Electrodeposition of Highly Enriched Uranium Thin Films for Advanced Fission Detectors

Inside the heart of a research reactor, where neutrons fly in a chaotic storm and temperatures soar, scientists need to know exactly how intense the radiation is at any given moment. They cannot rely on sensors that only tell them the total amount of radiation after an experiment is finished; they need a live feed, a constant stream of data that shows how the reactor's power rises and falls in real time. To get this, they use tiny devices called micro-pocket fission detectors. These sensors work like miniature ionization chambers. Inside each one is a very thin layer of special uranium, sitting on a platinum disk, surrounded by a small pocket of gas. When a neutron hits an atom in that uranium layer, it splits the atom apart. This split sends out two high-speed fragments. One fragment gets stuck in the metal disk, but the other flies out into the gas pocket, knocking electrons loose from the gas atoms and creating a tiny electrical spark. By counting these sparks, the sensor can tell the researchers exactly how many neutrons are passing by every second.

For this sensor to work perfectly, the layer of uranium must be just the right thickness. If the layer is too thin, there are not enough uranium atoms to catch enough neutrons to make a measurable signal. If the layer is too thick, the fragments from atoms deep inside the layer get stopped by the uranium itself before they can reach the gas, and the signal is lost. The sweet spot for these sensors is a mass of uranium between 0.3 and 1.0 micrograms. This is an incredibly small amount, roughly the weight of a single grain of dust, yet it must be deposited with extreme precision onto a disk only a few millimeters wide. The challenge is to put this tiny, precise amount of highly enriched uranium onto the sensor without wasting material or creating a layer that is uneven or too thick.

A team of researchers at Idaho National Laboratory set out to solve this problem by testing two different ways to build these uranium layers. They wanted to find the best method for coating platinum disks with 93 percent highly enriched uranium, a material chosen because its high concentration of the fissionable isotope uranium-235 makes the sensors much more sensitive, even with such a tiny total mass. The team compared two techniques: one that sweeps the electrical voltage back and forth over the disk, and another that holds the voltage steady at a single level. They also developed a strict new way to weigh the uranium after it was deposited, using two different scientific tools to make sure their measurements were accurate.

The researchers began by preparing a liquid solution containing the uranium and carefully controlling its acidity. They found that the acidity level was critical; if the liquid was too acidic, the uranium would dissolve back into the solution, but if it was too basic, it would form clumps that would not stick to the disk. By keeping the acidity at a specific, neutral level, they created a stable environment for the uranium to settle onto the platinum. They then applied the two different electrical methods to see which one could deposit the uranium more efficiently. The first method, which involved rapidly changing the electrical voltage, had been used before for smaller amounts of uranium, but the team wanted to see if it could handle the larger amounts needed for these new sensors. The second method, which kept the voltage constant, was a more straightforward approach that they hoped would build the layer faster and more consistently.

As they ran the experiments, they watched how the uranium accumulated on the disks. The method that swept the voltage back and forth worked, but it had a limit. As they added more layers by running more cycles, the rate at which new uranium stuck to the disk slowed down significantly. It was as if the surface became crowded, making it harder for new atoms to find a place to land. In contrast, the method that held the voltage steady was much more effective at building up the mass. It deposited uranium at a faster rate and continued to do so for longer periods without hitting the same wall of diminishing returns. The team found that by using the steady voltage method and stopping the process at just the right moment, they could reliably create layers that fell within the target range of 0.3 to 1.0 micrograms.

To be absolutely sure of their results, the team did not just trust the electrical data from the deposition process. They used two independent methods to weigh the uranium after it was on the disk. The first method involved measuring the natural radioactivity of a specific isotope, uranium-234, which is present in the material in a fixed ratio to the main uranium-235. By counting the alpha particles emitted by this isotope, they could calculate the total mass of the uranium. The second method involved dissolving a small piece of the uranium and analyzing it with a machine that separates atoms by their weight. This second method served as a check to confirm the first.

The results showed that the steady voltage method was the clear winner for creating these sensors. It produced consistent layers that were close to the target mass, and the isotopic signature of the uranium remained intact, meaning the process did not alter the chemical makeup of the material. However, the team also discovered a small but important discrepancy between their two weighing methods. The method that counted radioactivity consistently measured slightly less mass than the method that weighed the dissolved atoms. The researchers investigated this gap carefully. They looked at the shape of the energy signals from the radioactive counts and found no signs that the uranium layer was too thick to let the particles escape, which would have caused the radioactivity method to underestimate the mass. Instead, they concluded that the difference likely came from a small calibration issue in the equipment used to count the radioactivity, rather than a problem with the uranium layer itself.

This study confirms that the steady voltage method is a reliable way to build the ultra-thin uranium layers needed for advanced neutron detectors. It provides a clear path for making sensors that can survive the harsh environment inside a research reactor and provide the real-time data scientists need to understand how nuclear fuel behaves. While the team did not test the sensors inside an actual reactor as part of this specific study, the fabrication process is now proven to create the right kind of material. The next step will be to put these sensors into the reactor to see how they perform under real conditions, but the foundation for that work has been firmly laid. The researchers have shown that with careful control of the chemistry and the right electrical technique, it is possible to build these microscopic, high-precision tools that will help keep research reactors running safely and efficiently.