Article from the DLRmagazine 177: Meet Sarah Barnes, expert in muon tomography
Revealing a hidden world
Sarah Barnes from the DLR Institute for the Protection of Maritime Infrastructures
Sarah Barnes from the DLR Institute for the Protection of Maritime Infrastructures is investigating possible applications of muons. These elementary particles are part of cosmic radiation and are therefore present everywhere.
"Even as we stand here talking, thousands of muons are passing through our bodies," explains Sarah Barnes, a researcher at the DLR Institute for the Protection of Maritime Infrastructures, calmly pointing to her office – part laboratory, part warehouse. Colourful cables stick out from shelves and walls on the left, while a few desks with computers are arranged on the right. "These only came in yesterday," says the British-born scientist, referring to two black boxes stacked in the middle of the room "detector plates for muon tomography".
Barnes is a muon researcher. At DLR, she studies these elementary particles which are present all around us and form part of what is known as cosmic radiation. This high-energy particle radiation originates from deep space – from black holes, supernovae or even our own Sun. It reaches Earth's atmosphere in the form of protons, where it triggers the formation of many additional particles: electrons, protons, neutrons – and muons. This shower of new particles rains down on Earth's surface, and from here Barnes aims to use the muons that fall to examine objects tomographically – making hidden objects visible. The principle is similar to radiology – except that, unlike artificially-generated X-rays, naturally-occurring muons in cosmic radiation don’t come with negative health impacts.
Beyond the limits of X-rays
This new technology opens up a wide range of applications. For example, a muon scanner could be used to inspect the integrity of bridges without having to close them to traffic. In industry, prototypes made of particularly dense materials could be examined without having to disassemble them into their component parts. Muon tomography could also be used to check the condition of nuclear waste containers – providing vital information on whether the containers and their contents are still intact, which is important for long-term interim storage and necessary final disposal.
The technique is currently being used by archaeologists, helping them search for hidden chambers inside pyramids or to 'see' inside ancient urns that are too fragile to open. In geology, muon tomography is used to observe volcanic chambers and anticipate potential eruptions. At DLR, another application is currently under the spotlight: researchers in the EU-funded SilentBorder project are working on a muon tomograph to scan shipping containers.
First results of a muon scan
A scanner that utilises naturally occurring muon radiation ...
To conduct tomography, Barnes uses two opposing detector plates – known as hodoscopes. Each consists of three double layers of fibres arranged perpendicularly to each other. When a muon passes through one of the fibres, a flash of light is generated, which is recorded by a computer. This allows the position of the muon in the detectors to be determined. As the muon travels through the material, between the scanner plates, the muon is very slightly deflected from its path. "This happens almost at the speed of light," Barnes explains. The scanner measures the tiny variation between the elementary particle’s entry and exit angles, and complex algorithms and machine learning methods process the data to create an accurate three-dimensional image of what lies between the hodoscopes. Based on how the muons are scattered, it is even possible to infer which materials are located where.
SilentBorder
SilentBorder is an EU-funded project in which DLR researchers, together with colleagues at nine European universities, companies and public authorities, are developing an intelligent, cost-effective protoype scanner. The goal is for shipping containers to pass through the scanner in just two to five minutes – similar to baggage screening at airports. Since naturally occurring muon radiation poses no additional health risks, researchers hope to create a procedure that is more flexible, less bureaucratic and free from concerns about radiation regulations.
Active and passive methods
In imaging, a distinction is made between active and passive techniques. Active methods emit radiation that is then measured – such as ultrasound and X-rays. Passive methods, by contrast, use either radiation already being emitted by an object – such as temperature distributions revealed by a thermal imaging camera – or radiation that is already present, such as cosmic rays. One advantage of passive processes is their low energy consumption. A muon scanner panel consumes only about 100 watts – as much as a powerful laptop. What’s more, muon tomography enables three-dimensional imaging of the objects under investigation, which is otherwise only possible using complex and expensive techniques like computed tomography. The downside is the time required to scan very large objects, since the number of muons in cosmic radiation is limited. The rule is: the higher the resolution of the image required, the more muons must be tracked.
Less red tape, more security
Muon scanners may also help reduce bureaucracy, since radiation protection regulations for handling artificial sources of radiation would no longer apply. No special safety precautions need to be taken or documented, making the technology easier to use than, say, X-ray equipment.
Readout electronics of the muon detector
Sarah Barnes and her colleague Maximilian Pérez Prada investigate the readout electronics of the muon detector.
At container ports, currently only two to ten percent of all containers are randomly checked for illegal goods. Various methods are used for this purpose, the most common being X-ray scans and manual inspections, that require containers to be opened. In some cases, sniffer dogs are deployed to detect drugs or people. All of these methods are time-consuming and costly.
At the end of our conversation, Sarah Barnes shares a glimpse of the future: "We hope that our work will lead to the first muon scanners being installed in ports within five to ten years. The long-term dream is a large-scale muon scanning system that shipping containers pass through directly after unloading – scanned automatically in a few minutes, just like luggage at the airport – and without any radiation exposure."
