the Naryn-Kala fortress in Derbent, Russia, a mysterious subterranean vault – a buried structure whose original purpose has been unknown for decades. Scientists from three major Russian research institutes have made an important breakthrough in the field of muon tomography, creating tracking devices which allow geologists to ‘see through’ objects up to thousands of meters in diameter below the earth’s surface. By using this method to meticulously scan the subterranean structure, the team arrived at a suggestion it was once a vast church.
Michael Staib of Florida Institute of Technology and his team are harnessing this natural phenomenon to scan for hidden nuclear materials, and for any shielding hiding it. Muon tomography is a passive vehicle interrogation technique designed especially for detecting well-shielded nuclear contraband,” Staib said. “We simply use cosmic ray muons. Those are constantly being produced in the upper atmosphere and passing through us all the time.”
The new technology has other uses as well, Professor Polukhina said. “It is possible to non-invasively appraise a volcano’s vent, the reactor of a nuclear power plant, or a mountain glacier. [The technology can be used] to find new underground sources of natural gas, to catch a fire rising in a mountain used for coal mining long before it burns out from the inside, to predict the eruption of a volcano, or prevent the disastrous consequences of sinkholes in mines or city streets,” the scientist noted.
Muography
Muography is an imaging technique that profits from the penetrating power of elementary particles called muons, similar to electrons but with a mass about two hundred times larger.
In absorption muography, a muon tracker is located downstream of the body under investigation. By tracking back the muon trajectories, one obtains an angular map of their flux as seen from the location of the muon tracker itself. A comparison with the muon flux impinging on the Earth surface provides a map of the muon transmission (or equivalently absorption) in the traversal of the body being investigated. As the penetration of muons in matter depends on its density, such a map can provide a muographic image of its internal structures. These could be cavities or high-density zones. Muography (or muon radiography) is thus in principle similar to X-ray radiography, but capable of probing the interior of large bodies, thanks to the penetrating power of muons which is much higher than that of X-rays.
Italy Researchers have developed a new approach to the three-dimensional muography of underground structures, capable of directly localising hidden cavities and of reconstructing their shape in space. While exploring the cavities underground Mt. Echia they found that complexity of the system makes it difficult to identify hidden cavities without the ambiguities originating from the fact that standard 2D muographies are projective transmission maps, in which the shadows of other cavities are difficult to disentangle from one another.
To continue the investigation, they pursued a new development: a novel approach to three-dimensional (3D) muography, where the usual 2D muographies taken from at least three different locations are directly combined in a single analysis in order to localise cavities in space and reconstruct their shape.
The two muon trackers (called MU-RAY and MIMA) were used for the measurements real-time electronic devices that operate autonomously, with remote control and readout. The basic elements are bars of plastic material doped with a scintillating compound, following a simple and widespread technique for particle detection. The light generated by muons in the plastic scintillator bars is detected by Silicon Photomultipliers (SiPMs). These photosensors, recently developed, are solid-state devices that, as such, do not require any high voltage supply and have a very low power consumption. These features allow us to operate the detectors with relative ease also in remote environments.
Our measurements at Mt. Echia, the site of the earliest settlement of the city of Naples in the 8th century BC, have led us to the discovery of a hidden underground cavity, whose existence was not evident with the usual two-dimensional muography graphs, write the authors in Nature.
Russian Scientists Invent Device Allowing Them to ‘See’ Kilometers into Earth
Researchers from Russia’s National University of Science & Technology, the Russian Academy of Sciences’ Institute of Physics, and Moscow State University’s Institute of Nuclear Physics have joined their efforts to create a special tracking device which allows geologists to create accurate 3D maps of the environment deep beneath the planet’s surface. The trackers use a form of muon tomography, the technique using cosmic ray-generated muon particles, to radiograph underground objects and geological structures.
When a cosmic ray strikes the nucleus in the atmosphere, it produces a shower of subatomic particles, including pions and kaons which decay into longer-lived muons. Clarifying how muon tomography works, RIA Novosti explained that as cosmic ray-generated particles find their way into the earth’s atmosphere (40 km and below), they collide with molecules which make up the atmosphere. This creates new particles, some of which quickly become muons – unstable subatomic particles with a mean lifetime considerably longer than many other subatomic particles. Able to pass through earth’s entire atmosphere in the space of their ‘lifetime’, muons are also able to penetrate up to 8,500 meters below the water, or 2,000 meters into the earth’s surface.
