Part 2 of 5 Parts (Please read Parts 1 first)
Muons are fundamental charged particles that are created naturally when cosmic rays collide with atomic nuclei in the atmosphere. This results in the generation of pion particles which then decay. Muons are about two hundred times as massive as electrons. They can easily pass through the atmosphere to the surface of the Earth. About a thousand muons hit every square foot of the Earth’s surface every minute. They penetrate into rock and other dense materials but lose energy as they interact with electrons. They are absorbed eventually after traveling through a thousand feet of rock. Scientists can track the variation in muon flux to measure the density of the materials they are passing through.
British physicist Eric George first used atmospheric muons for measuring the thickness of ice above a mining tunnel in Australia in the 1950s. In 2003, Christopher Morris and his team at Los Alamos National Laboratory proposed using muon scattering as opposed to absorption to form images of dense objects, especially nuclear materials.
Muons are deflected by the dense concentration of charge inside an atomic nucleus. The higher the atomic number of an element, the greater the muon deflection will be. Uranium and plutonium will deflect muons much more than materials such as concrete and steel with lower atomic numbers. The composition of materials can be calculated by plotting the trajectory of muons before they enter an object and after they leave it. With a sufficiently long exposure time, muons can be recorded that have a wide range of incident angles and positions. This can provide a very accurate image of the contents of the object under study.
Morris and his team used muon detectors called drift tubes. These tubes contain gas and a wire that has a positive charge. When a muon passes through a drift tube, it liberates electrons which generate a signal at that point on the wire. Tubes are laid out in layers that are at right angles to each other. It is possible to map a trajectory of a muon by tracking a series of excited points on wires in the tubes. A minimum of two sets of perpendicular layers of drift tubes on either side of an object is required to plot the incoming and outgoing vectors of a muon.
Researchers at Los Alamos National Laboratory first carried out muon experiments with a tungsten cylinder. After those experiments, the team created muon detectors that were designed to identify nuclear materials concealed within cargo in trucks and shipping containers. Funding for this work was made available after the 9/11 terrorist attack in New York in 2001. Los Alamos developed a prototype of such a detector for nuclear materials for Decision Sciences Corporation, a company in California which is now selling such detectors in the U.S. and abroad.
Morris and his team then began working on monitors for spent nuclear fuel. They developed two muon trackers which each had twenty-four layers with twenty four-meter long drift tubes in each layer. They shipped these detectors to the Idaho National Laboratory (INL) for testing. The detectors were placed on either side of a dry cask containing fuel assemblies removed from a Westinghouse nuclear reactor in the 1980s.
Please read Part 3