Muon tomography

Muon tomography is a technique that uses cosmic ray muons to generate three-dimensional images of volumes using information contained in the Coulomb scattering of the muons. Since muons are much more deeply penetrating than X-rays, muon tomography can be used to image through much thicker material than x-ray based tomography such as CT scanning. The muon flux at the Earth's surface is such that a single muon passes through an area the size of a human hand per second.^[1] Since its development in the 1950s, muon tomography has taken many forms, the most important of which are muon transmission radiography and muon scattering tomography. Muon tomography imagers are under development for the purposes of detecting nuclear material in road transport vehicles and cargo containers for the purposes of non-proliferation.^[2] Another application is the usage of muon tomography to monitor potential underground sites used for carbon sequestration.^[1]

History

Cosmic ray muons have been used for decades to radiograph objects such as pyramids and geological structures. The technique of muon transmission imaging was first used in the 1950s by Eric George to measure the depth of the overburden of a tunnel in Australia.^[3] In a famous experiment in the 1960s, Luis Alvarez used muon transmission imaging to search for hidden chambers in the Pyramid of Chephren in Giza, although none were found.^[4] In both cases the information about the absorption of the muons was used as a measure of the thickness of the material crossed by the cosmic ray particles.

Muon transmission imaging

More recently, muons have been used to image magma chambers to predict volcanic eruptions.^[5] Nagamine et al.^[6] continue active research into the prediction of volcanic eruptions through cosmic ray attenuation radiography. Minato^[7] used cosmic ray counts to radiograph a large temple gate. Frlez et al.^[8] recently reported using tomographic methods to track the passage of cosmic rays muons through cesium iodide crystals for quality control purposes. All of these studies have been based on finding some part of the imaged material that has a lower density than the rest, indicating a cavity. Muon transmission imaging is the most suitable method for acquiring this type of information.

Mu-Ray project

The Mu-Ray project is funded by the Istituto Nazionale di Fisica Nucleare(INFN) (Italian national institute for nuclear physics) and the Istituto Nazionale di Geofisica e Vulcanologia (Italian national institute for geophysics and volcanology).^[9] The Mu-Ray project is committed to map the inside of Mount Vesuvius, located in Naples, Italy. The last time this volcano erupted was in 1944. The goal of this project is to "see" inside the volcano which is being developed by scientists in Italy, France, the USA and Japan.^[10] This technology can be applied to volcanoes all around the world, to have a better understanding of when volcanoes will erupt.^[11]

Muon scattering tomography

In 2003, the scientists at Los Alamos National Laboratory developed a new imaging technique: muon scattering tomography (MT). With muon scattering tomography, both incoming and outgoing trajectories for each particle are reconstructed. This technique has been shown to be useful to find materials with high atomic number in a background of high-z material such as uranium or material with a low atomic number.^[12][13] Since the development of this technique at Los Alamos, a few different companies have started to use it for several purposes, most notably for detecting nuclear cargo entering ports and crossing over borders.

The Los Alamos National Laboratory team has built a portable Mini Muon Tracker (MMT). This muon tracker is constructed from sealed aluminum drift tubes,^[14] which are grouped into twenty-four 1.2-meter-square (4 ft) planes. The drift tubes measure particle coordinates in X and Y with a typical accuracy of several hundred micrometers. The MMT can be moved via a pallet jack or a fork lift. If a nuclear material has been detected it is important to be able to measure details of its construction in order to correctly evaluate the threat.^[15]

MT uses multiple scattering Radiography. In addition to energy loss and stopping cosmic rays undergo Coulomb scattering. The angular distribution is the result of many single scatters. This results in an angular distribution that is Gaussian in shape with tails from large angle single and plural scattering. The scattering provides a novel method for obtaining radiographic information with charged particle beams. More recently, scattering information from cosmic ray muons has been shown to be a useful method of radiography for homeland security applications.^{[12] [16] [17] [18]}

Multiple scattering can be defined as when the thickness increases and the number of interactions become high the angular dispersion can be modelled as Gaussian. Where the dominant part of the multiple scattering polar-angular distribution is

The Fermi approximation, where θ is the polar angle and θ₀ is the multiple scattering angle, is given approximately by

The muon momentum and velocity are p and β, respectively, and X₀ is the radiation length for the material. This needs to be convolved with the cosmic ray momentum spectrum in order to describe the angular distribution.

