The scintillating fibre beam monitor for the CHNet-MAXI experiment at ISIS: characterisation of the light collection and detection system

In the quiet study of ancient history, lead is a silent storyteller. For centuries, artisans have shaped this metal into everything from Roman pipes to Renaissance sculptures, yet the specific origin of the ore used in these objects often remains a mystery. The different stable forms of lead atoms, known as isotopes, act like a chemical fingerprint, revealing the mine where the metal was first dug. However, reading this fingerprint without damaging the precious artifact has long been a challenge. A new approach, using a beam of subatomic particles called muons, offers a way to peek inside these objects non-destructively. When a muon is captured by a lead atom, it triggers a cascade of energy that reveals the atom's identity. To make this technique work, scientists must first master the art of guiding and counting these fleeting particles with extreme precision, ensuring that every measurement is accurate and every artifact remains untouched.

At the ISIS Neutron and Muon Source in the United Kingdom, a team of researchers is preparing for a major experiment called CHNet-MAXI, designed to unlock the secrets of lead in cultural heritage. Before they can begin their work on ancient artifacts, they had to build and test a specialized tool: a beam monitor. This device acts as a high-speed camera for the particle beam, tracking exactly where the muons are and how many are passing through at any given moment. The monitor is constructed from two flat layers of plastic fibers, each fiber being a tiny, square tube just three millimeters wide and ten centimeters long. These fibers are arranged in a grid, with one layer measuring the horizontal position and the other measuring the vertical position. When a particle strikes a fiber, it produces a flash of light, which is then captured by sensitive electronic eyes called silicon photomultipliers. The entire assembly is a delicate mesh of sixty-four fibers, each connected to its own electronic sensor, all housed within a custom 3D-printed case.

The researchers knew that for the experiment to succeed, this monitor had to be perfect. They needed to understand exactly how the light travels through the fibers and how the electronic sensors respond to it. To do this, they turned to powerful computer simulations. By creating a virtual model of the detector, they could watch how a beam of muons would behave as it passed through the fibers. The simulations showed that at the specific energy level chosen for the experiment, the muons would travel through the first layer of fibers and then stop within the second layer, depositing their energy in a predictable way. This computer modeling was crucial because it helped the team calculate a correction factor, a mathematical adjustment needed to convert the raw electrical signals into an accurate count of particles. The simulations suggested that because the fibers are packed so tightly together, almost every muon would hit at least one fiber, meaning the correction factor would be very close to one, simplifying the final calculations significantly.

With the theory in place, the team took the detector to a different facility in Italy, the CNAO center, to test it with a beam of protons. This was a dress rehearsal using a different type of particle, but one that allowed them to check the hardware and the data collection systems. They fired a beam of protons at the detector and watched the results on a screen. The device worked, successfully mapping the shape of the beam and confirming that the electronics could handle the task. However, the initial data revealed a problem: the response was not uniform. Some fibers were reporting a stronger signal than others, even when hit by the same amount of energy. This unevenness would have distorted the final images of the ancient artifacts, making some parts look brighter or darker than they really were. The team realized that before they could trust the device with real muons, they had to balance the sensitivity of every single fiber.

To fix this, the researchers performed a careful calibration using a source of gamma rays. They placed a small radioactive source above the detector and measured the light output from each of the sixty-four fibers. Because the source emits radiation in a predictable pattern, the researchers knew exactly how much light each fiber should produce based on its distance from the source. They compared the actual measurements against this expected pattern. Where a fiber was too sensitive, they applied a small reduction factor; where it was too weak, they applied a boost. This process, known as equalization, smoothed out the differences between the channels. After the adjustment, the detector produced a clean, uniform image of the beam, with no artificial hot or cold spots. The team verified that this calibration did not change the overall size or position of the beam, only the consistency of the signal across the surface.

The final step was to bring the detector to the ISIS facility, where it will be mounted on the beamline for the CHNet-MAXI experiment. The device is now ready to serve as the eyes of the experiment, counting every muon that passes through and ensuring that the data collected from the ancient lead samples is normalized correctly. By knowing exactly how many particles hit the sample, the scientists can accurately determine the isotopic ratios without ever touching the artifact. The successful testing and calibration of this scintillating fiber monitor represent a critical milestone. It transforms a complex assembly of plastic and electronics into a reliable scientific instrument, paving the way for a new era of non-destructive analysis in archaeology. The path from the computer simulations to the final calibration has ensured that when the first ancient object is scanned, the results will be as clear and true as the history they are meant to reveal.