Non-Destructive Beam Monitoring via Secondary Radiation Detection with Ce-Doped Silica Fibers

This paper demonstrates that a non-destructive external fiber monitor using Ce-doped silica scintillating fibers can effectively detect secondary radiation from an 18 MeV medical cyclotron beamline to provide linear intensity monitoring, beam-loss tracking, and decoupled spatial displacement measurements across various operational scenarios.

The Gist

Imagine you are trying to watch a high-speed race car zoom through a narrow tunnel. If you stick a camera or a sensor directly into the tunnel to measure the car's speed or position, you might accidentally hit the car, slow it down, or even crash it. In the world of particle physics, specifically with medical cyclotrons (machines that make radioactive isotopes for cancer treatment), this is a real problem. The "cars" are beams of protons, and the "tunnel" is a delicate beamline. If you put a physical sensor in the way, you ruin the beam's quality, which is bad news for making medicine.

This paper introduces a clever solution: The "Ghost Sensor."

Instead of sticking a sensor in the beam, the researchers built a system that watches the beam from the sidelines by detecting the "dust" and "sparkles" the beam kicks up as it travels.

The Core Idea: Watching the Wake, Not the Boat

Think of a speedboat moving through a lake. You don't need to jump into the water to know how fast the boat is going or where it's steering. You can just look at the wake (the waves) it leaves behind.

In this experiment:

• The Boat: A beam of protons (the particle beam).
• The Lake: The metal parts of the machine (collimators, targets, beam dumps).
• The Wake: When the protons hit the metal, they create a shower of invisible "secondary radiation" (neutrons and gamma rays). This is the "sparkle" or "wake."
• The Sensor: A special fiber-optic cable (like a glowing straw) coated with a material called Cerium-doped silica.

When this "glowing straw" is placed near the metal parts (but not in the beam's path), the secondary radiation hits it and makes it glow with a tiny bit of light. The researchers count these glimmers to figure out what the beam is doing.

The Three Tests They Ran

The team tested this "Ghost Sensor" in three different scenarios, like testing a new security camera in three different situations:

1. Measuring the "Engine Power" (Beam Intensity)

• The Goal: See if the sensor can tell how hard the beam is working (how many protons are flowing).
• The Analogy: Imagine trying to guess how fast a car is going by counting how many leaves it kicks up.
• The Result: They found a perfect straight-line relationship. The more protons flowing, the more "glow" the sensor saw. It worked accurately even when they turned the "engine" up or down by a factor of 1,000. It's like a volume knob that works perfectly from a whisper to a shout.

2. Spotting the "Leak" (Beam Loss)

• The Goal: Sometimes the beam gets messy and hits the walls of the tunnel where it shouldn't. This is a "beam loss," which is dangerous and wasteful.
• The Analogy: Imagine a garden hose that is supposed to spray water into a bucket. If the hose is kinked or the nozzle is loose, water sprays everywhere else. The researchers wanted to know if their sensor could tell how much water was spraying out the wrong way without getting wet.
• The Result: By focusing the beam tightly or letting it spread out, they created different amounts of "spray." The sensor's glow increased steadily as the "spray" (beam loss) got worse. This means they can use the sensor as an early warning system to say, "Hey, the beam is getting messy, fix the focus!"

3. Tracking the "Steering Wheel" (Beam Position)

• The Goal: If the beam drifts left or right, the machine needs to know so it can correct it.
• The Analogy: Imagine holding four flashlights around a target. If a ball hits the target, the flashlight closest to the hit will see the most "sparkles."
• The Setup: They put four fiber sensors around a target: Top, Bottom, Left, and Right.
• The Result: When they moved the beam to the Left, the Left sensor glowed brighter than the Right one, but the Top and Bottom stayed the same. When they moved it Up, the Top sensor glowed brighter.
• The Magic: By comparing the "Left vs. Right" glow and the "Top vs. Bottom" glow, they could pinpoint exactly where the beam was, completely independently. It's like having a GPS that tells you if you are drifting left or right without ever touching the car.

Why This Matters

This technology is a game-changer for medical cyclotrons because:

• It's Non-Destructive: It doesn't touch the beam, so the beam stays perfect for making medicine.
• It's Retrofittable: You can tape these sensors onto existing machines without rebuilding the whole thing.
• It's Smart: It can tell you if the beam is too strong, too weak, hitting the wrong spot, or drifting off course.

The Future: Making the "Ghost" Brighter

The researchers admit their current sensors are a bit like "night-vision goggles" that work, but could be better. They plan to swap the current fibers for a super-bright crystal (GAGG:Ce).

• The Upgrade: Think of swapping a dim flashlight for a high-powered laser pointer. This will make the signal much clearer, especially when the beam is very weak (like a whisper), and allow them to pinpoint the beam's location with even greater precision.

In a nutshell: This paper proves that you can monitor a high-speed particle beam by watching the "dust" it kicks up, using a special fiber-optic "glow-stick" that sits safely on the sidelines. It's a safe, smart, and non-invasive way to keep the machines that make life-saving medicine running perfectly.

Technical Summary

1. Problem Statement

In low-energy medical cyclotrons (typically accelerating protons to tens of MeV), reliable beam diagnostics are critical for radioisotope production and research. However, traditional diagnostic methods are often interceptive, meaning they physically interact with the beam (e.g., wire grids, scintillator screens, Faraday cups).