The denser the matter, the faster the muon particles’ presence wanes. With the help of muon tracking detectors, solid objects can be observed to track the passage of muons through its cavities. Three detectors placed around an object are usually sufficient to create a three dimensional map. Muons are detected with the help of a series of photographic plates layered with silver bromide, used to reveal and match the illuminated areas, building a trajectory of the exposure. The smaller the bromide granules and the more accurate the matching algorithms, the more correct the 3D picture of the object.
Speaking to RIA Novosti, MISiS Rector Alevtina Chernikova emphasized that this technology has a broad range of potential applications. “After deciphering the detector’s readings, it is possible to compile a three dimensional picture of a variety of objects, from a meter-sized cavity in the soil…to a map of the caves in a mountain,” she said.
Experiments to confirm the workability of the new muon tracking technology have been held in a mine belonging to the Geological Service of the Russian Academy of Sciences in Obninsk, western Russia. The detectors enabled scientists to ‘see’ the contours of the underground structure. Now, a suite of these detectors is being produced by Slavich Company, a Yaroslav-based firm specializing in technical photographic materials.
The good thing about our emulsion-based detectors is that they are easy to operate, do not require electricity, and in the case of geological prospecting, allow us to manage a much smaller number of underground holes, all while accurately distinguishing objects between one meter and one kilometer across with a high degree of accuracy,” Professor Polukhina said. MISiS specialists are presently working on software to improve the decoding of the trackers, and on the protection of tracker sensors from the harsh environment which can be found in underground holes.
Muon Detectors Hunt for Fissile Contraband
Hunting for a concealed nuclear weapon can be harder than looking for a needle in a haystack. Though exposed plutonium’s radiation is easy to detect, uranium’s is less so, and both can be shielded. At the APS Meeting, researchers presented an improved technique for using the natural radiation of cosmic rays to peer through solid objects and find any hidden fissile material.
A muon strikes every square centimeter of Earth once a minute on average. Gas electron multiplier (GEM) detectors can detect their location, and when several are stacked on top of each other, they can track the paths of the fast moving particles. The denser the material a muon passes through, the more its path is deflected. Uranium and plutonium are two of the densest elements in the periodic table, so the detectors are used to look for places where the paths of muons are the most disrupted.
“Uranium doesn’t have a very strong signal for radiation detection, but you simply use the fact that uranium is very heavy and very dense and so you can try to find a way to detect it using those characteristics,” Staib said. “No artificial radiation source [is needed] so there’s no exposure of an object to radiation beyond what it would be experiencing anyway.”
To look for nuclear materials, a shipping container is placed between two sets of large GEM detector plates. Two plates on top of the container track the paths of incoming muons, and two plates underneath track them on their way out. If there’s little or no dense material in the container, than the two parts of the muon’s path should line up. Even iron won’t deflect muons a great deal. However, if there’s a lot of dense material, like plutonium, uranium or lead shielding, the paths should veer sharply.
“If I can force them to put five tons of lead around it, I’m good because it’s easier to detect five tons of lead than the radiation,” said Michael Kuliasha from the Defense Threat Reduction Agency. “You have to have a robust radiation detection because it forces them to do something that’s actually easier to detect.”
He added that the difficulty of finding concealed nuclear weapons is not a new problem. “In 1945, Robert Oppenheimer, who was head of the Manhattan project, was actually asked in a congressional hearing by Senator [William] Milliken … how would you detect an atomic bomb hidden somewhere in a city. And [Oppenheimer] says, ‘I’d get a screwdriver and open each and every suitcase and crate,’” Kuliasha said.
GEM detectors were first developed at CERN to detect muons and other particles produced in collisions in accelerators. The idea to use passive scanning to find hidden fissile materials was first developed at Los Alamos in 2003, and has been developed further by the company Decision Sciences. Their method, which uses drift tube detectors, is about to undergo the first test commercial application in the Bahamas. Drift tubes are relatively inexpensive, but take longer to make a measurement than the GEM detectors.
Because both methods rely on the natural rate of muons traveling through the atmosphere, the only way to speed up the detection of illicit materials is by improving the sensitivity of the detectors. Right now Staib’s prototype takes about nine to ten hours to differentiate between different materials, but he says that with more development it should be able to get down to a few minutes.
In addition to scanning incoming cargo, Kuliasha said that the technology is promising for verification of arms reduction treaties like START. He said that a detector could be set up around a missile or submarine to see if nuclear warheads are still inside. At the same time there are limitations to the technology. Using it to scan an entire ship would be logistically impractical, and probably still wouldn’t be as effective as boarding and searching the vessel.