The Image can then be reconstructed by use of GEANT4.^[19] These runs include input and output vectors, in and out for each incident particle. The incident flux projected to the core location was used to normalize transmission radiography (attenuation method). From here the calculations are normalized for the zenith angle of the flux.

Nuclear waste imaging

Tomographic techniques can be effective for non-invasive nuclear waste characterization and for nuclear material accountancy of spent fuel inside dry storage containers. Cosmic muons can improve the accuracy of data on nuclear waste and Dry Storage Containers (DSC). Imaging of DSC exceeds the IAEA detection target for nuclear material accountancy. In Canada, spent nuclear fuel is stored in large pools (fuel bays or wet storage) for a nominal period of 10 years to allow for sufficient radioactive cooling.^[20]

Challenges and issues for nuclear waste characterization are covered at great length, summarized below:^[21]

• Historical waste. Non-traceable waste stream poses a challenge for characterization. Different types of waste can be distinguished: tanks with liquids, fabrication facilities to be decontaminated before decommissioning, interim waste storage sites, etc.
• Some waste form may be difficult and/or impossible to measure and characterize (i.e. encapsulated alpha/beta emitters, heavily shielded waste).
• Direct measurements, i.e. destructive assay, are not possible in many cases and Non-Destructive Assay (NDA) techniques are required, which often do not provide conclusive characterization.
• Homogeneity of the waste needs characterization (i.e. sludge in tanks, in-homogeneities in cemented waste, etc.).
• Condition of the waste and waste package: breach of containment, corrosion, voids, etc.

Accounting for all of these issues can take a great deal of time and effort. Muon Tomography can be useful to assess the characterization of waste, radiation cooling, and condition of the waste container.

Los Alamos Concrete Reactor

^[22] In the summer of 2011, a reactor mockup was imaged using Muon Mini Tracker (MMT) at Los Alamos. The MMT consists of two muon trackers made up of sealed drift tubes. In the demonstration, cosmic-ray muons passing through a physical arrangement of concrete and lead; materials similar to a reactor were measured. The mockup consisted of two layers of concrete shielding blocks, and a lead assembly in between; one tracker was installed at 2.5 m height, and another tracker was installed on the ground level at the other side. Lead with a conical void similar in shape to the melted core of the Three Mile Island reactor was imaged through the concrete walls. It took three weeks to accumulate 8 × 10⁴ muon events. The analysis was based on point of closest approach, where the track pairs were projected to the mid-plane of the target, and the scattered angle was plotted at the intersection. This test object was successfully imaged, even though it was significantly smaller than expected at Fukushima Daiichi for the proposed Fukushima Daiichi Tracker (FMT).

Left – Lead reactor core with conic void. Right – Observed core where average scattering angles of muons are plotted. The void in the core is clearly imaged through two 2.74 m concrete walls. The lead core of 0.7 m thickness gives an equivalent radiation length to the uranium fuel in Unit 1, and gives a similar scattering angle. Hot spots at the corners are artifacts caused by edge effect of MMT.^[23]

University of New Mexico UNM Research Reactor

After the Concrete reactor was successfully imaged, the Research Reactor at UNM was tested and imaged next. The University of New Mexico Research Reactor, AGN-201M, consists of 10.93 kg of polyethylene loaded with about 3.3 kg of uranium, enriched to 19.75% of U-235. Moderator and shielding consisting of graphite, lead, water, and concrete surround the core. Several access channels pass through and near the core. The core profile details how the fuel section is made of stacked cylindrical plates with access ports and control rod channels.

The data collection for muon tomography at the UNMRR ran over several months, though, due to different interruptions, total exposure amounted to 891 hours. The MMT's status was monitored remotely from Los Alamos, located 160 km (100 mi) from UNM, and the experimental data were collected in 3-hour increments. From this collected data a model of the UNMRR is created using the GEANT4^[19] toolkit, developed at CERN for the simulation of the passage of particles through matter.

