Double-differential neutron-induced cross sections on Fe-nat at GANIL-NFS

Inside the steel bones of a nuclear reactor, a quiet but relentless battle is being waged. When high-speed neutrons slam into the metal, they do not just bounce off; they shatter the atomic nuclei, creating tiny fragments of hydrogen and helium gas. Over years of operation, these gases accumulate within the solid metal, causing it to swell and become brittle, much like a sponge that has absorbed too much water. This damage threatens the structural integrity of the very vessels designed to hold the immense energy of fusion or fission. To predict how long a reactor can safely operate, scientists need to know exactly how these neutrons interact with iron, the primary ingredient in reactor steel. They need to measure not just how many particles are knocked loose, but exactly how much energy they carry and in which direction they fly.

For decades, scientists have struggled to get these measurements for the high-energy neutrons found in modern reactors. While data exists for lower energies, the picture becomes blurry and uncertain above 14 million electron volts, a range where the physics becomes complex and difficult to model. Without precise data in this high-energy zone, the computer simulations used to design safe reactors are forced to guess, leaving engineers with less certainty about the lifespan of their structures. A team of researchers recently set out to fill this gap, conducting a new experiment to map the behavior of neutrons hitting natural iron with unprecedented detail.

The experiment took place in 2023 at the Neutrons For Science facility in Caen, France, a site dedicated to producing beams of neutrons for research. The team used a specialized instrument called Medley, a device originally built at Uppsala University in Sweden and designed specifically to catch the light, charged particles that fly out when a nucleus breaks apart. The setup is essentially a large, empty vacuum chamber with a target of natural iron placed in the center. Surrounding this target are eight sets of detectors, arranged like the spokes of a wheel, each capable of tracking particles flying out at different angles. The iron target itself was a thin foil, just thick enough to interact with the beam but thin enough to let the resulting particles escape without losing too much energy.

To create the conditions needed for the study, the facility fired a beam of neutrons at the iron target. These neutrons were not all the same speed; instead, they formed a "white" beam, a broad spectrum containing energies ranging from low levels up to 44 million electron volts. As the neutrons struck the iron, they triggered nuclear reactions that ejected protons, deuterons, and other light particles. The detectors, arranged in telescopes, caught these escaping particles. By measuring how much energy each particle deposited in a series of silicon layers and a final scintillator crystal, the researchers could identify exactly what kind of particle it was and how fast it was moving.

A critical part of the process involved timing. The researchers measured the time it took for a particle to travel from the target to the detector, comparing it to the precise moment the neutron beam was generated. This time-of-flight measurement allowed them to calculate the energy of the incoming neutrons for every single event. Because the particles lose a small amount of energy as they pass through the iron target itself, the team had to apply a mathematical correction to reconstruct the true energy of the particles as they were first born. They used computer simulations to model how the particles would lose energy inside the metal and then used a statistical method to reverse-engineer the original data, effectively peeling back the distortion caused by the target material.

The result of this careful work is a new set of measurements for the double-differential cross section, a term that simply describes the probability of a particle being emitted at a specific angle with a specific energy. The team successfully measured these values for protons emitted from iron when hit by neutrons with an average energy of 14.5 million electron volts. They mapped out how many protons flew off at angles ranging from 20 to 160 degrees, creating a detailed picture of the emission pattern. The data showed that the experimental setup worked reliably, capturing the particles with high precision and allowing the team to correct for the energy losses within the target.

While the full analysis of all the data collected during the campaign is still underway, the preliminary results for the 14.5 million electron volt neutrons are already promising. The researchers found that their measurements could be fitted smoothly with standard mathematical curves, suggesting that the data is consistent and reliable. This success validates the use of the Medley setup at this facility and confirms that the complex corrections for energy loss and particle identification were handled correctly. The team is now working to extend these measurements to cover the entire energy range up to 44 million electron volts and to analyze other types of particles, such as helium nuclei.

This work provides a crucial piece of the puzzle for nuclear engineers. By providing hard data on how iron behaves under high-energy bombardment, the study offers a firmer foundation for the computer models used to design future fusion reactors and to assess the safety of existing ones. The ability to predict how structural materials will degrade over time is essential for the transition to cleaner energy sources, and this experiment has taken a significant step toward making those predictions more accurate. The researchers have proven that it is possible to measure these difficult interactions with high precision, opening the door for more rigorous testing of the materials that will one day hold the power of the stars.