They came from outer space

Materials World magazine
1 Sep 2018

The University of Glasgow, UK, has co-created a device that uses muons to detect radiation. Khai Trung Le talks to Dr Craig Shearer on its development.

Often described, not entirely inaccurately, as cosmic particles, muons are elementary particles – not formed of any other particles – similar to the electron, but with a greater mass. They are created when cosmic rays enter the Earth’s upper atmosphere, with around 10,000 muons per square metre every minute striking the Earth, and although they are radioactive, they decay with a half-life of 1.52 microseconds. While little known, they were instrumental in the discovery of an unknown chamber in Egypt’s Great Pyramid, mapping lava channels in Japan, and now to detect radioactive waste.

Muons typically move through matter with no distinguishable deviation before they are absorbed, making them ideal for imaging large and dense objects such as pyramids and volcanoes – muon technologies have so far focused on these areas. Dr Craig Shearer, Project Leader at the University of Glasgow’s National Nuclear Laboratory (NNL), UK, told Materials World, ‘These include large structures with a much lower image resolution limited to several metres, materials surrounded by air or a much less dense material, and the quick detection of illicit material inside a large container.’

Current X-ray technology is unable to examine the contents of radioactive waste containers, designed to prevent radiation penetration, whereas it will pass through containers of extremely dense material, including uranium, causing muons to scatter. Although the use of muon tomography has been posited for radiation detection for decades, it has taken an 11-year project from the University of Glasgow and nuclear fuel decommissioning site, Sellafield, to create one of the first commercial devices using muons to inspect radioactive material.

Overcoming the valley of death

Developed by Lynkeos Technology, a spin-off company from NNL, the Muon Imaging System (MIS) is able to inspect spent material to gauge whether it can be safely stored, imaging products of thermal treatment processes, and inspecting historic waste without needing to remove its concrete casing. Shearer said, ‘A key achievement of the MIS is the ability to resolve to images of materials with a high atomic number at sub-cm accuracy, such as pieces of legacy uranium stored within an inert matrix like concrete grout.’

Shearer attributes much of the developmental success of the MIS to the step-changes made in imaging software since 2009, in addition to the testing and design modifications working in collaboration with Sellafield. Shearer continued, ‘Since 2009, the technology has imaged various surrogate waste containers to demonstrate the capability of the technology, as it moved from a lab-based R&D system to a fully commercialised MIS, currently undergoing commissioning at the NNL Central Laboratory, ahead of full operational deployment on the Sellafield site.

‘When we first looked at this in 2009, we thought we had a 50/50 chance of turning this idea into a product that could be commercialised for the nuclear industry. But the results have surpassed expectations at every stage.’

However, with IP still being registered, Shearer was hesitant to provide further technical details on the MIS. ‘The Lynkeos system is based on a plastic-based scintillator technology incorporated into a design that meets the considerable challenges and stringent specifications of the international nuclear industry.’ In particular, Shearer noted, ‘We expect the technology will become economically competitive with current approaches while delivering a better analysis capability.’

Following a Royal Society workshop on cosmic-ray muongraphy in May 2018, Shearer said a more detailed overview and the different potential applications of the MIS would be available in a forthcoming issue of Philosophical Transactions of the Royal Society A.