Neutron Imaging: A Look Inside Solid Objects

Neutron imaging is a non-destructive method that can peer through solid metal as if it were glass, yet be stopped by a drop of water. This technique visualizes the internal structure of objects, allowing scientists and engineers to uncover hidden details and understand invisible processes. By using neutrons instead of light or X-rays, researchers can probe materials in a way that reveals complementary information about what lies within.

The Neutron Imaging Process

Creating a neutron image begins at a large-scale facility housing either a nuclear reactor or a particle accelerator. In a reactor, nuclear fission releases a stream of neutrons. A particle accelerator can also be used to fire protons at a dense target, causing it to shed neutrons in a process called spallation.

Once produced, these neutrons are guided into a controlled beam directed at the sample. Unlike light or X-rays that interact with an atom’s electrons, neutrons pass through the electron cloud to interact with the atomic nucleus. The likelihood of a neutron being scattered or absorbed depends on the type of nucleus it encounters, not the material’s density.

After passing through the object, the remaining neutrons strike a detector, often a scintillator screen. The screen converts neutron impacts into visible light, which is captured by a digital camera. The resulting image is a shadowgraph where areas that absorbed or scattered neutrons appear dark, while transparent areas appear light. This creates a map of the object’s internal composition.

Contrasting with X-ray Imaging

Neutron imaging is often contrasted with the more familiar X-ray imaging. X-rays are high-energy light that interacts with an atom’s electrons. Materials with many electrons, like dense metals or the calcium in bones, are effective at blocking X-rays. This is why medical X-rays show bones clearly against soft tissue.

Neutrons operate by a different principle, interacting with the nucleus itself. This interaction is not dependent on material density. Many dense metals opaque to X-rays, like lead and titanium, are transparent to neutrons. Conversely, light elements, particularly hydrogen, are effective at stopping neutrons, making them sensitive to materials like water, oil, and plastics.

A classic example highlights this contrast: a sealed lead box containing a flower. An X-ray image would show only the box’s outline, as the metal absorbs the X-rays, hiding the contents. A neutron image would be different. The neutrons would pass through the lead but be blocked by the hydrogen-rich water in the flower, revealing the biological structure inside the metal container.

Applications in Science and Industry

In engineering, the ability to see inside operating machinery without disassembly is an advantage. Researchers use neutron imaging to visualize the flow of oil and fuel inside a running engine in real-time. This allows them to study lubrication dynamics and fuel injection patterns under actual operating conditions to identify inefficiencies and design more effective engines.

In energy research, neutron imaging helps develop better batteries. As a battery charges and discharges, lithium ions move between the electrodes. Scientists use neutrons to track this migration of lithium in real-time, visualizing its distribution. This provides insight into how materials and designs affect battery performance, degradation, and safety.

Archaeologists and paleontologists use this technology for non-destructive examination. It can reveal the internal features of ancient artifacts, like the construction of a sword or the contents of a sealed vessel. For paleontologists, neutron imaging can show the unexposed structures of fossils encased in rock. This allows for the study of internal anatomy without risking damage to the specimen.

Accessing Neutron Imaging Facilities

Neutron imaging is not available in a typical lab or hospital, as it requires large, centralized research facilities. The sources that produce a useful neutron beam are either nuclear research reactors or spallation sources driven by particle accelerators. These are multimillion-dollar installations with only a limited number existing worldwide.

Access to these facilities is highly competitive. Researchers from academia, government, and private industry must submit detailed research proposals to use a neutron beamline. These proposals are then reviewed by a panel of experts who evaluate the scientific merit and technical feasibility of the experiment.

This proposal-based system ensures that the limited beam time is allocated to the most impactful research projects. Gaining access allows research teams to conduct experiments that would otherwise be impossible. The specialized nature of these facilities highlights the unique capabilities of using neutrons as a scientific probe.