Muon Tomography

Muon Tomography

Every second of every day, we are bombarded with thousands of particles that pass through our bodies without us noticing. Many of these particles are muons, second-generation leptons that are produced by cosmic rays and reach Earth’s surface at a rate of 1 per square centimeter per minute!

Cosmic rays are a form of high-energy radiation that originate from outside our solar system. When they reach Earth, the rays collide with particles in the upper atmosphere to produce a “shower” of particles, including muons. Muons are heavier than electrons and don’t get absorbed by materials as quickly as their less-massive relatives. They do not lose as much energy as they travel, allowing them to penetrate more deeply into materials than X-rays or other forms of radiation. Because of this, muons are excellent for probing unseeable objects.

Scientists today are finding ways to put these particles to use. One technique called Muon Scattering Tomography (MST) uses cosmic ray muons to construct three-dimensional models of the densities of obstructed objects or volumes.

MST is based on multiple Coulomb scattering, a phenomenon in which muons are deflected and slow down when they interact with material with a high atomic number, or “high-Z”. In multiple Coulomb scattering, particles are scattered due solely to the Coulomb force, the force that says opposite charges attract and like charges repel. As negatively-charged muons pass through a volume, they interact with the negatively-charged electrons in the material and are deflected. Researchers can analyze their angles of deflection before and after passing through a volume to gather information about the mass they are inspecting.

Below, you can read about different ways researchers are using muon tomography and technology from the CMS to safeguard our cities, preserve our environment, and protect human health.

Security and Environmental Protection

Spare parts from the CMS are being used to study how muons can help keep our cities and environment safer and more secure.

A group in Padova, Italy, at the INFN National Laboratory of Legnaro is heading the Cosmic Muon Tomography project (CMTp). They have obtained two spare drift tube chambers from the CMS to use in experiments using muon scattering tomography (MST).

There are lots of potential applications of cosmic muon detection using several techniques, including MST, from homeland security and industry to environmental protection and building stability monitoring. For example, MST cargo scanners have been installed in Freeport, Bahamas, to detect contraband trying to enter the port. Other cities are following their lead. These portals use cosmic ray muons and drift tube technology to “scan” the contents of a truck or container quickly and reliably so as not to disrupt the flow of the transport chain.

Another very promising application of MST is related to the control of spent nuclear fuel containers. Nuclear fuel bundles, once used in a nuclear reactor, must be permanently sealed in special containers and stored in secure sites since they can still be used to build nuclear weapons. As of now, we do not have a way to inspect the contents of storage containers without opening them and risking harm. But the CMTp has shown, based on simulated cosmic muons that cross both the containers and dedicated muon detectors, that it is possible to devise a method to detect or exclude the presence of fuel bundles.

The research being done by CMTp and similar groups will be invaluable in furthering the field of muon tomography. Its many uses give incredible potential to solve many of today’s looming problems.

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The MST scanner at the INFN National Laboratory of Legnaro near Padova. Source: CMTp

The MST scanner at the INFN National Laboratory of Legnaro near Padova.

Source: CMTp

Homeland Security

Researchers at the Florida Institute of Technology in Melbourne, Florida, USA, are working on Gaseous Electron Multiplier (GEM) detectors that utilize cosmic ray muons for homeland security. This research began under a grant from the U.S. Department of Homeland Security in 2007.

To protect our borders from nuclear contraband, existing radiation monitors search incoming cargo for radiation emitted by special nuclear material, which includes plutonium and some isotopes of uranium. This radiation, however, can be blocked by shielding made of iron or lead, posing a special challenge for homeland security and our current monitoring technology. 

Fortunately, muon tomography is a promising alternative. Experiments have already shown that we can probe shielded material by measuring the deflections of cosmic ray muons using GEM detectors.

Inspired by the design of GEMs in CERN’s COMPASS experiment, the team at Florida Tech designed and constructed their own GEM detectors for use in homeland security. These GEMs are made of an electrically-insulating foil sandwiched in copper and dotted with microscopic holes. Each hole is a mere 60 millionths of a meter in diameter—about the average width of a human hair! The completed detector measures 30 x 30 x 1 cm.

When a particle (in this case, a cosmic ray muon) enters the detector and passes through one of the holes, an “avalanche” of electrons is produced. The electric field generated across the copper electrodes then pulls the electrons to an electronic amplifier where they are read out as an electrical signal. This voltage gives us information about the position of the muon, which subsequently informs us about its deflection. With this data, we can learn about the material within an enclosed area, like a shipping container. “Triple-GEMs” provide even higher gain because electrons avalanche through three layers of multipliers before being read out.

In addition to homeland security, GEMs will also play a major role in the upcoming CMS upgrade. This upgrade involves the installation of large-area GEM detectors, called GE1/1, in a certain region of the forward muon endcap. The prior endcap system only uses cathode strip chambers in this specific region, which are unable to handle the large inundations of muons that are expected after the upgrade.

But thanks to the GEM detector’s high spatial resolution, using GE1/1 in conjunction with the CSC system will greatly improve deflection measurement capabilities. In addition, GE1/1 is relatively inexpensive, very efficient, and can handle large fluxes of muons. 

In the two years leading up to their 2019 installation, the GE1/1 detectors are being constructed at CERN, Florida Tech, and other sites worldwide. In the future, GEMs are also planned to be installed in other areas of the endcap to further improve muon reconstruction.

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