• The Issue: At low energies, even thin interceptive devices can significantly perturb the beam's transverse profile or degrade its energy. This makes them unsuitable for sensitive applications, such as cross-section studies near reaction thresholds.
• The Goal: Develop a non-destructive monitoring system capable of measuring beam intensity, losses, and position without introducing material into the beam path.

2. Methodology

The authors investigated an External Fiber Monitor (EFM) based on Cerium-doped (Ce-doped) silica scintillating fibers. The system operates by detecting secondary radiation (neutrons and gamma rays) generated when the primary proton beam interacts with existing beamline components (collimators, targets, dumps).

Experimental Setup:

• Location: Bern Medical Cyclotron (18 MeV proton accelerator) Beam Transfer Line (BTL).
• Detector: Ce-doped silica fibers (3 cm sensitive length) arranged in specific geometries around beamline components.
• Readout: Scintillation light is transported via 20-meter multimode optical fibers to a single-photon detector (IDQ) connected to a pulse counting system (Vertilon MCPC618), located outside the radiation area.
• Three Use Cases Evaluated:
  1. Beam Intensity Monitoring: Fibers mounted around a water-cooled, electrically isolated beam dump.
  2. Beam Loss Monitoring: Fibers mounted around a 10 mm collimator while varying beam focusing (FWHM).
  3. Beam Position Monitoring: Four fibers arranged in a collar around a beam dump to detect horizontal and vertical beam displacements.

3. Key Contributions

• First Systematic Assessment: This work provides the first comprehensive evaluation of the EFM concept across three distinct operational scenarios (intensity, loss, and position).
• Non-Interceptive Architecture: Demonstrates a viable method for continuous beam monitoring without perturbing the beam, suitable for retrofitting onto existing infrastructure.
• Decoupled Position Sensitivity: Proved that opposing fiber pairs can provide independent, monotonic sensitivity to horizontal and vertical beam displacements.
• Empirical Calibration: Established a correlation between fiber signals and electrical loss proxies, enabling the estimation of beam losses without direct interception.

4. Key Results

A. Signal Linearity (Intensity Monitoring)

• Performance: The summed signal from four fibers exhibited a linear dependence on the beam current over nearly three orders of magnitude (up to ~92 µA).
• Residuals: Relative residuals remained within ±3%.
• Limitations:
  • Low Current: Deviations were attributed to beam instability and the inability of the upstream UniBEaM detector to monitor position at high currents.
  • High Current: A systematic drift was observed, attributed to the activation of the aluminum target (production of short-lived $^{27}\text{Si}$, $T_{1/2} = 4.15$ s). If irradiation time is shorter than the equilibrium time (~25 s), the signal yield is reduced.
• Signal-to-Noise Ratio (SNR): Ranged from ~4.4 at the lowest currents to ~94 at the maximum current, setting a practical lower limit for quantitative linear response.

B. Beam Loss Monitoring

• Method: The beam focus was varied (FWHM from 3.5 mm to quasi-flat) to induce controlled losses on a collimator.
• Correlation: A normalized EFM signal proxy ($S_{EFM}$) scaled monotonically with an electrical loss proxy ($S_I$, derived from current measurements).
• Outcome: A quadratic empirical calibration curve was established, allowing beam losses at the collimator to be inferred directly from the EFM signal in similar geometries.

C. Beam Position Monitoring

• Method: A 6.5 mm × 6.5 mm beam spot was steered horizontally and vertically across a beam dump while monitoring four fibers (Top, Bottom, Left, Right).
• Result:
  • The ratio of opposing fibers ($R_H = S_{Left}/S_{Right}$ and $R_V = S_{Top}/S_{Bottom}$) showed a clear, decoupled dependence on beam position.
  • Horizontal steering changed $R_H$ while $R_V$ remained constant, and vice versa.
• Conclusion: The system can distinguish between beam focusing changes and beam drifts, providing a direct measure of position along each axis.

5. Significance and Future Outlook

Significance:
The EFM offers a practical, non-destructive solution for medical cyclotron operations. It complements existing interceptive diagnostics (like SEMs or wire grids) by enabling continuous monitoring during radioisotope production without stopping the beam or degrading its quality. It provides critical observables such as beam losses near apertures and position trends.

Limitations & Mitigations:

• Geometry/Material Dependence: The response is specific to the component geometry and material, requiring site-specific calibration.
• Transport Noise: Signal pickup in transport fibers must be mitigated via careful routing or differential readout.
• Target Activation: For short-pulse beams, target activation (e.g., $^{27}\text{Si}$) can skew results; this must be accounted for in time-dependent measurements.

Future Work (Outlook):

• Material Upgrade: Replacing Ce-doped silica with GAGG:Ce crystals to increase light yield by an order of magnitude. This would improve SNR, extend the dynamic range (especially at low currents), and allow for smaller sensor volumes to improve spatial precision.
• Particle Discrimination: Implementing signal discrimination between neutron- and gamma-induced contributions to better quantify target activation effects.

In summary, the paper validates the EFM as a robust, non-invasive diagnostic tool that can be integrated into routine medical cyclotron operations to enhance beam quality assurance and safety.