Advances in Neutron Imaging Create Opportunities for Low Power Research Reactors

Neutron imaging is a non-invasive technique for examining internal structures that is carried out using research reactors or accelerator-based neutron sources. “It is an amazing tool, with endless possibilities for scientific and industrial research and development, as well as for forensics and the study of cultural artefacts,” said Molly-Kate Gavello, Associate Project Officer at the IAEA. Neutron imaging can be used to test motors, shock absorbers and turbine blades. It can show how water moves within a living plant, or examine the inside of a fossilized dinosaur skull filled with ferrous rock.  

Although neutron imaging has been in use since the 1950s, two dimensional (2D), film-based images were the main format until the 1990s. With the advent of digital technologies, including sophisticated digital cameras, neutron imaging now utilizes computed tomography (CT), which uses hundreds of images taken from different angles to create a highly detailed three dimensional (3D) image.

Until recently, neutron imaging with CT, or 3D imaging, was not feasible for low flux neutron sources, such as low power research reactors, for both technical and financial reasons.

High-quality images at low power

This changed in 2021, when Jana Matoušková, a PhD student at the Czech Technical University in Prague (CTU), and her supervisor Lubomír Sklenka demonstrated the capability to perform neutron imaging with CT at 500 watts (W) of research reactor power.  

The breakthrough followed two developments. First, low-cost, high-quality astronomy cameras had become available in the previous decade. Second, researchers at the Heinz Maier-Leibnitz Research Neutron Source (FRM II) at the Technical University of Munich, Germany, had realized the potential of these new cameras, and in 2016 they presented the first mini facility for neutron tomography, including for low power reactors. Led by Burkhard Schillinger, the team developed and built a low-cost, high-quality neutron imaging system with a 3D printed detector housing and a downscaled version of the professional control software used at the Advanced Neutron Tomography and Radiography Experimental System (ANTARES) imaging facility at the FRM II research reactor. The image quality of the new detectors matched that of the state-of-the-art system usually employed at the ANTARES facility.

Matoušková wanted to test neutron imaging with low power neutron sources, such as the CTU’s 500 W VR-1 training reactor — by comparison, the 20 megawatt FRM II reactor has 40 000 times more power and therefore produces 40 000 times more neutrons than the CTU reactor. This would prove to be challenging, since she was unable to access the CTU facilities for experiments due to COVID-19 restrictions. 

Sklenka contacted Schillinger for advice on replicating the low-cost system that FRM II had developed, and Schillinger advised Matoušková in video calls and supplied her with information on the system’s design and where to source the necessary parts. Step by step, she built a neutron imaging system in her own home and tested it with visible light.  

Once the COVID-19 restrictions had been lifted, Matoušková installed her system at the CTU reactor and successfully generated the CTU’s first digital neutron 2D image, followed by a neutron CT with a 12-hour exposure at 500 W. This means that results can be obtained within one day and with significantly less power — the power of research reactors where the technique is also used ranges from hundreds of kilowatts to tens of megawatts. 

Matoušková is now refining the CTU’s neutron imaging system as part of her PhD studies. The system is mainly being used for educational purposes, but also for carrying out research, for example, to examine cultural artefacts in collaboration with the National Gallery in Prague. 

Sharing technology and expertise

FRM II and the CTU’s experience has demonstrated that a mini facility could be used at any neutron source, including extremely low power research reactors. Schillinger said that his team is ready to provide the design plans and software for free and to help with installation and set-up internationally. 

With parts made by a 3D printer, software downscaled to fit on a laptop and a drop in prices for astrophotography cameras, the whole package can be assembled for under €5000 and can be transported easily. In 2022, Schillinger and Aaron Craft, a research scientist at the Idaho National Laboratory in the United States of America, led an IAEA expert mission to install a digital neutron imaging system at the Chilean Nuclear Energy Commission’s RECH-1 research reactor. Schillinger brought the components in a suitcase, and the system was set up within two days. 

“The IAEA plays a key role in making this technology available for low power research reactors,” Schillinger said. “With the new sensitive detectors, it opens an entire new field of application for those reactors, which do not provide enough neutrons for complex neutron scattering experiments. Neutron imaging makes them more accessible for education, research and collaboration with museums.”  

The IAEA supports technical cooperation with research reactors, including expert missions and equipment procurement. It also publishes guidebooks on neutron imaging, provides regional training and is expanding its e-learning opportunities. The IAEA also enabled Matoušková to spend four months at the RA-6 research reactor in Argentina to help install and test a low-cost neutron imaging system in 2022.  

A similar dual neutron–X-ray system has been installed and commissioned at the IAEA’s Neutron Science Facility in Seibersdorf, Austria, where it is being used for training. 

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What is neutron imaging? 

Neutron imaging is a non-invasive method for examining the internal structures and composition of opaque objects. It is based on principles similar to those of X-ray imaging. However, in contrast to X-rays, which are absorbed by dense materials such as metals, neutron beams penetrate most metals and rock, and they are attenuated by some light elements, such as boron, carbon, hydrogen and lithium. Neutrons can also help visualize magnetic fields, as well as strain in technological and structural materials.