Fukushima application

On March 11, 2011, a 9.0-magnitude earthquake, followed by a tsunami, caused an ongoing nuclear crisis at the Fukushima Daiichi power plant. Though the reactors are stabilized, complete shutdown will require knowledge of the extent and location of the damage to the reactors. A cold shutdown was announced by the Japanese government in December, 2011, and a new phase of nuclear cleanup and decommissioning was started. However, it is hard to plan the dismantling of the reactors without any realistic estimate of the extent of the damage to the cores, and knowledge of the location of the melted fuel.^[24][25] Since the radiation levels are still very high at the inside of the reactor core, it is not likely anyone can go inside to assess the damage. The Fukushima Daiichi Tracker (FDT) is proposed to see the extent of the damage from a safe distance. A few months of measurements with muon tomography, will show the distribution of the reactor core. From that, a plan can be made for reactor dismantlement; thus potentially shortening the time of the project many years.

In August 2014, Decision Sciences International Corporation announced it had been awarded a contract by Toshiba Corporation (Toshiba) to support the reclamation of the Fukushima Daiichi Nuclear complex with the use of Decision Science's muon tracking detectors.

Decision Sciences International Corp

Decision Sciences International Corporation has implemented muon tracker technology in a Multi-Mode Passive Detection System (MMPDS). This port scanner located in the Freeport, Bahamas can detect both shielded nuclear material, as well as explosives and contraband. The scanner is large enough for a cargo container to pass through, making it a scaled-up version of the Mini Muon Tracker. It then produces a 3-D image of what is scanned.^[26]

Decision Sciences was awarded the 2013 R&D 100 award for the MMPDS. The R&D 100 award recognizes the best and most unusual high-technology products of the year.^[27]

Non-proliferation

Tools such as the MMPDS in Freeport, Bahamas can be used to prevent the spread of nuclear weapons. The safe but effective use of cosmic rays can be implemented in ports to help non-proliferation efforts. Or even in cities, under overpasses, or entrances to government buildings.

The Nuclear Non-proliferation Treaty (NPT) signed in 1968 was a major step in the non-proliferation of nuclear weapons. Under the NPT, non-nuclear weapon states were prohibited from, among other things, possessing, manufacturing or acquiring nuclear weapons or other nuclear explosive devices. All signatories, including nuclear weapon states, were committed to the goal of total nuclear disarmament.

The Comprehensive Nuclear-Test-Ban Treaty (CTBT) bans all nuclear explosions in any environments. Tools such as muon tomography can help to stop the spread of nuclear material before it is armed into a weapon.^[28]

The New START^[29] treaty signed by the US and Russia aims to reduce the nuclear arsenal by as much as a third. The verification involves a number of logistically and technically difficult problems. New methods of warhead imaging are of crucial importance for the success of mutual inspections.

Muon Tomography can be used for treaty verification due to many important factors. It is a passive method; it is safe for humans and will not apply an artificial radiological dose to the warhead. Cosmic rays are much more penetrating than gamma or x-rays. Warheads can be imaged in a container behind significant shielding and in presence of clutter. Exposure times depend on the object and detector configuration (~few minutes if optimized). While SNM detection can be reliably confirmed, and discrete SNM objects can be counted and localized, the system can be designed to not reveal potentially sensitive details of the object design and composition.^[30]

CRIPT detector

The Cosmic Ray Inspection and Passive Tomography (CRIPT)^[31] detector is a Canadian muon tomography project which tracks muon scattering events while simultaneously estimating the muon momentum. The CRIPT detector is 5.3 m tall and has a mass of 22 tonnes. The majority of the detector mass is located in the muon momentum spectrometer which is a feature unique to CRIPT regarding muon tomography.

After initial construction and commissioning^[32] at Carleton University in Ottawa, Canada, the CRIPT detector was moved to Atomic Energy Of Canada Limited's Chalk River Laboratories.^[33]

The CRIPT detector is presently examining the limitations on detection time for border security applications, limitations on muon tomography image resolution, nuclear waste stockpile verification, and space weather observation through muon detection.

References