Development and Performance Study of a Capillary Liquid Scintillator Neutron Detector

In the quest to understand how stars are born and how we might harness their power, scientists often look to the most violent events in the laboratory: the implosion of tiny fuel pellets in fusion experiments. When these pellets collapse, they release a flood of high-energy neutrons that carry a secret map of the explosion's shape and symmetry. To read this map, researchers need detectors that can see these invisible particles with extreme sharpness, distinguishing details that are mere fractions of a millimeter apart. The challenge lies in capturing the faint flash of light these neutrons create without that light blurring into a smear, which would destroy the fine details needed to understand the fusion process.

A team of researchers in China has taken a significant step toward solving this problem by building and testing a new type of neutron detector. Instead of using solid blocks or fibers, they constructed a device made of thousands of tiny, hollow glass tubes packed together like a honeycomb. Each tube, or capillary, is filled with a special liquid that glows when a neutron hits it. The goal was to see if this structure could keep the light from each individual neutron confined to its own tube, allowing the detector to pinpoint exactly where the particle struck. While previous computer models suggested this would work, the real world often introduces complications, such as light leaking from one tube into its neighbor, which can blur the image and confuse the data.

To test their design, the team created a detector using a hexagonal array of quartz glass capillaries, each with an inner diameter of about 50 micrometers. They filled these tubes with a liquid scintillator called EJ-309, a substance known for its ability to convert neutron energy into visible light. Because the liquid has a slightly different optical density than the glass walls, light tends to bounce inside the tubes, much like a fiber optic cable guides a signal. However, the researchers knew that in a real device, some light would inevitably escape and spill over into adjacent tubes. To stop this, they coated the outside of the glass tubes with a dark, light-absorbing material, essentially painting the outside of the honeycomb to catch any stray photons before they could travel further.

The team first tested the concept using a green laser to simulate a point of light entering the tubes. Without the dark coating, the light spread out widely, creating a blurry spot that covered an area roughly 260 micrometers across. When they used the coated tubes, the light stayed tightly confined, shrinking the spot to just 90 micrometers. This simple but effective modification proved that the coating could successfully suppress the lateral spread of light, keeping the signal sharp.

Next, they moved to the real test: exposing the detector to neutrons from an Americium-Beryllium source. They recorded thousands of frames of data, carefully filtering out background noise to identify genuine neutron events. The results were promising. The detector successfully identified individual neutrons and measured their energy with a high degree of accuracy. The researchers found that the detector captured about 10% of the neutrons hitting its surface, a figure that rose to roughly 13.5% when calculated based only on the area occupied by the active liquid, excluding the glass walls. This level of efficiency is considered strong for a device of this size and complexity.

Perhaps most importantly, the team analyzed how well the detector could distinguish between one neutron hitting a single tube and multiple neutrons hitting nearby tubes simultaneously. For events involving one or two tubes, the detector's response was nearly perfectly linear, meaning the signal strength doubled when two tubes were hit compared to one. However, when three or more tubes were involved, the signal became slightly distorted. This happened because the light from the central tubes spread into the neighboring ones, and the larger area used to measure the signal accidentally captured some of this extra, scattered light. While this revealed a limit in the detector's ability to handle complex, overlapping events, it also provided a clear physical understanding of how the light behaves, allowing future designs to account for this effect.

The study also measured the detector's ability to pinpoint the location of a neutron. By analyzing the shape of the light spots, the researchers determined that the core of the signal was incredibly sharp, with a width of about 12.7 micrometers. This allowed them to calculate the center of the light spot with a precision of roughly 5.5 micrometers. However, the ultimate limit on how close two separate neutrons could be and still be seen as distinct points was set by the physical spacing of the tubes themselves, which was about 54 micrometers. This means that while the electronics can find the center of a spot very precisely, the detector cannot distinguish two neutrons if they land in tubes that are too close together.

These findings offer a practical roadmap for building better neutron imagers for future fusion research. The work confirms that coating the outside of the capillaries is a viable way to keep light from blurring the image, and it provides a clear picture of the detector's strengths and limitations. By understanding exactly how the light spreads and where the resolution limits lie, engineers can now design next-generation detectors that are even sharper, potentially unlocking a clearer view of the fusion process and bringing us closer to the goal of clean, limitless energy.